Soil Science - Concepts and Applications - HandoutsEt

Managing Editors

Iqrar Ahmad Khan & Muhammad Farooq

Soil Science Concepts and Applications

Muhammad Sabir

Javaid Akhtar

Khalid Rehman Hakeem

University of Agriculture Faisalabad, Pakistan

Muhammad Sabir

Javaid Akhtar Institute of Soil and Environmental Sciences

University of Agriculture, Faisalabad, Pakistan

Khalid Rehman Hakeem Department of Biological Sciences,

Faculty of Science, King Abdulaziz University,

Jeddah, Kingdom of Saudi Arabia

ISBN 978-969-8237-93-6

This work is subject to copyright. All rights are reserved by the Publisher, whether

the whole or part of the material is concerned, specifically the rights of translation,

reprinting, reuse of illustrations, broadcasting, reproduction on microfilms or in any

other physical way, and transmission or information storage and retrieval,

electronic adaptation, computer software, or by similar or dissimilar methodology

now known or hereafter developed. Exempted from this legal reservation are brief

excerpts about reviews or scholarly analysis or material supplied specifically for

being entered and executed on a computer system, for exclusive use by the

purchaser of the work. Duplication of this publication or parts thereof is permitted

only under the provisions of the Copyright Law of the Publisher’s location, in its

current version, and permission for use must always be obtained from the

University of Agriculture, Faisalabad, Pakistan. Permissions for use may be

obtained in writing to the Office of the Books and Magazines, University of

Agriculture, Faisalabad, Pakistan. Violations are liable to prosecution under the

respective Copyright Law. The use of general descriptive names, registered names,

trademarks, service marks, etc. in this publication does not imply, even in the

absence of a specific statement, that such names are exempt from the relevant

protective laws and regulations and therefore free for general use. While the advice

and information in this book are believed to be true and accurate at the date of

publication, neither the authors nor the editors nor the publisher can accept any

legal responsibility for any errors or omissions that may be made. The publisher

makes no warranty, express or implied, with respect to the material contained

herein.

Foreword

The digital age has its preferences. The reading time has been encroached upon by

a watching time. The access to information is easy and a plenty where Wikipedia

has emerged as the most powerful encyclopedia ever. Yet, a book is a book! We

wish to promote the habit of reading books. Finding books is not difficult or

expensive (www.pdfdrive.com) but a local context and indigenous experiences

could be missing.

The University of Agriculture, Faisalabad (UAF) has achieved global rankings of

its flagship programs and acceptance as a leader in the field of agriculture and

allied sciences. A competent faculty, the stimulating ecosystem and its learning

environment have attracted increasing attention. Publication of books is an

important KPI for any institution of higher learning. Hence, UAF has embarked

upon an ambitious ‘books project’ to provide reference texts and to occupy our

space as a knowledge powerhouse. It is intended that the UAF books shall be made

available in both paper and electronic versions for a wider reach and affordability.

UAF offers more than 160 degree programs where agriculture remains our priority.

There are about 20 institutions other than UAF who are also offering similar degree

programs. Yet, there is no strong history of indigenously produced text/reference

books that students and scholars could access. The last major effort dates back to

the early 1990’s when a USAID funded TIPAN project produced a few multiauthor

text books. Those books are now obsoleted but still in demand because of lack of

alternatives. The knowledge explosion simply demands that we undertake and

expand the process anew.

Considering the significance of this project, I have personally overseen the entire

process of short listing of the topics, assemblage of authors, review of contents and

editorial work of 29 books being written in the first phase of this project. Each

book has editor(s) who worked with a group of authors writing chapters of their

choice and expertise. The draft texts were peer reviewed and language corrected as

much as possible. There was a considerable consultation and revision undertaken

before the final drafts were accepted for formatting and printing process.

This series of books cover a very broad range of subjects from theoretical physics

and electronic image processing to hard core agricultural subjects and public

policy. It is my considered opinion that the books produced here will find a wide

acceptance across the country and overseas. That will serve a very important

purpose of improving quality of teaching and learning. The reference texts will

also be equally valued by the researchers and enthusiastic practitioners. Hopefully,

this is a beginning of unleashing the knowledge potential of UAF which shall be

continued. It is my dream to open a bookshop at UAF like the ones that we find in

highly ranked universities across the globe.

vi Foreword

Pakistani soils are derived from alluvial deposits of the river Indus and its

tributaries. These soils are productive but endangered by salinity/sodicity, nutrient

depletion, soil erosion and desertification and low organic matter contents. The

rising population and urbanization are also pouching on fertile agricultural lands.

This book highlights all aspects of Pakistani soils comprehensively and fills the gap

of availability of a proper text in soils sciences.

Before concluding, I wish to record my appreciation for my coworker Dr.

Muhammad Farooq who worked skillfully and tirelessly towards achieving a

daunting task. Equally important was the contribution of the authors and editors of

this book. I also acknowledge the financial support for this project provided by the

USDA endowment fund available to UAF.

Prof. Iqrar A. Khan (Sitara-e-Imtiaz)

Vice Chancellor

Unviersity of Agriculture, Faisalabad

Preface

Soil is the most important and basic natural resource which supports life on the

earth. Soil is developed due to the weathering of rocks, addition of organic matter

and subsequently the profile development. Soil serves as a natural habitat for

microorganisms which are responsible for different ecological and agricultural

functions of the soil. Soil being a natural medium for plants, provides anchorage,

nutrients and water for growth and development of plants. Being a natural and

universal sink for variety of the pollutants, soil occupies the pivotal position in the

environment and maintaining its quality. This book focuses on the beginners in the

discipline of Soil Science and will endeavor to acquaint the students with basic

concepts of soil, its origin, properties and application particularly in agriculture and

environment in general.

This book comprises of 12 chapters, each chapter covers important concepts of

soils, their origin, properties and applications. By reading this book, the students

will be able to understand the soil as a natural medium for plant growth, Soil

Sciences as a scientific discipline, its branches, environment and its branches and

geology and its branches. In the next section of book, the concepts about the soil

formation and different factors affecting soil formation, profile development and

soil morphological features are well explained. The chapter titled “Soil

Classification” introduces the students with soil classification, scientific basis of

soil classification, hierarchies of soil classification and land capability

classification.

After introduction to soils, soil formation and soil classification, next chapter titled

“Chemical Properties of Soils” introduces the students with soil colloid as the most

important and active soil constituents which influence wide range of soil physico-

chemical properties and soil fertility. Soil microbes are very important biological

entity of soils which interact with each other and plants and thus greatly influence

properties of soils. The chapter “soil microbial ecology” provides basic concepts

about soil microbes, their activities and role in cycling of different nutrients and

maintenance of soil fertility. Provision of nutrients to the plants is the basic

function of the soil which highlights its importance in agriculture. Nutrients

cycling, availability to plants, absorption by plants, their different functions,

sources and management are explained in the chapter “Plant Nutrients and Soil

Fertility Management”.

The problem soils particularly the salt-affected soils are affecting agricultural

productivity in arid/semi-arid environment of Pakistan. This important aspect of

soils is explained in the chapter “Salt-affected Soils: Sources, Genesis and

Management”. Environmental pollution is one of the major problems being faced

by the world posing significant health hazards. The chapter titled “Environmental

Pollution and Management” is dedicated to the commonly reported environmental

pollutants, various components of environment (biotic and abiotic), types of

viii Preface

pollutants and their management. Climate change has adverse impact on all the

spheres of life and is the most serious issue among all the global environmental

challenges for human beings. Soil being the sink for carbon plays an important role

in mitigating climate change by sequestering carbon in relatively stable form. The

concepts of climate change and carbon sequestration are well explained in the

chapter “Climate Change and Carbon Sequestration”. Conservation of soil and

water resources is important for sustainability of agriculture and environment. The

last chapter covers this important area and comprehensively introduces the students

with concepts of soil erosion, its types, and management through conservation of

water resources.

This book will encompass all the processes, functions and behavior of soils

comprehensively to acquaint the early stage learners about origin of soils, their

formation and properties and their role to perform different agricultural and

environmental functions. Conclusively, we hope that this book will fulfill the

requirements of undergraduate students regarding the basic concepts of soils, their

origin, their properties, their different types and their role in sustaining agricultural

productivity and environmental quality with special focus on Pakistani soils.

Muhammad Sabir

Javaid Akhtar

Khalid Rehman Hakeem

Chapter 1

Soil, Earth and Environment

Asif Naeem, Muhammad Sabir, Saifullah and Sadia Bibi*

Abstract

The chapter “Soil, Earth and Environment” discusses soil as a natural medium for plant growth as well as explains its other functions including nutrient cycling, regulation of water and carbon dioxide supplies and a medium for landscaping. Approaches to the study of soil i.e., soil science and its branches, have been elaborated in detail. Elemental composition of Earth crust, types of rocks (igneous, metamorphic and sedimentary), their mineralogical composition and occurrence in Earth’s crust have been described. The term “mineral” has been defined, and different minerals, their properties and their role in soil genesis have been described. Classification of minerals is based on their physical properties, like crystal structure and habit, hardness, luster, diaphaneity, color, streak, cleavage and fracture, and specific gravity. This chapter also describes the four spheres of Earth, namely atmosphere, lithosphere, hydrosphere and biosphere in terms of their properties and effects on environmental conditions. In addition to that, geology and its branches (physical geology, mineralogy, petrology, mining geology and hydrology), environment and its elements, and environmental science and its branches (ecology, atmospheric science, environmental chemistry, environmental engineering and geosciences) have been covered in detail.

*Asif Naeem Soil Science Division, Nuclear Institute for Agriculture and Biology, Faisalabad, Pakistan. For correspondance: scoutuaf@gmail.com Muhammad Sabir, Saifullah and Sadia Bibi Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

2 A. Naeem, M. Sabir, Saifullah and S. Bibi

Keywords: Soil, Rocks and Soil Minerals, Geology, Spheres of Earth, Environmental Science.

1.1. Soil

In its traditional meaning, “soil” is the natural medium for plant growth, whether it has well defined layers, i.e., the soil horizons. This is the common meaning for the term “soil” and the greatest interest is aligned to it. Since soil supports plants that provide food and cater other human needs, filters water and recycles all sort of wastes, it is essential for life on earth. Excluding bare rocks, deep water, areas of everlasting frost, or the sterile ice of glaciers, soil covers the surface of Earth as a continuum. Soil, in this context is a natural body comprising of solids, liquid, and gases occurring on the surface of land and is characterized by the presence of one or both of following: i) horizons, that are discernible from the original material as a result of additions, losses and transformations of energy and matter; ii) the ability to support the rooted plants in a natural environment (Soil Survey Staff 1999). Thus, soil may be defined as “The unconsolidated mineral material on the immediate surface of the Earth that functions as a natural medium for plant growth on land surface”. There are two approaches to study of soil: First approach considers soil as a natural body, weathered and synthesized product, while the other treats soil as a medium for growth of plants.

1.1.1. Pedological approach

Pedology (from Greek “pedon”, means soil or Earth) is the study of origin, classification and description of soils as they occur in their natural environment. A pedologist examines and classifies soil as a natural body and does not emphasize on its practical use.

1.1.2. Edaphological approach

The properties of soil in relation to plant growth, reasons for variation in soil productivity and methods to improve soil productivity are studied in Edophology (from Greek word “edaphos”, means soil or ground). Edaphologists are more practical and their ultimate goal is to study soils in relation to their production of food and fiber.

1.2. Functions of Soils

Soils perform many agricultural and non-agricultural functions in the global ecosystem which are described as follows:

1.2.1. Medium for plant growth

As an anchor for plant roots and as a water holding tank, soil provides a hospitable place for a plant to take root. Soil properties those affect plant growth include its texture (coarse or fine), aggregate size, pH, salt concentration, porosity, and water

Soil, Earth and Environment 3

holding capacity. The ability to store and supply nutrients to plants is an important function of soil and is referred to as soil fertility. Soil fertility is influenced by the amount of clay and organic matter content in the soil. High clay and organic matter contents generally lead to a highly fertile soil.

1.2.2. Nutrient recycling

Being the major "switching yard", soil stores and regulates the transformation of plant nutrients and other elements in the ecosystem. A generalized explanation of nutrient cycling is shown in Fig. 1.1. These biological and geochemical processes are governed by nutrients’ transformation into forms available for plant absorption, adsorption onto soil, and loss into air or water. Decomposition of organic matter by soil microorganisms liberates elements from the complex materials, drives them back into rotation and hence is pivot of all the transformations. During the decomposition of organic matter, generally but not always, complex compounds are converted into simpler ones.

Cycling of nutrients can be assessed from soil fertility status, organic matter content and soil reaction (pH) indicators. Soil fertility indicators are mineral nitrogen and plant available phosphorus, potassium, sulfur, calcium, magnesium, zinc and boron. Organic matter indicators are soil organic matter content, carbon to nitrogen ratio, particulate organic matter, microbial biomass carbon and activity of soil enzymes.

Fig. 1.1 Schematic diagram of nutrient cycling

Producer Consumers

Secondary

consumer

Decomposer

1.2.3. Regulation of water supplies and environmental

interactions

Water falling on land in the form of snow or rain is absorbed and stored by soil for later use. The stored pool of water remains available for plant absorption and use by soil organisms to sustain their life between precipitation or irrigation happenings. This ability of soil to hold a specific amount of water against the pull of gravity is known as its water holding capacity (WHC). Upon soil saturation, some of water will drain down into its profile and the leftover will be held in the soil until it evaporates or is drawn into plant roots, which ultimately transpire it through leaves. Infiltration of water into a soil profile is influenced by its texture, structure and the amount of vegetative cover on it. Coarse textured (sandy) soils have lesser water retention ability and therefore allow its rapid infiltration, whereas fine textured soils have an abundance of micro-pores causing low rate of infiltration and allowing the soil to retain a lot of water. Thus, some salient soil properties can regulate drainage and storage of water and its dissolved solutes, including plant nutrients, pesticides and other compounds. In short, soil’s function is to partition water for groundwater recharge and use by plants and animals.

Qualities of soil and water are strongly linked to each other, and, to a large extent, soil properties are governed by the quality of water. When water passes through a soil profile, dissolved nutrients and other compounds are retained in it and the water gets filtered and purified. Consequently, aquifer recharge takes place with clean water and risk of eutrophication (a process whereby presence of excess nutrients in water bodies stimulates growth of algae and other aquatic plants) of lakes and other water bodies is reduced.

1.2.4. Landscaping and engineering medium

Soil serves as foundation for all civil structures including houses, buildings, roads and other structures which are set upon it but there is great variation in physical properties of different soil types. Landscape applications range from the gardens and lawns of residential houses to the bridge and roadway construction around highway interchanges. Both the physical and ecological functions of soils must be considered in all these instances. Soil properties including compressibility, bearing strength, shear strength, consistency and shrink-swell potential of soil are of concern in engineering and construction works. These engineering properties in turn are governed by the most basic soil physical properties including type of clay minerals, texture and structure.

1.2.5. Carbon storage and maintenance of gaseous balance in

Organic carbon is an extremely important component of soil which is originally created through photosynthesis by plants. It plays significant roles in improving nearly all properties of soil including moisture retention, soil structure, drainage and nutrient storage. Storage of carbon in soil is essential in decreasing the amount of carbon dioxide (CO2) in the atmosphere, thereby regulating climate change. Complex

organic molecules are continually broken down by soil organisms into simpler organic and inorganic molecules/elements. Some of them are released into atmosphere as gases such as CO2. However, humification, a biological transformation whereby more complex and stable organic matter is formed, is also carried out by some soil microorganisms. The amount of organic carbon in a soil is determined by the balance between input and loss of carbon. Globally, soil contains about three times as much carbon as vegetation cover on it and twice as much as in the atmosphere. Land use practices such as the conversion of grasslands to arable lands, arable lands to commercial ones and cultivation of organic soils are the major source of CO2 emission from soil.

1.3. Soil Science

Soil science may be defined as “The branch of science dealing with soil as a natural resource on surface of the Earth, including pedology (soil genesis, classification and mapping) and the physical, chemical, biological and fertility properties of soil and their relation to crop production.

Soil Science is further divided into six well defined and developed disciplines which reflect its scope.

1.3.1. Soil mineralogy

Soil mineralogy deals with the primary and secondary minerals and their contribution to chemical, physical and biological properties, and fertility of soils.

1.3.2. Soil fertility

It addresses the ability of soil to supply essential nutrients and water to plants in sufficient amounts and proportions for their growth and development in the absence of toxic substances. Nutrient supplying power of a soil primarily depends on how much of the nutrients are inherently present, in which forms these are present, organic matter content and reaction (pH) of the soil and the rate of organic matter mineralization.

1.3.3. Soil chemistry

Soil chemistry primarily deals with chemical reactions in soil contributing to soil development and those affecting plant growth. Since concerns have grown about soil health risks, the recent emphasis in soil chemistry has shifted from agricultural soil science to environmental soil science. Soil pH, anion and cation exchange capacity, clay mineralogy, sorption and precipitation reactions, oxidation-reduction and soil chemical equilibria are the key areas studied under soil chemistry.

1.3.4. Soil physics

Soil physics deals with physical properties and processes in soils. It deals with the dynamics of physical soil components by drawing on the principles

of physics, physical chemistry, engineering, and meteorology. Soil physics applies these principles to address practical problems of agriculture, ecology, and engineering.

1.3.5. Soil microbiology

Soil microorganisms include bacteria, fungi, actinomycetes, algae and protozoa. Microorganisms in soil are important because they affect the structure and fertility of soils. Soil microbiology deals with microorganisms, their population, classification and role in soil transformations and how they affect soil properties. The soil organisms have different characteristics which determine their role in the soil they live in.

1.3.6. Soil conservation

It involves how to safeguard the soil against physical loss by erosion (by water and wind) or chemical deteriorations. Thus, soil conservation is concerned with a combination of all management and land-use methods that protect the soil from deterioration caused by human or natural factors.

1.3.7. Pedology

Pedology deals with soil genesis, classification and its survey for different land uses. Soil genesis deals with weathering of rocks and minerals and factors affecting the weathering process.

1.4. Earth

Earth can be described as a sum total of land, air, water and its organisms. The land includes valleys and mountains, while air is a mixture of different gases with nitrogen (N2) and oxygen (O2) being the dominant ones. Water occurs as oceans, lakes, rivers, streams, rain, snow and ice. The mass of Earth, approximately 5.98×1024 kg, is mostly composed of iron (32 %), oxygen (30 %), silicon (15 %) magnesium (14 %), sulfur (3 %), calcium (1.5 %) and aluminum (1.4 %) and 3.2 % of trace elements (of which 2% is nickel). Due to mass segregation, primarily the core region of Earth is composed of iron (88.8%), sulfur (4.5%) and minor amounts (6.8%) of trace elements (including 5.8% nickel). Since more common rocks of the Earth's crust are nearly all oxides, oxygen constitutes more than 47% of it. Silica, alumina, iron oxides, lime, magnesia, potash and soda are principal oxides in the Earth Crust (Morgan and Anders 1980).

1.4.1. Minerals

Any naturally occurring abiogenic solid substance that is stable at room temperature, has a chemical formula and ordered atomic structure, is known as mineral (Dyar and Gunter 2008). Exceptions to the rule of stability at room temperature include mercury and water which are liquid at room temperature. Similarly, the criteria of being abiogenic and with a structured arrangement are also controversial and some organic

compounds have been assigned a separate class. Therefore, many scientists proposed to amend the definition of mineral so that biogenic or amorphous substances may also be considered as minerals. That’s why, International Mineralogical Association (IMA) now defines a mineral as "an element or chemical compound that is normally crystalline and has been formed because of geological processes” (Nickel 1995). In the light of this definition, organic class of minerals has again been included in both of Dana and the Strunz mineral classification schemes (Skinner 2005). Over 60 biominerals had been published prior to the listing of IMA (Veis 1990).

Minerals that persisted or changed little in their chemical composition since they were formed from molten lava are called primary minerals, e.g. quartz, micas and feldspar. They are dominant in sand and silt fractions of soil. Secondary minerals are those which were formed by breakdown of less persistent primary minerals and tend to dominate in clay and to lesser extent in silt fraction of soil, e.g., silicate clays and iron oxides (Weil and Brady 2016).

1.4.1.1. Physical properties of minerals

In classification, minerals range from simple to complex ones. Some minerals can be fully identified based on their physical properties only while others require complex methods, for their identification, like X-ray diffraction analysis. Physical properties used in classifying the minerals include crystal structure and habit, hardness, luster, diaphaneity, color, streak, cleavage and fracture, and specific gravity.

i. Crystal structure and habit

Every mineral has a regular and geometric internal arrangement of atoms or ions resulting in a specific crystal structure. Crystal structure is always periodic in nature even when the mineral grains are indiscernible and irregular in shape. Crystals are classified into six families based on relative lengths of the crystallographic axes and the angles between them.

The six crystal families are restricted to 32 classes (point groups) which differ in their symmetry. Due to restriction of 32 classes, minerals of different composition may have identical crystal structure and are called isomorphic minerals. The examples are halite (NaCl), galena (PbS), and periclase (MgO) which all are isometric (hexaoctahedral) minerals. Polymorphs, on the other hand, is group of minerals having same chemical formula but different structure. Pyrite and marcasite, for example, are isometric and orthorhombic forms of iron sulfides (FeS2). The behavior of minerals to have same chemical formula but different crystal structure is named as polymorphism (Dyar and Gunter, 2008).

Minerals differing in crystal structure greatly differ in other physical properties. The common example is diamond and graphite allotropes of carbon, the former is the hardest mineral, has adamantine luster and crystallizes in the symmetry, whereas the later is very soft, has greasy lustre and belongs to hexagonal crystal family.

Crystal habit defines the overall shape of a crystal. Common shapes include bladed, acicular (needle like common in natrolite), equant, (typical example is garnet), dendritic (tree-pattern, as in native copper), tabular, prismatic (Chesterman and Lowe 2008).

ii. Hardness

Resistance of a mineral towards scratching is named as its hardness and it is determined by the chemical composition and crystalline structure of the mineral. All sides of a mineral do not necessarily show the same hardness. Crystallographic strength renders some directions harder than others as in kyanite, which has a Mohs hardness of 5½ parallel to [001] but 7 parallel to [100]. The ordinal Mohs scale is the most common scale of hardness measurement. It is defined by ten minerals so that a mineral with a higher index scratches those below it (Table 1.1) (Dyar and Gunter 2008).

iii. Luster and diaphaneity

How light reflects from the mineral's surface regarding its quality and intensity is known as luster. It may either be metallic/sub-metallic which is identified by high metal like reflectivity, as in galena and pyrite or non-metallic which may be vitreous (a glassy luster, in silicate minerals), adamantine (in diamond), pearly (in apophyllite and talc), silky (in fibrous minerals like asbestiform chrysotile) and resinous (in members of the garnet group).

The diaphaneity is defined as the ability of a mineral to pass light through it. Mineral may be transparent (muscovite), translucent (nephrite) and opaque (graphite) (Busbey et al. 2007; Dyar and Gunter 2008).

Table 1.1 Mohs hardness scale

Mohs hardness Mineral Chemical formula

1 Talc Mg3Si4O10(OH)2

2 Gypsum CaSO4·2H2O

3 Calcite CaCO3

4 Fluorite CaF2

5 Apatite Ca5(PO4)3(OH, Cl, F)

6 Orthoclase KAlSi3O8

7 Quartz SiO2

8 Topaz Al2SiO4(OH, F)2

9 Corundum Al2O3

10 Diamond C Source: Dyar and Gunter (2008)

iv. Color and streak

Color of minerals is the most obvious and usually, not always, a non-diagnostic property. The minerals in which color is diagnostic property e.g. malachite (green) and azurite (blue), have dichromatic elements in their composition. In contrast, allochromatic elements are present in colorless minerals in trace amounts. Examples

of such minerals are ruby and sapphire varieties of corundum mineral. The colors of pseudochromatic minerals (opal, bornite) are due to of interference of light waves. Play of color when the mineral is turned, as in opal, is due to variation in reflection of different colors. The phenomenon of change in color as light passes through a mineral in a different orientation is known as pleochroism (Busbey et al. 2007; Dyar and Gunter 2008).

The color which a mineral give in powder form is known as its streak. Streak plate made of porcelain and colored either black or white is most commonly used for testing this property. Streak may be different or identical to the body color of mineral. The body color of hematite is black, silver or red but its streak is cherry-red to reddish-brown (Busbey et al. 2007; Dyar and Gunter 2008).

v. Cleavage, fracture and tenacity

The breakage of a mineral along the planes of weakness of crystal structure is known as cleavage. The quality of cleavage is determined by how clean and easy mineral breakage takes place. In decreasing order of quality, "perfect", "good", "distinct", and "poor" are common descriptors of cleavage. Cleavage can be seen as series of parallel lines marking the planar surfaces in transparent mineral or in thin-section. Cleavage is not a universal to all minerals, for example, quartz does not cleave due to absence of crystallographic weakness. Contrarily, in micas, weekly bounded silica tetrahedra sheets perfectly cleave along basal line. Since cleavage is a function of crystallographic weakness, cleavage may be one, two, three, four, or six directional cleavage (Chesterman and Lowe 2007; Dyar and Gunter 2008).

Fracture is breakdown of mineral that does not follow the cleavage plan. There are several types of fracture including conchoidal (whereby rounded surfaces are formed as in quartz), splintery, fibrous, and hackly (Dyar and Gunter 2008).

Resistance of a mineral towards both cleavage and fracture is known as tenacity. Minerals can be ductile, malleable, brittle, sectile, flexible, or elastic in tenacity (Dyar and Gunter 2008).

vi. Specific gravity and other properties

Specific gravity is defined as the ratio of the mass of the mineral to the difference between its weight in air and water. Specific gravity of rock forming minerals - especially silicates and sometimes carbonates ranged from 2.5 to 3.5. Due to having elements with higher atomic mass in composition, oxides and sulfides have a higher specific gravity. Various other properties such as effervesce, magnetism, smell, radioactivity can be used as diagnose to minerals (Dyar and Gunter 2008).

1.4.2. Rocks

Geologically, a rock is coherent assembling of minerals or mineraloids of variable composition by the action of heat and water into a solid aggregate that forms a part of the Earth crust. For example, granite (a common rock) is composed of quartz, feldspar and biotite minerals. Rocks form nearly whole of the outer solid layer of Earth crust, i.e., lithosphere.

1.4.2.1. Types of rocks

The minerals forming the rocks are held together by chemical bonds in an orderly manner. Rock forming processes determine the types and abundance of minerals in a rock. Since most of the rocks contain silica, its proportion determines the properties and name of a rock (Wilson 1995). In addition to chemical composition, rocks are geologically classified according to texture and size of the constituent particles (Blatt and Tracy 1996). Geological model, also called as the rock cycle, explains the transformation of one type of rock into another with the course of time. Based upon the mode of formation, rocks are categorized into three major types, i.e. igneous, sedimentary and metamorphic which are explained in Chapter 2.

1.4.3. Spheres of Earth

The Earth’s environment comprises of atmosphere, lithosphere, hydrosphere and biosphere.

1.4.3.1. Atmosphere

A layer of gases held around the Earth by force of gravity is known as atmosphere. The first 32 km above the Earth's surface contain about 99% of the total atmospheric mass. Atmosphere is divided into several concentric strata or layers, because of variations in temperature which result mainly from differential absorption of solar radiations.

Troposphere is the atmospheric layer closest to the Earth and due to vigorous convective air currents within this stratum, it is named so which mean the "region of mixing". It contains about 80 % of the total atmospheric mass. Troposphere retains about 99 % of atmospheric water vapors which play a major role to regulate air temperature owing to their ability to absorb solar radiations and heat energy from the Earth’s surface. The stratosphere extends after the troposphere to an altitude of about 50 km above the Earth's surface. Water vapors in this region are very low and ozone gas, about 90 % of which resides within 15-25 km of this layer. Ozone absorbs solar ultraviolet radiation ranging in wavelength from 290-320 nm and, thus, regulates thermal regime of the stratosphere.

The mesosphere extends from approximately 50 to 85 km above the surface and is characterized by low temperature. The stratosphere and mesosphere jointly are sometime referred to as the middle atmosphere. The thermosphere is located above the mesosphere. Above this altitude, atmosphere becomes too thin to support aircraft and vehicles. Beyond about 160 km altitude, atomic oxygen becomes the major atmospheric component. The most distant atmospheric region from Earth's surface is the exosphere. The exosphere is a transitional zone between atmosphere and space (Skinner and Porter 1987).

1.4.3.2. Lithosphere

The term lithosphere is derived from two Greek words: lithos, means rocky, and sphaira, means sphere. It is the rigid shell of Earth and can be identified based on its mechanical properties (Skinner and Porter 1987). The upper part of the lithosphere chemically interacts with hydrosphere, atmosphere and biosphere, and is

called as the pedosphere. The difference in response to stress between the lithosphere and the underlying asthenosphere defines the boundary between these two layers. There are two types of lithosphere; the one which underlies the ocean basin, named as oceanic lithosphere, and the other is continental lithosphere lying below continental crust.

1.4.3.3. Hydrosphere

Hydrosphere (from Greek word - hydōr, "water" and - sphaira, "sphere") defines the whole amount of water over and under the Earth’s surface. There are 1.386 × 109 km3 of water on Earth with a total mass of 1.4 × 1018 tons (0.23 % of Earth’s mass). About 20 × 1012 tones of total water is in the Earth’s atmosphere. Of the total ware, saline water is 97.5 % and rest is freshwater. About 68.7% of freshwater is in the "form of ice whereas the remaining 29.9% is found as groundwater. Only 0.26% of the fresh water is concentrated in lakes, reservoirs and rivers which is accessible for our economic use. Approximately 75% of the Earth's surface is covered by ocean. Transfer of water from one state or reservoir to another, known as Hydrological cycle (Fig. 1.2), is driven by solar energy and force of gravity (de Villiers 2003).

Fig. 1.2 Schematic diagram of water cycle

1.4.3.4. Biosphere

The term "biosphere" was introduced by geologist Eduard Suess in 1875, which he described as “the place on Earth's surface where life dwells”. The biosphere, the global sum of all ecosystems, is referred to as a self-regulating zone of life on Earth and it is a closed system. So, it is the global ecological system in which all organisms integrate with each other and with other elements of the system such as atmosphere, hydrosphere and lithosphere. The biosphere is hypothesized to have been evolved at least 3.5 billion years ago, either from non-living matter (biopoesis) or by biogenesis, i.e., creation of life from living matter (Campbel et al. 2006).

1.5. Geology

Geology (from Greek “Geo”, means Earth, and Logos means Science) is known as the Earth Science and it deals with the study of origin, structure, composition and history of Earth which includes the development of life and nature of the processes occurring on Earth. Geologists discover new deposits of rocks and minerals of economic value and thus play a significant role in the development of a nation. For

Rainfall

Soil Clouds

Ocean and Rivers

ease of studying, science of Geology has been categorized into different branches as detailed below:

1.5.1. Physical geology

The study of the processes occurring on Earth by the action of physical agents, including water, wind, glaciers and sea waves, is known as physical geology. Physical geology contributes a significant role in civil engineering through revealing constructive and destructive processes driven by physical agents such as erosion, transportation and deposition of particulate matter. It helps in selecting a suitable site for different types of projects to be undertaken at a specific site.

1.5.2. Mineralogy

Mineralogy deals with the study of chemical composition, crystal structure and physical properties of minerals. More specifically, scientific studies of the processes of mineral formation and classification, geographical distribution and utilization of minerals are undertaken in mineralogy. Mineralogy with emphasis on minerals of economic importance is known as economic geology.

1.5.3. Petrology

Petrology is the branch of geology which deals with the study of rocks. It is further subdivided into: (i) Structural geology also named as tectonics, deals with identification and arrangements of structures found in rocks. It plays an important role in the selection of suitable sites for dams, tunnels, multistoried buildings, etc.; (ii) Stratigraphy deals with stratified rocks.

1.5.4. Paleontology

The branch of geology which involves the study of fossils, the prehistoric remains of organisms, is called as paleontology. Study of fossils describes how animals and plants had evolved and migrated through ages and climate of an area.

1.5.5. Mining geology

Mining geology deals with study of application of geological principles to mining engineering for selection of sites suitable for quarrying and making mines.

1.5.6. Hydrology

Both the qualitative and quantitative study of water which is present under and/or above the surface of Earth and in the atmosphere, is known as hydrology.

1.6. Environment

In literal sense, environment refers to external conditions or stimuli influencing development of people, animals or plants. By this definition, environment consists

of intellectual, physical, economic, social, cultural, political, moral and emotional forces acting upon an organism from its conception to its death. Thus, environment has great influences on behavior, nature, growth, development and maturity of organisms.

Environment is a system of physical, biological and cultural elements inter-related to each other both individually as well as collectively in various ways. Physical elements include space, climate, water bodies, landforms, rocks, minerals and soils. These determine the character and opportunities as well as limitations of humans. Biological elements include micro-organisms, plants, animals, and people whereas cultural elements are artificial features such as social, economic and political values.

1.6.1. Environmental Sciences

As a branch of science, environmental science examines physical and natural environments of Earth and their complex interactions with humans. It is a multidisciplinary approach whereby it involves integrated application of atmospheric and Earth science, ecology, biology and chemistry to study the environment.

In the recent scenario, environmental pollution may be defined as the study of environmental pollutants, their sources and channels to various environments and remediation of the contaminated environments. The branches of environmental science include atmospheric science, ecology, environmental chemistry, environmental engineering and geosciences.

1.6.1.1. Ecology

Ecology is study of the interactions among organisms and with their environment. The concept of ecology is well explained by biodiversity, population of living organisms and competition between organisms and components of ecosystems. Ecosystems consist of interactions among biotic and abiotic components of the ecosystem and their environment.

1.6.1.2. Atmospheric science

Atmospheric science deals with the study of atmosphere, its processes, the effects of other systems on the atmosphere, and vice versa. It includes meteorology, climatology, aeronomy and planetary science. Meteorology makes use of atmospheric chemistry and atmospheric physics for weather forecasting while climatology addresses the changes in atmosphere that define change in climate over time and, hence, the average climate. The study of the upper atmospheric layers is known as aeronomy. Planetary science deals with the study of atmospheres of the planets of the solar system.

1.6.1.3. Environmental chemistry

Environmental chemistry is an interdisciplinary approach that makes use of atmospheric, aquatic, soil and analytical chemistry to study chemical and biochemical processes occurring in the environment and are impacted by humans.

1.6.1.4. Environmental engineering

Environmental engineering is the integration of environmental science and engineering principles to provide healthy natural environments, including air, water and land by cleanup of pollution sites for safe living of humans and animals. It can also be defined as the branch of applied science and technology that focuses on issues of energy preservation and control of waste productions by human and animal activities. The major areas addressed under environmental engineering are management and control of wastewater and air pollution, disposal and recycling of various wastes, radioprotection and public health, industrial hygiene and environmental sustainability.

1.6.1.5. Geoscience

Geoscience is a special branch of planetary science dealing with planet Earth. The reductionism approach (a complex system is a collection of its parts) of Earth sciences includes the study of biosphere, hydrosphere and atmosphere as well as the solid Earth. Typically, Earth scientists build a quantitative understanding of how the Earth system works, and how it had evolved to its current state by applying principles of mathematic, physics, chemistry, biology and chronology.

1.7. Conclusions

In this chapter, we have discussed the soil as a natural medium for plant growth. Soil is characterized by the presence of well-developed profile consisting of soil horizons. Soil supports plants, filters percolating water and recycles organic waste by serving as a sink. Origin of parent material, soil minerals, development of soil profile, soil formation, physical, chemical and biological properties of soils, nutrient availability to plants and problem soils studied under the umbrella of different branches of soil science are explained in this chapter. The origin of rocks from lava, different types of the rocks and their properties greatly influence the soil formation processes and different biogeochemical properties the soils thus formed. Geology and its different braches which provide insight to all these processes are also discussed in this chapter. Environment being sum of all the factors influencing life cycle of a living organism is greatly influenced by different processes being occurred in soil and geological perspectives. After studying this chapter, students would be able to understand the nexus between soils, rocks and the environment.

References

Blatt, H. and R.J. Tracy (1996). Petrology, 2nd Edition. W.H. Freeman Company, New York, USA.

Weil, R.R. and N.C. Brady (2016). The Nature and Properties of Soils, 15th Edition. Pearson Education Inc., Upper Saddle River, New Jersey, USA.

Busbey, A.B., R.E. Coenraads, D. Roots and P. Willis (2007). Rocks and Fossils. Fog City Press, San Francisco, USA.

Campbell, N.A., B. Williamson and R.J. Heyden (2006). Biology: Exploring Life. Pearson Prentice Hall Boston, Massachusetts, USA.

Chesterman, C.W. and K.E. Lowe (2008). Field Guide to North American Rocks and Minerals. Toronto, Canada.

de Villiers, M. (2003). Water: The Fate of Our Most Precious Resource. McClelland and Stewart, Toronto, Ontario, Canada.

Dyar, M.D. and M.E. Gunter (2008). Mineralogy and Optical Mineralogy. Mineralogical Society of America, Chantilly, Virginia, USA.

Morgan, J.W. and E. Anders (1980). Chemical composition of Earth, Venus, and Mercury. Proc. Nat. Acad. Sci. 77: 6973–6977.

Nickel, E.H. (1995). The definition of a mineral. Can. Minerol. 33: 689–690. Skinner, B.J. and S.C. Porter (1987). Physical Geology. John Wiley and Sons, USA. Skinner, H.C.W. (2005). Biominerals. Mineral Manage. 69: 621-641. Soil Survey Staff (1999). Soil Taxonomy: A Basic System of Soil Classification for

Making and Interpreting Soil Surveys, 2nd Edition. Natural Resources Conservation Service, U.S. Department of Agriculture, Washington D.C., USA.

Veis, A. (1990). Biomineralization. In: Simkiss, K. and K.M. Wilbur (Eds.), Cell Biology and Mineral Deposition. Academic Press, San Diego, CA, USA.

Wilson, J.R. (1995). A Collector’s Guide to Rock, Mineral and Fossil Localities of Utah. Utah Geological Survey, Utah, USA.

Chapter 2

Concepts in Soil Genesis and

Morphology

Shamsa Kanwal, Hamaad Raza Ahmad and Irshad Bibi†

Abstract

Soil is a natural body that comprises of solid, liquid and gaseous phases, and is characterized by horizons/layers, different from the initial material from which it has been formed. Soil supports plant growth and has properties resulting from the integrated effect of climate and living organisms acting upon the parent materials determined by relief/topography for a certain time. Soil forming factors and processes work in integration to create a soil with unique properties. This concept of soil formation was given by the Russian school in 1870. Soil genesis starts with the weathering/disintegration of rocks (i.e., igneous, sedimentary or metamorphic) which creates the initial material for its development. However, it is a long time requiring process and it may take hundreds to thousands of years to develop a complete soil profile. Morphology of a soil is the reflection of the processes responsible for its formation. Morphology encompasses the form, structure and organization of soil material responsible for development of soil horizons, color and other features. Every soil has its own distinct morphology which is a consequence of interaction among different soil forming factors and processes. Soil morphology not only tells what is there and where it is but also how it was formed. Hence, by interpreting soil morphological features one can have a good idea about its genesis.

Keywords: Soil Morphology, Genesis, Horizons, Pedogenic Processes, Rocks and Minerals, Climate, Soil Profile

†Shamsa Kanwal*, Hamaad Raza Ahmad and Irshad Bibi

Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: skanwal1375@gmail.com Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

18 S. Kanwal, H.R. Ahmad and I. Bibi

2.1. Introduction

The basic concept of soil has evolved through a long period of scientific studies. Various concepts were put forward at different times, however, the latest concept favored by the American school of thought is generally considered more appropriate. According to this concept, soil is defined as a “collection of natural bodies occupying a portion of the Earth’s surface that supports plants or is capable to support plants out of door and that have properties due to integrated effect of climate and living matter acting upon parent material, as conditioned by relief over periods of time” (Brady and Weil 2007). At places, a soil may be modified or even made of earthy materials anthropogenically. It extends into shallow water or air on upper side and into deep water or to barren areas comprising of rocks or ice at its margins. However, its lower limit is the most difficult to define. Soil includes layers/horizons relatively close to its surface, which differ from the underlying rock or parent material, formed because of interactions of climate, living organisms, parent material and relief, over a geological time period. Commonly, soil grades to its lower margin to a hard rock or to an earthy material virtually devoid of roots, animals or marks of any other biological activity. The lower limit of soil, therefore, is normally the lower limit of biological activity.

The biological activity or pedogenic processes may extend to depths greater than 2 m; consequently, the lower limit of soil for classification purposes is arbitrarily set to 2 m. Thus, a soil is a three-dimensional body having length, width and depth. A soil is unconsolidated material that results from the weathering of hard, consolidated materials like rocks, several soil forming processes are involved which convert the material into a soil. The sequence of soil genesis processes can be conceptualized as below:

Rocks → Weathering → Parent material → Soil formation factors and processes → Soil

2.2. Rocks and Minerals

Rocks are hard consolidated masses formed by the assemblage of one or more mineral grains. Rocks can be classified into different types depending upon their chemical composition as well as their mode of formation.

2.2.1. Igneous rocks

These are the most abundant rocks present in the Earth crust, originated from the solidification of molten lava or magma. Geologists consider that igneous rocks were the first rocks formed on the Earth whereas sedimentary and metamorphic rocks were formed later, from igneous rocks. The word igneous is derived from a Latin word meaning fire, which provides a clue to the origin of these rocks. There are many weak points in the earth crust, from where molten material may make its way through these points the surface of earth. Once the molten material has escaped from the conditions of high temperature and pressure which exist deep under the Earth’s surface, it begins

Concepts in Soil Genesis and Morphology 19

to cool down. Eventually, the flowing molten material slows down, stagnates, solidifies and becomes a solid igneous rock. The characteristics of igneous rocks are determined by the composition of the original molten material from which it was formed. Salient examples of igneous rocks include Granite, Gabbro and Basalt.

2.2.2. Sedimentary rocks

Sedimentary rocks are formed from the layers of accumulated sediments. These rocks are formed by the deposition and re-cementation of weathered products of other rocks. For example, quartz sand weathered from granite and deposited at the bottom of pre-historic sea may, through geological changes, have become cemented into solid mass called sandstone. Similarly, re-cemented clays are termed as shales. Pre-existing rocks of any group (igneous, metamorphic and sedimentary) are broken down by the continuous process of weathering to form debris known as sediments. The process continues over millions of years and early sediments are gradually buried and more sediment accumulates. The accumulated sediment is finally compacted through the weight of the sediment layers one above the other; the water squeezes off and the minerals become cemented. In the process of sedimentary rocks formation, layers of sediments are built up, layer over layer. Layering is, therefore, a fundamental characteristic of these rocks. Each layer is separated from the one above and the one below by a line of demarcation known as bedding plane. This generally represents the sudden change in the grain size or in the composition of the sediment being laid down. Examples of sedimentary rocks include shale, mudstone and limestone.

2.2.3. Metamorphic rocks

Meta means change and Morph means shape or form. Therefore, metamorphic rocks are those rocks which are formed due to change in shape of pre-existing igneous or sedimentary rocks under very high temperature and pressure. The process of change is called metamorphism. Igneous and sedimentary masses subjected to tremendous pressure and high temperature succumb to metamorphism. This process of metamorphism generally occurs deep within the Earth’s outer layers. This is the reason that most of metamorphic rocks appear after the considerable erosion of overlying material. The metamorphic rock “slate” is derived from sedimentary shales and mudstone whereas sedimentary limestone can be metamorphosed into marble. Some examples of metamorphic rocks are given below;

Igneous/Sedimentary rock transformed into Metamorphic rock

Granite → Gneiss Shale → Slate Mudstone → Slate Limestone → Marble

2.3. Soil Minerals

Soil minerals are naturally occurring inorganic substances present in soils, having definite chemical composition and crystalline structure. These may either be primary minerals or secondary minerals.

2.3.1. Primary/original minerals

These are the original minerals which have not changed since their formation from molten lava or magma. They are abundantly found in igneous and metamorphic rocks, e.g., quartz, microcline, orthoclase, muscovite, biotite, augite, hornblende, etc.

2.3.2. Secondary minerals

These are the minerals which have been transformed from the primary minerals. During weathering, primary minerals break down into their respective elements, these elements then recombine to produce secondary minerals. These are commonly found in sedimentary rocks, e.g., calcite, dolomite, gypsum, apatite, limonite, hematite, gibbsite and layer silicate clay minerals.

2.4. Weathering

Weathering refers to physical and chemical transformation of rocks or minerals, present in the Earth’s crust. The rocks and minerals which are not in equilibrium with the Earth’s surface temperature and pressure are transformed and broken down to smaller fragments or into secondary minerals. A number of physical, chemical and biological activities are responsible for carrying out weathering either of the rocks and/or for developing of soil profiles. Some authors make a distinction between weathering within and beyond the soil profile. Weathering that takes place in the soil solum is referred to as pedochemical weathering and that which takes place below the soil solum is termed as geochemical weathering.

2.4.1. Physical/Mechanical weathering

Physical/mechanical weathering refers to breakdown of larger rock fragments (Fig. 2.1) into smaller ones without any change in chemical composition of rocks or minerals (Schaetzl and Anderson 2005). As particles are broken down into smaller fragments, their surface area is increased and they become more prone to chemical weathering. Several biotic and abiotic factors are involved in physical weathering, like temperature, moisture, wind, ice, plants, animals and human beings. The role of various factors in physical/mechanical weathering is elaborated below.

2.4.1.1. Temperature

Temperature is an important agent that causes a considerable change in rock fragments as well as within a developing soil profile. Generally, rocks are composed of more than one mineral resulting in differential expansion and contraction of minerals in rocks when rocks heat up and cool down during day and night,

respectively. As some minerals expand more than others, temperature changes induce differential stresses which eventually cause the rocks to crack apart. Furthermore, the outer surface of a rock is often warmer or colder than the inner portion of the rock, slowly resulting in peeling away of the outer layers. This process is called exfoliation.

2.4.1.2. Water

Water is also an important weathering agent and, thus, has a deciding influence on soil formation and profile development. An area having heavy rainfalls is prone to accelerated rates of weathering. Water enters cracks present in rocks. When temperature lowers, water is transformed into ice crystals and exerts tremendous force, of up to about 1465 Mg cm-3. On rise of temperature, the process is reversed. Alternate freezing and thawing helps to widen the cracks; resultantly, the rocks are broken down into smaller fragments.

2.4.1.3. Abrasion by Water, Wind, and Ice

When flowing water is loaded with sediments, it has tremendous cutting power. These sediments colloid with each other and breakdown into smaller particles. The rounding of riverbed rocks and beach sand grains is an evidence of the abrasion that accompanies water movement. Wind-blown dust and sand can break down rocks by abrasion in certain arid regions. In glacial areas, huge moving ice masses carrying soil and rock fragments, grind down rocks n their path and can carry away large volumes of material.

Fig. 2.1 After effects of physical weathering

2.4.2. Chemical weathering

Chemical weathering is a decomposition process in which chemical changes take place in rocks and minerals. In this process, soluble materials are released, new minerals are synthesized and some resistant products remain as such. Several processes are involved in chemical weathering; however, water is the main agent which is responsible for bringing chemical changes in rocks and minerals.

2.4.2.1. Hydration

Hydration refers to the addition of water molecules/hydroxyl groups to a mineral without causing any decomposition of that mineral. Hydrated minerals are more prone to decomposition due to their increased volume which makes it softer and more stressed. In case of layer silicate clays, like Mica, hydration may occur on surfaces and broken edges while in simple salt it may occupy the entire surface.

5 Fe2O3 + 9 H2O → Fe10 O15.9 H2O

CaSO4+ 2H2O → CaSO4.2H2O

2.4.2.2. Hydrolysis

In hydrolysis reactions, water molecule splits into its hydrogen and hydroxyl components and the hydrogen so released often replaces a cation from the mineral structure. Depending upon the soil and climatic conditions, the released cation may go to the cation exchange sites on clay minerals, combine with anions to form new mineral or it may leach down to underground water. Hydrolysis may result in complete decomposition or transformation of the original mineral structure, e.g., hydrolysis of calcium carbonates produces OH- ions which cause alkalinity in soils:

CaCO3+ 2H2O → Ca2++ HCO3--+ OH-

2.4.2.3. Dissolution

Dissolution refers to the dissolving of simple mineral salts, like chlorides or carbonates, by water. Water dissolves many minerals by hydrating the cations and anions until they become dissociated and get surrounded by water molecules. These dissolved minerals are converted into solution, which permits greater chemical changes than could occur in a unionized (generally solid) state. This process is more important in areas where calcium carbonate (limestone) is present either in soil or in rocks.

2.4.2.4. Oxidation

Oxidation refers to the loss of electrons from an element. It plays very important role in weathering process in those areas where plenty of oxygen and Fe, Mn and S containing minerals are present. Oxidation results in decomposition of the mineral by destabilizing the mineral structure due to imbalance of charge created by oxidation process, e.g., ferrous is oxidized to ferric resulting in destabilization of the mineral crystal structure.

Fe2+ → Fe3+

2.4.2.5. Reduction

Reduction is the gain of electrons and takes place under water saturated/submerged soil conditions. Minerals that contain Fe, Mn, or S are especially susceptible to this reaction. When Fe is transformed into its reduced form (i.e., Fe2+), it becomes highly mobile and susceptible to leaching if free drainage occurs. Furthermore, it imparts a characteristic green or blue green color to the soil.

Fe3+ → Fe2+

Various chemical weathering processes occur simultaneously and interdependently.

2.4.3. Biological weathering

Biological weathering refers to the breakdown of larger rock fragments and chemical transformation of minerals by living organisms. Lower plants, like mosses and lichens, grow on exposed rocks, catch dust particles, accumulate organic matter and encourage further plant growth. The pressure exerted by roots of higher plants assists in disintegration of rocks by opening the spaces for penetration of water which may later freeze and expand and thus accelerating the weathering process. Moreover, plant roots also secrete certain organic acids which help to solubilize the minerals through chelation process and thus help in mineral weathering (Van Hees et al. 2000)). In addition to that, plant roots produce CO2 during respiration, acidity the soil environment, which in turn results in increased mineral weathering through dissolution process. When plants grow, they uptake nutrient cations from the soil minerals, hence promote more soil weathering, as the soil solution will attempt to attain equilibrium. Burrowing animals, such as earthworms, ants and rodents, and hoofed animals through their actions also contribute slowly to the disintegration of rocks. Humans accelerate the slow process of physical weathering by ploughing and cultivating.

2.5. Types of Soil Parent Material

Depending upon the location and mode of transportation, parent material can be of many different types.

2.5.1. Sedentary/Residual

The parent material formed in place and still at the original site is called sedentary/residual (Fig 2.2). This parent material develops in its original place from the rocks, generally by long and intense weathering. In Pakistan, the Pothohar plateau is covered with such materials. The nature of these materials depends upon the nature of rocks from which these materials have been formed.

Fig. 2.2 Residual parent material

2.5.2. Transported parent material

2.5.2.1. Colluvial

Colluvial is the parent material transported by gravity, and is deposited at the base of foothills or mountains. These deposits are extremely variable in composition. The soils developed from these parent materials may be coarse in texture and even stony. Being coarse, such parent material has good drainage. It is abundantly found in Northern areas of Pakistan, like Murree, Abbottabad, and Nathiagali, etc.

2.5.2.2. Alluvial deposits

The parent material which has been transported/deposited by the action of rivers and streams is termed as alluvium. The alluvial soils are finely layered, to great depths. This kind of parent material is somewhat sandy (coarse textured) near river banks to is more clayey away from river banks. These soils developed of alluvial parent material are generally level, fertile and highly productive; however, clayey soils may suffer with drainage problem. Alluvial soils are more prevalent in Punjab and Sindh provinces of the country.

2.5.2.3. Glacial deposits

This refers to all parent materials of glacial origin, whether deposited directly by the ice melt or by the associated water. Glacial deposits consist of heterogeneous mixture, varying in size from coarse fragments to clay particles. This type of parent

material is generally present in northern parts of the Punjab province as well as in Khyber Pakhtunkhwa province.

2.5.2.4. Eolian deposits

These are materials which are transported and deposited by the action of wind. Depending upon the type of original material that was subjected to such actions, such wind-blown materials are differentiated into loess, sand dunes, adobe and volcanic ash.

2.5.2.3.1. Loess

Loess is generally silty in nature, with some sand and clay. Whereas mostly agricultural soils in Pakistan are alluvial in nature, some soils, like most soils of Pothowar plateau, are formed from loess. Loess-derived soils are generally productive, provided adequate water is available; these soils are quite open and porous in nature.

2.5.2.3.2. Sand dunes

Sand dunes are sandy materials transported and deposited by the action of wind. The coarser particles deposit near the source and accumulate in the form of sand dunes. Salient examples are the vast soils in Thal, Thar and Cholistan deserts. Because of their coarse texture and air climate (leading to water scarcity) most of the sand dune soils possess low productivity potential.

2.5.2.3.3. Adobe

Adobe parent martial is like loess, but more calcareous in nature.

2.5.2.3.4. Volcanic ash

Volcanic ash is transported by wind from active volcanoes. Such parent material in not present in Pakistan, but is very common in Japan, Mexico, Indonesia, etc.

2.6. Stability Indices and Weathering Sequences

Stability indices is the arrangement of soil minerals according to their stability or weatherability. It is very useful in determining the pace of weathering of a particular soil material, indigenous soil minerals and the nutrients’ status of soils. Moreover, this stability indices also indicates the effect of biotic and abiotic factors on the course of soil genesis/formation. It is also helpful in predicting the soil’s behavior under the applied forces as soil physical properties are related to clay minerals which are dominant in that soil. Mineral stability indices for sand and silt sized fractions, proposed by Goldich (1938) are olivine > hypersthene>biotite> feldspar> mica>quartz. For detailed information about these stability models, readers are referred to Soil Genesis and Classification by Boul et al. (2003).

2.6.1. Factors affecting stability indices/mineral weathering

2.6.1.1. Nature and composition of minerals

Mineral weatherability/stability depends upon their chemical composition as well as on structural arrangement of the minerals. Simple salt minerals, like NaCl, CaCO3, and CaSO4

, are more susceptible to weathering than complex minerals, like layer silicates, comprising of tetrahedral and octahedral sheets. Tetrahedral coordination also influences mineral weathering. Olivine comprising of a single tetrahedron only, weathers more rapidly compared to quartz which has maximum sharing of its tetrahedral oxygen. Furthermore, isomorphic substitution also plays an important role on the rate of mineral weathering. During isomorphic substitution, Al3+ ions replace Si4+ ions in tetrahedral sheets; this substitution destabilizes the mineral structure and results in mineral disintegration. In addition to that, Fe, S and Mn contents in the minerals also affect the pace of mineral weathering due to oxidation-reduction processes.

2.6.1.2. Climate

Water is a crucial requirement for all weathering processes, like hydration, hydrolysis and dissolution. It not only dissolves minerals but also helps in translocation of elements within the developing soil profile; hence is important for the soil development process. Temperature affects the rate of biochemical reactions. Weathering processes are very slow in the low temperature areas or even in the high temperature regions, if there is moisture shortage.

2.6.1.3. Particle size and exposure time

Smaller particles having large surface area weather more rapidly compared to larger sized particles. Quartz is the most stable mineral in sand and silt sized fractions but quartz present in the clay sized fraction is susceptible to relatively rapid weathering. The extent of weathering also depends upon the time duration for which rocks have been exposed to certain climatic/atmospheric conditions. About 75 % of the sedimentary rocks are exposed to weathering at the Earth’s surface (Greensmith 1978).

2.7. Soil Formation

Soil formation is a sequence of processes involved both in production of parent material by weathering processes and in soil profile development. Soil formation ultimately leads to the development of horizons within the soil profile. Another obvious evidence of soil formation is that organic matter and soil colloids are transported downward in the soil profile, resulting in the development of clay, carbonates, iron and manganese oxides, humus, and/ or gypsum zones within the soil profile.

2.7.1. Factors affecting soil formation

The process of soil formation and properties of the produced soil are controlled by many spatial factors. Jenny (1941) had described the influence of various factors on the process of soil formation by the following simple equation:

S(xy) = f(cl, o, r, p, t,…….)

Where “S” is soil property, “xy” are locational coordinates, “f( )” means ‘as a function of’ “cl” stands for climate, “o” is indication of organisms, “r” is relief/ topography, “p” is parent material (i.e., condition of soil formatting material at time zero), and “t” is time (i.e., age of soil). Brief descriptions of the role of these factors in soil formation is described in this section.

2.7.1.1. Parent material

Parent material is an early state of a soil system. It is the initial material from which soil develops. The soil’s parent material has profound effect on soil properties, like texture which in turn controls many physico-chemical properties of the soil. Soil parent material may include a bed rock, inorganic and organic constituents, water, wind, ice, gravity, volcanic or material moving down a slope. The rate of weathering is influenced, to a large extent, by the type of parent material (Creda 2002; Olson 2005). The type of soil developed as a result of weathering processes depends largely upon the type of rocks (i.e. parent material) and how those rock minerals had reacted to external environmental factors, like rainfall, temperature, pressure, wind velocity and erosive forces. The parent material contributes to soil properties in three ways:

1) Intensity of weathering

2) Elements present in parent material

3) Particle fraction

In hot and humid areas of the world, like the tropical regions (i.e., Thailand, Indonesia, etc.), intense weathering results in development of acidic soils due to excessive leaching of basic cations (like calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K)) from the soil profile. Contrarily, in arid climate regions (like countries of the Middle East), because of less leaching of basic cations, the soils formed are alkaline in nature with dominance of free carbonates (primarily CaCO3) and lesser profile depth than acid soils. Soil mineral “olivine” weathers rapidly compared to quartz because the Si tetrahedra are held together only by O-metal cation bonds, whereas in quartz it consists entirely of linked Si tetrahedra.

2.7.1.2. Climate

Climate is the average weather conditions at a specific place over a period of time as determined by temperature, precipitation, wind velocity and other morphological factors of the area. Key climatic components in relation to soil formations are the amount of rainfall, evaporation, temperature, wind, solar radiation (White 2006; Gray et al. 2011) and type of vegetation. These sub-factors influence the rates of chemical, physical and biological processes responsible of soil profile development. In general, as rainfall increases the rate of physical and chemical weathering increases resulting in deep soil profile development. It also influences leaching of

colloids in the soil profile and the chemical weathering processes like hydration, dissolution and hydrolysis. The sustenance of plant life on the rock surface is also influenced by climate. Temperature controls not only rate of weathering but also intensity of biological activities. Warm, moist climate encourages rapid plant growth and hence more organic matter production; the opposite is true for cold and dry climate. Generally, temperature changes with latitude and altitude. Temperature controls the rate of weathering and biological processes of growth and decomposition. Rates of reaction are roughly doubled for every 10°C rise in temperature, although enzymatic reactions are sensitive to high temperatures and mostly attain a maximum between 30 and 35°C. Climate also influences the growth of natural vegetation. Vegetation is the main contributor of organic matter to soil; most soils of arid zone regions contain much lesser than 5% organic matter. Whereas humid region environment is favorable for the growth of perennial trees, grasslands as native vegetation are dominant in semiarid regions and various kinds of shrubs grow in arid lands. Wind influences soil formation directly, by erosion and deposition processes. The windblown material is called eolian. The eolian soils are not productive due to their low organic matter content, water and nutrient holding capacity.

2.7.1.3. Biota

An ecosystem consists of soil and the living organisms. The activities of living plants, animals and decomposition of their wastes and residues have marked influence on soil development. The soils having large numbers of these animals have fewer but deeper horizons because of constant mixing/over turning within the soil profile. Symbiotic association of fungi, algae and or cyanobacteria have a major role in weathering, because they produce chelates that trap metallic ions, released by the weathered rocks, to form organo-metallic complexes. Burrowing animals, like mole, ants, termites, earthworms, influence soil formation when they are present in large numbers. Similarly, soil microorganisms like Rhizobia can fix atmospheric nitrogen (N2) into compounds useable by plants.

2.7.1.4. Relief

Relief has strong influence on local climate, plants/vegetation, erosion and drainage of a landscape (Fig. 2.3). Any variations in relief will affect the temperature (a rough idea is that for every 100 meters increase in height, temperature will decrease by 0.5°C). Impermeable B horizons, generally are developed on undulating lands. The Northern hemisphere having slopes with south-facing aspects have more solar radiation; hence, more vegetation is present on the north facing slopes and scarce and smaller shrubs are present on the south facing slopes. Thus, relief can increase or decrease the pace of weathering processes. On smooth or flat surfaces water is removed less rapidly than in the areas with steep slopes. The soils with steep slope are susceptible to water erosion resulting in shallow soil profile development; the opposite is true for soils with gentle slope.

Fig. 2.3 Difference in vegetation at different topographical positions

2.7.1.5. Time

Time duration is another important soil-forming factor. The time required for soil development depends upon so many other factors, like climate, parent material, organisms and relief. It is difficult to quantify the soil formation as development of soil profile is continuously changing with time. (Hugget 1998) Soil horizons tend to develop more rapidly under warm, humid and forested conditions where there is enough water available to move clay, humus and other soil colloids downwards through the soil profile. Soils can be classified according to their age as under:

i. Young soils

The soil with no developed horizon and having recently developed material. Usually, young soils have thinner A horizon and no B horizon. Soil Order Inceptisols is comprised of young soils with no accumulation of clay material (i.e., lacking illuvial clay).

ii. Immature soils

The soils which are at early stage of development with poorly developed horizons, because these soils were exposed to soil forming factors for short periods of time. Such soils are usually shallow in depth and are alluvial in nature.

iii. Mature soils

The older soils which are almost in equilibrium with the environment. These soils have fully developed genetic horizons, like O, A, E, B and C. Salient examples are the soils belonging to Oxisol and Ultisol Orders.

2.8. Basic Processes of Soil Formation

During soil genesis, the unconsolidated weathered material undergoes many soil forming processes. These processes are brought about with variations in the four-basic soil forming processes. These four basic processes are often referred as basic soil forming or soil formation processes. They are responsible of soil formation under all kinds of environment (Brady and Weil 2009). These processes are:

2.8.1. Addition

Addition of materials to the developing soil profile from outside sources is called as additions. For example, addition of dissolved ions and dust particles through rainfall, fallen plant leaves, twigs, nitrogen fixation, animal dung, salts or silica dissolved in groundwater and deposited near or at the soil surface when the rising water evaporates.

2.8.2. Losses/Removal

Materials are lost from soil profile by volatilization, denitrification, leaching to groundwater, erosion of surface material, or by any other form of removal. Leaching causes the loss of water and dissolved substances such as salts or silica, weathered from the parent materials. Grazing of vegetation by animals or harvesting of crops by farmers result in removal of large amounts of both organic matter and nutrient elements from the soil.

2.8.3. Transformation

Transformation involves physical or chemical modification of soil constituents, i.e., some materials are broken down (are fragmented) and others are synthesized. The transformation process is governed by hydrolysis, hydration, redox reactions, dissolution and carbonation. For example, weathering of primary minerals results in disintegration and alteration of various kinds of silicate clays. As primary minerals decompose, the decomposition products recombine to form new minerals which include additional type of silicate clays and hydroxides of iron and aluminum. The organic residues decompose to give rise to organic acids, humus and other products

2.8.4. Translocation/Transfer

It involves the internal movement of inorganic and organic materials horizontally within a horizon or vertically above or below the horizon. Water, either percolating down with gravity or rising up by capillary action, is the most common translocation agent. The material moved within the profile includes dispersed clay particles,

dissolved salts and dissolved organic substances. Soil organisms also play a major role in translocation of soil materials, e.g., incorporation of surface organic matter into A and B horizons of soil profile by certain earthworms, and transport of B and C horizon materials to the soil surface by termites and rodents.

2.9. Specific Pedogenic Processes

These are also called soil building processes. These control the sequence of rearrangement of matter that affects soil formation. The followings are main soil pedogenic processes:

2.9.1. Alluviation

Alluviation is the deposition of alluvial parent material by means of river, streams, etc. Most agricultural soils of Pakistan, especially within the Indus river system, are developed of alluvial materials.

2.9.2. Bioturbation

Bioturbation is disturbance of soil by living organisms. It includes soil displacement by plant roots and mixing of soil profile by earthworms, ants or rodents and moving down of soil organic matter. For example, uprooted vegetation may result in breakdown of bed rock and deposition of sediments resulting in slowing down of soil profile development.

2.9.3. Eluviation

Eluviation is the mobility of materials, like clay, organic matter, Fe and Mn oxides, out of a soil horizon, usually from E to B horizons. In some unplowed soils the eluviated (E) horizon found below the A horizon is light-colored with coarser-texture, because of its leaching over time.

2.9.4. Illuviation

Illuviation is accumulation of material like sand, silt, clay, salts and organic matter into a horizon; e.g., in subsurface Argillic and Spodic horizons there is accumulation of phyllosilicate clays and organic matter. In Pakistan, soil series Bhalwal, Hafizabad and Lyallpur have evidences of clay illuviation.

2.9.5. Leaching

Removal of soluble materials from a soil profile is called leaching. As water moves down through the soil profile it can carry away salts, clays, silt and sand particles and humus; e.g., heavy irrigation with good quality water is usually recommended for saline soils in order to leach down excess salts from the root zone

2.9.6. Gleization

Gleization is derived from Russian word “Glei” meaning blue, green or grey clay. This process occurs under anaerobic conditions when ferric iron (Fe3+) is reduced to ferrous (Fe2+), it results in red-brown/grey mottled appearance because both reduced (Fe2+, grey) and oxidized (Fe3+, red/brown) iron species being present together. Under saturated conditions the iron reduced to soluble ferrous form.

2.9.7. Podzolization/Silication

The migration of Al3+ and organic matter with or without Fe, to the B horizon, resulting in the relative concentration of silica in E horizon (i.e., silication). This process dominates in coniferous forest soils where climate is cold and humid causing leaching of sesquioxides and humus from upper horizon to lower horizon. The process is responsible for acidification and release of many organic compounds of acidic in nature.

2.9.8. Depodzolization

The gradual removal of silica out of profile and accumulation of sesquioxides, usually due to change in climate or vegetation that weakens or stops the podzolization process. It is basically degradation of existing podozol features.

2.9.9. Decarbonation

Loss of carbonates from a soil, usually by leaching. It is reverse of carbonation resulting removal of carbonates from soil by leaching.

2.9.10. Re-carbonation

Adding carbonates to a soil that had previously been leached, usually by increased Ca and Mg cycling or as additions of calcareous dust.

2.9.11. Calcification

Processes leading to the accumulation of secondary calcium carbonate in soils. In this case, there is accumulation or precipitation of calcium carbonates occurs in part of soil profile resulting development of calcic horizon. Hafizabad and Lyallpur soil series contains calcic horizon.

2.9.12. Decalcification

Decalcification refers to removal of calcium ions or CaCO3 from one or more soil horizons (acidification). In Pakistan, non-calcareous soil series include Gujranawala and Guliana.

2.9.13. Salinization

The accumulation of soluble salts such as sulfates and chlorides of calcium, magnesium, sodium and potassium in soils. It mostly occurs in arid and semi-arid regions having high temperature and low rain fall resulting rise in saline groundwater through capillary action and leaving salts on the surface of soil after evaporation. The process of salinization dominates in arid region of Pakistan where calcium sulfate is precipitated. Pitafi and Jarwar soil series have surface salinity due to calcium chloride whereas saline coastal belt having Jati and Nangin soil series have sodium chloride salinity (GOP 2010)

2.9.14. Desalinization

The leaching of salts from the upper solum to the lower solum, resulting in a leached upper profile above a B horizon with columnar structure. This is done by improving drainage condition followed by application of good quality irrigation water to leach down salts from root zone.

2.9.15. Solonization/sodication

Accumulation of sodium (Na) salts on exchange sites in a natric horizon. This process resulting poor soil aeration, low infiltration rate and dispersion of soil particles. Soils developed by this process are called sodic soils.

2.9.16. Solodization/Desodication

Leaching of sodium (Na) salts from exchange sites of a horizon. These soils usually reclaimed by the application of gypsum (CaSO4) resulting removal of Na from exchange site with Calcium

2NaX + CaSO4 → Na2SO4 (leach down) + CaX

X = Exchange site/complex

Adilpur, Jhakkar and Missan soil series have evidence of soil sodicity.

2.10. Soil Morphology

It refers to the form, structure and organization of the soil materials within a soil profile/pedon. Morphology of soil is the reflection of the processes responsible for its formation (Fanning and Fanning 1989). Thus, from the morphological characteristics, one can make fair estimates of the soil forming processes and factors that have been acting upon a soil. Soil morphology is defined as the physical constitution and structural properties of a soil. It is the study of the shape and nature of the soil profile and its horizons.

2.10.1. Soil profile description

The purpose of soil profile description is to provide information which will enable readers to obtain an understanding of the morphological characteristics of a soil and to compare these characteristics with those of other soils of which they have descriptions or personal knowledge. Comparison of soil profile description is greatly facilitated if, in each description, data are presented in the same order. For this reason, the standard outlines of soil profile descriptions which define even the order in which separate characteristics of individual horizons should be described has been proposed by FAO soil science staff members. In preparing a profile description a soil surveyor should assume that the readers will have no knowledge of the soil or its location and thus must provide as much detail as possible on both of these aspects.

The following order of presentation is proposed for description of individual soil profiles:

1) Information on the site described and sampled

a) Profile number

b) Soil series name

c) Higher category classification

d) Date of examination/description

e) Author of description

f) Location

g) Elevation

h) Landform

• Physiographic position of the site

• Landform of the surrounding region

i) Slope on which profile is located

j) Vegetation /land use

k) Climate (Rainfall, Temperature, Wind etc.)

2) General information on the soil

a) Parent material

b) Drainage

c) Moisture conditions in the soil

d) Depth of ground water table

e) Presence of surface stones/rocks out crops

f) Evidence of erosion, if any

g) Presence of salt affected soils

h) Human influence

3) Brief general description of the profile

4) Description of individual soil horizons

For each horizon

a) Horizon symbol

b) Depth of top and bottom of horizon

c) Color

• Moist

• Dry

d) Color mottling

e) Texture

f) Structure

g) Consistence

• Wet

• Moist

• Dry

h) Cutans

i) Cementation, if any

j) Pores

k) Content of rock and mineral fragments

l) Content of mineral nodules

m) Pans

n) Content of carbonates/soluble salts

o) Artefacts

p) Features of biological origin (krotovines)

q) Content of roots

r) Nature of boundary with horizon below

t) Number of samples, taken for analysis

Soil morphology can be described by soil profile description

2.10.2. Master horizons/layers

It is strongly recommended that the appropriate symbol from the system of horizon nomenclature should precede all other data in the description of each individual horizon (Fig. 2.4). Such designations provide a better understanding of the probable relationships between horizons in a single profile. When the correlation of several soil descriptions is attempted, they serve to indicate which horizons can be compared most validly. It is recognized that use of the system calls for interpretation of genetic significance of the observed soil characteristics, and that it is not always easy. Nevertheless, the surveyor who describes a soil profile would certainly be in a better position to make such interpretation than anyone else who may attempt to do so later from the description alone.

2.10.2.1. O horizon or layer

It refers to the organic matter accumulation on surface of the soil that has been deposited either by falling leaves/twigs from plants and trees or animal wastes and residues over long periods of time. Therefore, O horizon is generally present on the soil surface and is darker in color compared to the horizons below it. However, in case of organic soil, O horizon may occupy the entire thickness of the soil.

2.10.2.2. A Horizon

A horizon is generally the mineral horizon, present either below O horizon or at the soil surface. The soils’ A horizons have well decomposed organic materials and are characterized by the presence of humified organic matter along with mineral material. Generally, these horizons/layers represent the physical disturbance of soil caused by ploughing / cultivation.

2.10.2.3. E Horizon

These mineral horizons are present below A horizon. Generally, these are lighter in color compared to the soil horizons below and above it. These are characterized by the presence of more resistant minerals, like quartz in sand and silt fractions, as most of the soluble minerals and clay particles have been leach down from this horizon.

2.10.2.4. B Horizon

Soil’s B horizons are the mineral horizons which are present below O, A and E horizons. These are the horizons which have been most affected by pedogenic processes and represent all or much of the obliteration of parent materials from which the soils are developed. Soil’s B horizon is characterized by having illuvial accumulations of silicate clays, sesquioxides, humus and carbonates.

2.10.2.5. C Horizon

This mineral horizon/layer of a soil exhibits very little effect of pedogenic processes on the parent rock. C horizons do not exhibit the properties of any other master horizon, like A, E, B and O. However, they may contain decomposed plant roots in cracks at wider space intervals.

2.10.2.6. R Layer

Hard bedrock layer lying beneath one or more of the above described master horizons is termed as R layer. This material is sufficiently hard that it can be dug out with a spade even under moist conditions. However, small cracks filled with either soil or roots may be present in it.

2.10.2.7. W Layer

It represents water layer in the soil. The water layer may be in frozen or in liquid form.

Fig. 2.4 Schematic sketch of a typical soil profile

2.10.3. Subordinate horizons

The subordinate horizons represent further variations within a master horizon. For example, Oa, Oe and Oi are all organic horizons but the subscript small letters indicate different stages of decomposition of organic material within O horizon.

1) “a” is used only with O master horizon and represents well decomposed organic material.

2) “b” represents buried mineral genetic horizon. It cannot be used with buried O horizon.

3) “c” represents concretions or nodules. It is specifically used for iron, aluminum, manganese or titanium cemented nodules or concretions.

4) “d” is used for physical root restriction. Roots cannot penetrate these hard layers which are either natural or humanly induced like plough pan or basal till etc.

5) “e” represents intermediate decomposition of organic material and only used with master horizon O.

6) “f” is used for frozen soil horizons generally C or W which have permanent ice

7) “g” represents gleying under anaerobic soil conditions. In water saturated soils, iron is converted into ferrous form, resultantly; soils with low chroma color or mottled pattern appear. This subordinate distinction is generally used along with master horizon B and C. It cannot be used with master horizon E which is also of low chroma color or along with C horizon which have low chroma color due to the nature of parent material.

8) “h” is used with master horizon B and represents illuvial accumulation of organic matter or organic matter-sesquioxide complex.

9) “i” represents slightly decomposed organic material and used only with master horizon O.

10) “j” represents presence of jarosite minerals (yellow colored Fe-K sulfate). Generally, it is used along with B and C master horizons.

11) “k” represents the accumulation of alkaline earth carbonates like Ca and Mg carbonates along with B and C horizons.

12) “l” represents cementation/induration. It can be used along with any master horizon if it is more than 90 % cemented except R.

13) “m” represents accumulation of higher concentration of exchangeable sodium and can be used along any master horizon.

14) “n” represents residual accumulation of sequioxides. It can be used along with any master horizon where sequioxides have been accumulated after leaching of other soluble materials during intense weathering.

15) “o” represents ploughing/cultivation disturbances in soil. It is used only with surface horizons like A or O even if the material has been mixed by cultivation from underlying horizons.

16) “p” represents accumulation of secondary silica and can be used along with any master horizon.

17) “q” represents soft or weathered bedrock and can be used with only master horizon C.

18) “r” indicates the illuvial accumulation of sesquioxides and organic matter and can be used along with master horizon B only. If the chroma and color value is less than 3 than it can be used in combination with h like Bhs.

19) “s” represents accumulation of silicate clays either in the form of peds coatings or lining of pores. It is mostly used with B horizon; however, it can also be used along with C and R horizons.

20) “v” indicates presence of plinthite: a humus poor, iron-rich material and may have mottling of yellow, red and grey colors.

21) “w” represents development of color and structure in B horizon. It may have blocky structure and darker color compared to horizons below and above it without any indication of illuvial accumulations.

22) “x” represents fragipan characteristic i.e. genetically developed hardness/brittleness without any appreciable cementing agents in B and C master horizons.

23) “y” This letter designates the accumulation of secondary gypsum/CaSO4 in B and C master horizons only. If a horizon is more than 90 % cemented by gypsum than it can be written as Bym.

24) “z” indicates accumulation of soluble salts in B and C master horizons.

2.10.4. Transitional horizons

Transitional horizons are layers of soil between two master horizons within a soil profile. Horizons dominated by properties of one master horizon that also have some properties of an adjacent master horizon. Each transitional horizon is designated with two letters; the first letter indicates the dominant master horizon and the second letter indicates the presence of another master horizon. For example, AB is a transitional

horizon between horizons A and B, but it is more like horizon A than horizon B. Sometimes, separate components of two master horizons are recognizable within a transitional horizon and at least one of the component materials is surrounded by others. Such transitional horizon is designated with two capital letters, with a slash in between. The first letter indicates higher volume in the transitional horizon, e.g. A/B transitional horizon would qualify as A horizon except for inclusions of less than 50% by volume of material that would qualify as B horizon.

2.11. Conclusion

Soil sustains life on this planet by providing food, feed and fiber to its inhabitants. It is a unique natural body that also acts as a filter for the environmental contaminants and keeps air and water clean. However, its formation is a long-term process and depends upon several pedogenic factors and processes. Weathering of rocks leads towards the creation of parent material, initial material on which other soil forming factors like living organisms and climate work depending upon the topography (relief) and time. Soils in different parts of the world vary widely in their properties. The properties of a soil are true indicator of its course of development/formation. Generally, soil development is rapid in areas having permeable parent material coupled with levelled topography, tropical humid climate and high density of living organisms.

References

Boul, S.W., R.J., Southard, R.C. Graham and P.A. McDaniel (2011). Soil genesis and classification, 11th Edition. John Wiley and Sons. NJ, USA.

Brady, N.C. and R.R. Weil (2009). Elements of the Nature and Properties of Soils. 3th Edition. Pearson Education. Inc. CA, USA.

Brady, N.C. and R.R. Weil (2007). The Nature and Properties of Soils,14th Edition. Pearson Education. Inc. CA, USA.

Creda, A. (2002). The effect of season and parent material on water erosion on highly eroded soils in eastern Spain. J. Arid Environ. 52: 319–337.

Fanning, D.S. and M.C.B. Fanning (1989). Soil: Morphology, Genesis and Classification. John Wiley and Sons, NJ, USA.

Goldich, S.S. (1938). A study in rock weathering. J. Geol. 46: 27–23. GOP. (2010). Soil Compendium of Pakistan. National. Inst. Res. Soils and

Geomatics. Ministry of Food and Agri. Soil Survey of Pakistan Lahore, Pakistan.

Gray, J.M., G.S. Humphreys and J.A. Deckers. (2011). Distribution patterns of World Reference Base soil groups relative to soil forming factors. Geoderma 160: 373–383.

Greensmith, J.T. (1978). Petrology of Sedimentary Rocks, 6th ed. George Allen and Unwin, London, UK.

Huggett, R.J. (1998). Soil chronosequences, soil development, and soil evolution: a critical review. Catena 32: 155–172.

Jackson, M.L. (1964). Chemical composition of soils. In: F.E. Bear (Ed.) Chemistry of the Soil. Rheinhold Pub. Corp., NY, USA. pp. 71–141.

Olson, K. R. (2005). Factors of Soil Formation / Parent Material. In: Hillel, D. (Ed.), Encyclopedia of Soils in the Environment, 1st Edition. Academic Press. NY, USA. pp. 532-535.

Jenny, H. (1941). Factors of Soil Formation. McGraw-Hill, New York, USA. Schaetzl, R. and S. Anderson (2005). Soils Genesis and Geomorphology. Cambridge

University Press, Cambridge, UK. VanHees, P.A.W., Lundström, U.S. and R. Giesler (2000). Low molecular weight

organic acids and their Al-complexes in soil solution—composition, distribution and seasonal variation in three podzolized soils. Geoderma 94: 173-200.

White, R.E. (2006). Principle and Practices of Soil Science, 4th ed. Blackwell Publishing Company, MA, USA.

Chapter 3

Soil Classification

Irshad Bibi, Hamaad Raza Ahmad, Shamsa Kanwal, Rob

Fitzpatrick, Attar Khan Jarwar, Mahmood Sadiq and Nadeem

Ahmad†

Abstract

Soil is important for supporting plants and supplying food, fibres, drugs and other things of human need as well its ability to filter water and recycle wastes. Soils are independent natural bodies, each with a unique set of properties, resulting from a rare combination of climate, living matter, parent material, and age of the landforms etc. The properties of each soil, as expressed by a soil profile through the different horizons, reflect the combined effect of the genetic factors responsible for the development of that soil. Thus, soil consists of horizons near the Earth’s surface that have been altered by physical, chemical and biochemical processes in contrast to the underlying parent material that remains unaltered. Although it is difficult to define the depth of the lower boundary as the depth of soil may vary depending on soil forming factors and processes, a depth of 200 cm is a limit for lower boundary of soil. The main objective of classifying soils is to establish hierarchies of classes that clarify the relationship among soils and the factors responsible for their formation. Soil classification is also a mean of communication between the scientists working on different aspects of soils around the globe. For example, soil survey classes are

†Irshad Bibi*, Hamaad Raza Ahmad and Shamsa Kanwal Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: irshad.niazi@uaf.edu.pk Rob Fitzpatrick CSIRO Land and Water, Private Bag. No. 2, Glen Osmond, South Australia-5064, Australia. Attar Khan Jarwar, Mahmood Sadiq and Nadeem Ahmad Soil Survey of Punjab, Multan Road, Lahore, Pakistan. Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

42 I. Bibi, H.R. Ahmad, S. Kanwal, R. Fitzpatrick, A.K. Jarwar, M. Sadiq and N. Ahmad

needed that can be grouped or sub-divided to make predictions about soil response to management and manipulation activities.

Keywords: Classification, Soil Taxonomy, Soil orders, Soil series, Groups, USDA classification

3.1. Introduction

It is vital to accept a formal system of soil classification and description, in order to determine the different materials found in soil investigation. Soil classification is the categorization of soil into classes or groups with similar features and theoretically similar behaviour. There are mainly two types of soil classification (i) technical classification, which is for specific and practical purpose; (ii) natural classification, in which grouping of soil is made based on many natural properties. All features of population in natural system of soil classification are considered and various classes are defined and segregated based on those features. Soil taxonomy is defined as a process of categorizing, describing and naming the soils. Like the taxonomy of living organisms, soil taxonomy is formulated to make it easy so that people can communicate data about various kinds of soils, their properties, how these are used, and what are the areas where these are located.

3.2. Systems of Soil Classification

Soil classification gives generalized information about the generalized behavior and nature of soils belonging to a region or location. It also gives scientists, engineers, and agriculturists, information about the kind of soils. There are many classification systems that classify soils into different categories. Different countries have different classification parameters and classification standards that divide soils into different categories based upon their physical and chemical properties. In the USA, the Unified Classification System and AASHTO Classification system are very popular. The Natural Resources Conversion Service is another classification system that works exclusively for agriculture. The details of the different classification systems are discussed below:

3.2.1. AASHTO classification system

In 1929, the Bureau of Public Roads proposed a classification system for soils. This system was developed by Hogentogler and Terzaghi (1929) and now is known as the AASHTO soil classification system. This classification system considers the Atterberg Limits and particle size while categorizing soils into seven different groups namely A1, A2, and so on, ending at A7. This system is used for designing highways and it gives a clear idea about roadway pavement characteristics based upon the soil beneath the surface of road.

Soil Classification 43

3.2.2. Unified soil classification system (USCS)

The Unified Soil Classification System was developed by Cassagrande in 1948 which categorizes soils into different groups, like well graded soils, poorly graded soils, organic soils, and many other types. It is used by geotechnical engineers to determine particle size distribution and texture of soils. Each type of soil is characterized with a two-letter symbol, the first letter defining the type of soil and the second letter designating its quality in terms of plasticity and grading. For instance, if more than 50% of soil passes through a 0.075 mm sieve it belongs to the fine-grained group. If the soil has a liquid limit less than 50 it will be termed as ML or CL meaning silt or clay with low plasticity. If the soil is organic in nature, it will be simply termed as OL (both for silt and clay).

3.2.3. USDA classification system

The United States Department of Agriculture (USDA) classification system, known as Soil Taxonomy, developed by the Natural Resources Conservation Service, categorizes soils from an agricultural perspective (Soil Survey Staff 2003). In Soil Taxonomy, all soils are divided into 12 different Orders based on factors such as topographical conditions, water table, time of soil formation, vegetation, etc.

3.2.4. Canadian system of soil classification

Canadian system of soil classification is mainly used in Canada. Because of their cold climate, in this classification system provision is not made for the soils of thermic and warmer soil temperature regimes and the soils in torric and xeric soil moisture regimes. This system is strengthened by its classification of the cold soils and salt-affected Canadian soils (Soil Classification Working Group 1998)

3.3. Soil Taxonomy – A System of Classification

Soil taxonomy is based on measureable and definable soil features; therefore, it is based on morphological system of taxonomy but with strong genetic information. It is a hierarchical system with six categories, i.e., order, suborder, great group, subgroup, family and series, starting from the lowest to the highest category.

3.4. Diagnostic Soil Features

Some important criteria which are used as backbone for classification are the absence or presence of certain diagnostic horizons and specific soil features.

3.4.1. Diagnostic epipedons or surface horizons

Epipedons are made at the surface and are noticeably darkened by organic matter (OM). An epipedon is not the synonym of A horizon as it could also comprise part or all the B horizon. Out of the seven epipedons recognized, only two (mollic and ochric) have been recognized in Pakistani soils.

3.4.1.1. Mollic epipedon

In this type of epipedon, fine stratification is not present. It has dark colour with the Munsell colour of 5 or lower when dry and 3 or lower when moist with chroma of 3 or lower when moist. It has a thickness of more than 18 cm and massive hard structure is not present. It has 1% or more organic matter while more than 50% base saturation and above 250 mg P2O5 kg-1 soil soluble in 1% citric acid.

3.4.1.2. Umbric epipedon

Fine stratification is not present. It is like mollic epipedon except that its base saturation is less than 50%.

3.4.1.3. Anthropic epipedon

It is also like mollic epipedon but contains above 250 mg P2O5 kg-1 soil soluble in 1% citric acid. It is not considered in terms of base saturation.

3.4.1.4. Plaggen epipedon

In this type of epipedon, artifacts are present. It is a human-made horizon generated by long-term application of manure. It is more than 50 cm thick. Its colour and organic C content vary depending on the source of materials used for manuring.

3.4.1.5. Ochric epipedon

In ochric epipedon, rock structure and fine stratification are absent. It is not like mollic, umbric, anthropic, plaggen or histic epipedon.

3.4.1.6. Melanic epipedon

Melanic is a thick horizon which occurs at or close to the soil surface with high contents of of organic carbon present with the aluminium-humus complexes. It has the properties of low bulk density and high phosphate retention.

3.4.1.7. Histic epipedon

Histic epipedon has a thin structure where organic matter is dominant if it is not ploughed. And if it is ploughed and other mineral material is mixed with it, it has above 12-18% organic C based on clay content and it remains saturated for minimum of 30 successive days during some period of a year, unless it is artificially drained.

3.4.2. Diagnostic subsurface horizons

Subsurface horizons are formed underneath the surface horizons. Often, subsurface horizons are exposed to the surface if the surface of profile is eroded

3.4.2.1. Agric horizon

Agric horizon is formed under cultivation and contains illuvial silt, clay and humus. Organic matter, clay and silt are present as mixture. Worm channels and similarly ped surfaces are potentially coated with the dark coloured mixture of clay and silt. However, the coatings have a lower colour value and chroma compared to the soil matrix itself.

3.4.2.2. Argillic or natric horizon

Argillic horizon is formed under an eluvial horizon and contains illuvial clay. The horizon is at least 15 cm in thickness. The clay content increases vertically within a distance of 30 cm. Argillic horizon has ~ 3% more clay than the surface horizon if the surface horizon has < 15% clay content. Natric horizon like argillic except it contains > 15% exchangeable sodium.

3.4.2.3. Calcic horizon

Calcic horizon has accumulation of calcium and magnesium carbonates and has a thickness of ≥15 cm. By volume, it has 5% secondary carbonate concretions or soft powdery forms. The 15% lime requirement is waived off in case of coarse texture (i.e., sandy or coarse loamy).

3.4.2.4. Albic horizon

Albic horizon is free iron oxides, and clay is removed. It is ashy white in colour because of the dominance of primary sand and salt particles. It may lie above a spodic, argillic or natric horizon.

3.4.2.5. Sombric horizon

Sombric horizon is formed under free drainage conditions by the illuviation of humus not associated with sodium or aluminium. This horizon is dark in colour and it has base saturation value of <50%. It is not underlined by albic horizon.

3.4.2.6. Spodic horizon

Spodic horizon is composed of illuvial accumulation of active amorphous materials. The active amorphous materials are made up of organic matter and aluminium with or without iron. It underlies an O A, Ap or E horizon.

3.4.2.7. Duripan

A horizon cemented by silica into a subsurface hardpan is known as duripan. It is firmly cemented and therefore restricts soil management. The pan shows evidence of accumulation of opal or other forms of silica such as laminar caps, partly filled interstices etc. The fragments of duripan do not slake even after soaking for long time in water or HCl.

3.4.2.8. Oxic horizon

Rock structure or sesquioxide coated lithorelics make less than 5% of oxic horizon. It has highly weathered minerals (kaolinite, smectite, vermiculite) and the cation exchange capacity value is also low. The minimum thickness of this horizon is 30 cm and its clay content is higher compared to the surface horizon (i.e., less than that necessary to be defined as an argillic horizon).

3.4.2.9. Kandic horizon

It is a horizon, which has low activity clays i.e., <16 cmolc kg-1 clay., and it also meets the clay increase needs of an argillic soil horizon.

3.4.2.10. Cambic horizon

The texture of cambic horizon is loamy fine sand, very fine sand or even finer, with weak development of colour and structural features that do not express it as a mollic or umbric epipedon.

3.4.2.11. Salic horizon

The thickness of salic horizon is at least 15 cm and has salt accumulation, which is more stable than gypsum. It has 2 % or more salt content and has 60% or more salt contents.

3.4.2.12. Sulfuric horizon

Sulfuric horizon is made up of minerals (e.g., iron oxides) and organic matter with pH less than 3.5 and it contains jarosite mottles.

3.4.2.13. Fragipan

It has fragile sandy or loamy subsurface horizon which may have cambic, argillic, spodic or albic horizon. It has very low organic material and high percentage of bulk density compared to overlying horizon or horizons and it is cemented when it becomes dry but it is brittle when it is moist (Rashid and Memon 2001).

3.5. Other Diagnostic Soil Features

There are some other diagnostic soil features in addition to the above discussed horizons; these are also used to classify soils.

3.5.1. Abrupt textural change

It is defined as textural change from an albic or ochric horizon beneath argillic horizon within 7.5 cm or less. Clay contents usually lower than 20%.

3.5.2. Andic soil properties

The materials in Andic soils are volcanic glass in the form of silt size particles with lower than 25% organic C. Their bulk density is 0.9 g cm-3 measured at 33 kPa water tension or lower and the amount of phosphate retention is ≥ 85%.

3.5.3. Durinodes

Durinode soils are lightly cemented to the indurated nodules. This cementation is caused by microcrystalline forms of SiO2. They are not slaked in water.

3.5.4. Gilgai

This is micro-relief type of clayey soils with increased expansion co-efficient with changes in the moisture content.

3.5.5. Slickensides

Slickensides are grooved and polished surfaces which are produced by one soil mass sliding verses another one and some other diluents. Slickensides aregenerally present as dark red mottles in polythene or platy patterns.

3.5.6. Permafrost

Permafrost is a soil layer in which temperature is 0°C perennially or lower all the year.

3.5.7. Lithic contact

It is expressed as the contact coherent and soil beneath the material which is sufficiently hard to do it by hand digging with a spade impractical but it might be scrapped or chipped.

3.5.8. Paralithic

It is like a lithic contact excluding the coherent materiallying beneath it; paralithic contact has Moh’s scale hardness of lower than 3 (Razzaq and Rafiq 2001).

3.6. Nomenclature

The nomenclature system was developed by classical linguists J.L. Heller, and A.L. Leemans. This was developed, so that each class had a name that was mnemonic – that is, to help remember, and to know about the properties of different soils of each class. The name also defines a class so that one can determine both the category of the classes and the higher categories of classes to which it relates. The following aspects are considered while establishing the nomenclature in soil taxonomy: Each taxon needs a name to be used in speech; a very good name easy to pronounce, is typical in meaning and short as it is helpful if taxon name indicates its location in the classification. Above the series category, each taxon name reflects its class, in all other categories of which it is a member. Soil series name specifies just the category of series hence a series name might be considered as a series but it does not elaborate the order, suborder, great group, subgroup, or family.

3.7. Categories of Soil Taxonomy

A category of soil taxonomy is defined as a group of classes which is nearly at the same level of abstraction and generalization and which includes all the soils on Earth. Soil taxonomy consists of six categories (Fig. 3.1). In the order of descending level and ascending number of classes, these categories include: order, suborder, great group, subgroup, family, and series. It is considered that soil taxonomy is a kind of categorizing phenomenon. In the highest category, i.e., order, soils of the Earth are arranged or sorted into a small number of classes. The process of sorting soils must be distinct to make it eloquent for our objectives. When differentiation is made for

all the soils into a small number of classes (12 orders), each order becomes heterogeneous compared to the properties which are not recognized in the process of sorting. The order ranking may offer enough information about properties of soil as obtained from classifying a soil at the lowest taxonomic level (Schaetzl and Anderson 2005). This is main benefit of multi-categoric classification system to convey sufficient information. For sorting in the next category, it is required to decrease the heterogeneity which is low in suborder.

The process of sorting is continued in the remaining soil taxonomy categories going down to soil series level. The soils within a soil series have similar properties and are approximately homogeneous and can be easily understood. These categories of soil taxonomy are discussed below:

3.7.1. Order

It is the highest level and most generalized category of soil taxonomy system. All sort of soils around the globe belong to any of the 12 orders of Soil Taxonomy system. The presence or absence of diagnostic features or soil forming processes differentiates soils in these 12 orders.

3.7.2. Suborder

Suborders helps in differentiating the soils within an order because of effects of soil temperature, soil moisture and chemical or textural characteristics. In World total, there are 64 suborders. Fourteen sub orders have been documented in Pakistan (Ahmad and Amin, 1986).

3.7.3. Great group

It is helpful to differentiate soils within in a suborder because of presence of horizons or other characteristics. These differences segregate the soils which have the following common features, i.e., close similarities in kind, arrangement, and degree of expression of horizons. However, thin soil surface horizons have some exceptions that could be mixed by ploughing the soil or damaged by the process of erosion and transitions of some horizons to other great groups may occur. Types and number of suborders within an order change with various orders. In total, there are more than 300 great groups (Larson et al. 1980).

3.7.4. Subgroup

Each great group is categorized into subgroups which are of three types: (i) typic subgroups within a great group elaborates the core idea of any great group, (ii) those having the intergrading features of another order or suborder or great group, (iii) those having distinct properties which separate them from others that are neither intergrades nor typical. In total. there are approximately more than 2,400 subgroups.

3.7.5. Family

Soils are differentiated within a subgroup due to the soil properties which are vital to plant growth or engineering aspects. The soils are categorized within a subgroup with similar chemical or physical properties. These properties provide us the important descriptive information, such as aeration and retention and movement of water, particle size, calcareousness, temperature, soil depth, slope, consistence, cracks, texture, mineralogy, pH etc. In some cases, features and properties of soils are considered without their importance as indicators for the process of soil formation. The soils belonging to a soil family are similar in their management requirements.

3.7.6. Series

Soil morphological features differentiate soil series. It is considered the lowest category which has greater than 19,000 series that have been devised in the USA (USDA, 2014). Up till 2009 in Pakistan there are 890 soil series have been identified (Amin et al. 2010). The procedure employed to differentiate soils between series is usually similar to those which is used in classifying soils for other (higher) categories, however the range allowed for one or multiple features within a soil series is the smallest compared to range allowed in higher categories (Soil Survey Staff 1999).

Fig 3.1 The flow chart of the US Soil Taxonomy Soil Classification system

3.8. Formative elements

Each order name contains a formative element derived from Latin and Greek indicating characteristics of order followed by vowel and ending with sols (Table 3.1: Brady and Weil 2002)

Table 3.1 Soil orders with formative elements and pronunciation

Order Formative element Pronunciation

Entisols Ent Recent Inceptisols Ept Inception (beginning) Alfisols Alf Pedalfer (Clay Accumulation, B Horizon) Spodosols Od Greek Spodous (Wood Ash) Ultisols Ult Ultimate (Loss) Oxisols Ox Oxide Gellisols El Jelly (Freeze) Histosols Ist Histos (Tissue) Vertisols Ert Invert (Cracks, Swelling Clays) Andisols And Ando (Black Soils) Mollisols Oll Mollis (Soft) Aridisols Id Aridus (Dry)

3.9. Soil Orders

3.9.1. Gelisols

Gelisols (from Latin gelare, "to freeze") have extremely cold climates with permafrosting feature nearly 2 meters above the soil surface. Geographically, Gelisols are limited to high latitude. Gelisols support only ~0.4% of total population in the world due to their cold weather. The decomposition of organic matter proceeds slowly due to low temperature. Hence, most Gelisols deposit huge concentration of organic C, just the wetland soils have more organic material. Gelisols occurring in the dry areas of Antarctica are excluded as they are present in a desert system with less or no vegetation, thus having low levels of organic C.

3.9.2. Histosols

Histosols (from Greek histos, "tissue") consist of organic matter mainly and are called as peats and mucks. These soils have at least 20-30% of organic matter by weight with greater than 40 cm of thickness. Most of these soils are developed in settings like wetlands in which restricted flow inhibits the breakdown of organic matter, allowing it to accumulate with passage of time. Histosols are become more productive when drained as this process promote decomposition of organic material. The physical properties of these soils limit their use for engineering purposes.

3.9.3. Andisols

Andisols is derived from the word ando meaning “black soil”. These soils are formed due to volcanic activities. Colloidal weathering outputs are dominated such as imogolite, allophane and ferrihydrite which make them different from other orders. Therefore, these soils have unique physical and chemical characters with high water holding capacity and the capacity to 'fix' huge amounts of phosphorus.

3.9.4. Oxisols

Oxisols (derived from French word meaning “oxide”) are present in the inter-tropical areas of the planet and are highly weathered soils. Excess amounts of Fe and Al minerals are present in these soils. They cover an ice-free area of 5-7.5% of the Earth. These soils are categorized by low nutrient status, extremely low fertility as in these soils more leaching of nutrients occurs and clay size particles mainly have Fe and Al oxides instead of Ca, Mg or K. The addition of lime and fertilizers can make these soils productive.

3.9.5. Spodosols

Spodosols (from the Greek spodos, meaning “wood ash”) are soils of acidic nature featured by humus accumulation in subsurface that is complexed with Fe and Al. The Spodosols are usually formed from the coarse-textured materials with a light coloured and leached E horizon covering a reddish-brown spodic horizon. This phenomenon that forms horizons is called as podzolization.

3.9.6. Entisols

Entisols are soils of recent origin. The main feature in these soils is that they are formed by unconsolidated material, excluding an A horizon with generally no genetic horizons. These soils are identified by large diversity, inland use and environmental settings. Entisols exist in rocky and steep settings. Shore deposits produce habitation and cropland for many people globally in large river valleys. This is the most widely distributed soil order present on ~16% of ice-free area of land (Brady and Weil, 2002).

3.9.7. Vertisols

Vertisols (origin from Latin word verto, meaning “turn”) are clay-rich soils that have ability to shrink and swell with a change in their moisture content. Wide cracks are formed during dry conditions as the volume of soil is reduced. The volume of soil increases when the clay in these soils absorbs moisture. Well developed and distinct horizons are not formed due to the swelling and shrinking process of these soils. Vertisols generally comprise ~2.5% area in the world (Brady and Weil, 2002).

3.9.8. Aridisols

Aridisols (origin from Latin word aridus, meaning “dry") are calcium carbonate containing soils present in arid regions of the world, with subsurface development in some horizons. Aridisols contain clays, silica, salts, calcium carbonate and gypsum accumulated in them. Aridisols cover ~12% of ice-free area of the land. These soils are good for range, recreation and wildlife. Due to the dry climate, they cannot be used for agricultural activities unless water is available for irrigation.

3.9.9. Ultisols

Ultisols are acidic in nature and are leached soils with low fertility. They are present in temperate and humid regions, naturally prevalent in stable and older landscapes. Primary minerals are extremely weathered and leaching of much of K, Mg and Ca have occurred from these soils. Ultisols have high acidity and low amount of plant available K, Mg and Ca. These soils are unfit for agriculture, without heavy investment on their acidity and fertility management by way of liming and fertilization.

3.9.10. Alfisols

Alfisols have comparatively high native fertility and are moderately leached soils. Alfisols are mostly present in temperate humid and subhumid regions of the world. Alfisols comprise ~10.1% of the world ice-free area. They occupy 8350 km2 of the land area in Pakistan. (Rafiq, 2001).

Clays are accumulated under the subsurface horizon of Alfisols and are usually present under forest. The world’s 17% population is supported by Alfisols. Alfisols are productive both for agriculture and silviculture because of favorable climate and high native fertility.

3.9.11. Inceptisols

Inceptisols (origin from Latin word inceptum, "beginning") are soils which have least horizon formation. These soils are more developed compared to Entisols, but many features are absent that are present in soils of other orders. They are commonly present on comparatively young geomorphic surface steep slopes, surfaces, and with resistant parent materials. Inceptisols are present over an estimated 15% the world ice-free area. A huge percentage of these soils is present in mountainous regions which are used for recreation, watershed and rangeland purposes. Inceptisols support almost 20% of the population of the world – the highest percentage amongst all soil orders (Windwolf, 2016).

3.9.12. Mollisols

Mollisols (origin from Latin word mollis, "soft") are grassland environment soils. Dark and thick surface horizon is their characteristics. These are due to the addition of organic matter. Mollisols are mainly present in the middle latitudes such as in Great Plains of the USA. They occupy globally ~7.0% of ice-free area of the Earth. These soils are one of the very important and productive soils of land area and are widely used for agriculture.

3.10. Suborders

Suborders are mainly differentiated based on difference in soil moisture regimes or some dominant soil horizon or properties. In Pakistan, only 14 suborders have been recognized, including Orthids, Argids, Aquents, Fluventus, Orthents, Psamments,

Ustalks, Udalfs, Ochrepts, Usterts, Torrerts, Udolls, Ustolls and Borolls (Rafiq, 2001).

3.11. Great Groups

Great groups are subdivisions of suborders, mainly based on soil features and soil horizons. There are > 300 great groups which are currently recognized in soils worldwide. Some of the salient properties used in identifying great groups include the kind, arrangement and degree of expression of horizons, and base status.

In Pakistan, 21 great groups have been recognized, namely Camborthids, Haplargids, Calciorthids, Natargids, Salorthids, Torrifluvents, Ustorthents, Torripsamments, Ustochrepts, Eutrochrepts, Haplutstalts, Natrustalfs, Chromusterts, Torrets, Hapludolls, Haplustodolls and Haploborolls (Rafiq 2001).

3.11.1. Subgroups

Subgroups are subcategories of great groups. There are more than 2400 subgroups worldwide (USDA 2014).

Subgroups are of three types:

1) Typic – it represents the central concept of great groups.

2) Those that have properties intergrading to another order, suborder, or great group,

3) Those having some properties that differentiate them from other subgroups.

• Typic Camborthids: It represents typic subgroup of Camborthids. These soils have minimal horizon development for Aridisols. In general, these are the soils that have developed on most recent deposits or eroded soils.

• Xerollic Camborthids: It represents Camborthids with the xeric (dry) moisture regime.

• Typic Torrifluvents: It represents soils with the central concept of Typic subgroup of Torrifluvents.

• Udic Haplustalfs: Soils that are like Typic Haplustalfs, however they have Udic moisture regime.

• Typic Ustochrepts: It represents soils in the Inceptisols soil order with the central concept of Ustochrepts.

3.12. Families

Soil families within a subgroup are divided on those properties of soil which are important for plant growth and engineering purposes. These properties include particle size, soil mineralogy, calcareousness, soil pH, temperature, soil depth, slope, consistence and cracks.

3.13. Series

Soil series are separated based on morphological features of soils. They are recognized using the following criteria (Soil Survey Staff, 1951):

• Parent material (texture and mineralogy)

• Kind, thickness and arrangements of soil horizons

• Colour of B horizon or subsoil

• Texture of B horizon or subsoil

• Structure of B horizon or subsoil

• Consistence of B horizon or subsoil

• Pores in the B horizon

• Soil reaction (pH)

• Calcareousness, contents of carbonates

• Types of salt and salt content

• Organic matter content

3.14. Soil Orders and Sub groups in Pakistan

Among the 12 soil orders recognized in Soil Taxonomy (Soil Survey Staff 1951), soils belonging to only six orders occur in Pakistan. The type and extent of soil groups found in Pakistan are given in Table 3.2. Some salient soil profiles found in Pakistan along with the identified series and the area landscape are also shown in Fig. 3.2 to Fig. 3.14.

3.14.1. Aridisols

Aridisols occur in arid and semiarid climates. Soils of this order have salic, cambic, calcic and argillic diagnostic horizons. The soil groups within Aridisols are Camborthids, Haplargids, Calciorthids, Natargids and Salorthids.

3.14.2. Entisols

The soils of this order lack any soil profile development, except for some humification and homogenization in the surface horizons. The soil groups of this order include Torrifluvents, Ustifluvents, Torripsamments and Ustipasamments.

3.14.3. Inceptisols

These soils have cambic horizon and mainly occur in humid, subhumid and semiarid regions. The groups of these soils include, Ustochrepts and Eutrochrepts.

Table 3.2 Soil orders and groups found in Pakistani soils

Soil Order Sub-order Extent in

Pakistan (km2)

Aridisols Argids (Clay) 2559710 Calcids (Carbonates) Cambids (Formed with Alluvial fans) Cryids (Cold) Durids (Duripan) Gypsids (Gypsum) Salids (Salty) Entisols Aquents (Wet) 177000 Arents (Mixed horizons) Fluvents (Alluvial deposits) Orthents (Lack of horizon development and steep

slope)

Psamments (Sandy) Inceptisols Anthrepts (Human made, High P) 27700 Aquepts (Wet) Cryrpts (Very Cold) Udepts (Humid) Usteps (Semi Arid) Xerepts (Dry Summar and Wet Winter) Alfisols Aqualfs (Wet) 8350 Cryalfs (Cold) Udalfs (Humid) Ustalfs (Moist/Dry) Xeralfs (Dry Summar and Moist Winter) Vertisols Aquerts (Wet) 8350 Cryerts (Cold) Torrerts (Hot Summar very dry) Uderts (Humid) Usterts (Moist/Dry) Xererts (Dry Summar and Moist Winter) Mollisols Albolls (Albic horizon) 6100 Aquolls (Wet) Cryolls (Very Cold) Rendolls (Calcareous) Udolls (Humid) Ustolls (Moist/Dry) Xerolls (Dry Summar and Moist Winter)

Source: Brady and Weil (2002), Rafiq (2001)

Fig. 3.2 (a) Sultanpur series soil profile-relatively younger soil with profile development to limited depth and horizonation not much conspicuous. Substratum comprises unaltered parent material exhibiting depositional stratification (b) Sultanpur series landscape-level plain under diversified irrigated cropping.

Fig. 3.3 (a) Pacca series soil profile- A double storeyed profile, the upper one (Pacca) is similar to Sultanpur soil in development (b) Pacca series landscape-a broad basin used for irrigated general cropping

Fig. 3.4 (a) Miranpur series soil profile-deep, clayey basin-filled with grey colour and weakly expressed vertic characters, cloddy surface due to long rice cultivation, (b) Miranpur series landscape-a broad basin, fallow after rice cultivation, no trees

Fig. 3.5 (a) Hafizabad series soil profile-very deeply developed with slight melanization of the topsoil and strong braunification in the subsoil, (b) Hafizabad series landscape-level plain under diversified irrigated cropping

Fig. 3.6 (a) Lyallpur series soil profile-very distinct colour horizonation. Zone of secondary lime accumulation occurs below about 90 cm depth, (b) Lyallpur series landscape-level plain. Deep, well aerated root zone reflected in healthy shisham (Dalbergia sissoo) trees

Fig. 3.7 (a) Bhalwal series soil profile- well humified topsoil resulting from long continued manuring, strong braunification of the subsoil, clearly visible calcic horizon, (b) Bhalwal series landscape- intensively cropped level plain with healthy trees.

3.14.4. Alfisols

These include saline-sodic soils which are non-calcareous or slightly calcareous, having argillic horizons. The groups in this order are Hapludalfs, Haplustalfs and Natrustalfs.

Fig. 3.8 (a) Bhalwal series soil profile- well humified topsoil resulting from long continued manuring, strong braunification of the subsoil, clearly visible calcic horizon, (b) Bhalwal series landscape- intensively cropped level plain with healthy trees

Fig. 3.9 (a) Naziabad series soil profile- development similar to Rasulpur but stronger and deeper due to a wetter environment, (b) Wazirabad series landscape- mostly tubewell irrigated, partly dry-farmed land

Fig. 3.10 (a) Gujranwala series soil profile-very deeply developed soil with well humified upper part of solum, (b) Gujranwala series landscape-level plain used for diversified irrigated crop production.

Fig. 3.11 (a) Guliana series soil profile-very deeply developed soil with distinct braunification of the sub-soil. Lighter colour of the surface horizons due to depletion of humus through clean cultivation (b) Guliana series landscape-nearly leveled table land, tarraced for dry-farmed cropping

Fig. 3.12 (a) Pindorian series soil profile-well developed horizon of secondary lime accumulation below 150 cm depth formed by leaching from the overlying solum (b) Pindorian series landscape-soil conditions favorable for vegetable production.

Fig. 3.13 (a) Peshawar series soil profile-soil developed on the piedmont plain surface in the Peshawar valley (b) Peshawar series landscape-nearly leveled plain under irrigated general cropping.

Fig. 3.14 (a) Warsak series soil profile-weak profile development, stratified substratum, grey colour inherited from parent material (b) Warsak series landscape-leveled plain landscape, diversified irrigated crop production.

3.14.5. Mollisols

These soils have a calcium rich organic matter. These are grass land soils containing humus rich surface horizon of 60 to 80 cm in depth. Some Mollisols are also present in cold mountain regions and the soil groups of these soils include, Hapludolls, Haplustolls and Haploborolls.

3.14.6. Vertisols

Vertisols are clayey soils with the expanding type clay, predominantly montmorillonite, and occur in arid and subhumid areas. These are slightly to moderately calcareous in nature. The soil groups of these soils include Chromusterts and Torrets (Rafiq 2001).

3.15. Soil Temperature Regimes

Soil temperature regimes are defined in terms of temperature measured at the depth of 50 cm or at para-lithic and lithic contact if it is shallower. In practice, soil temperature regimes knowledge is important to:

1) Identify the development and establishment of definite soils.

2) Constantly classify and precisely map soils.

3) Use this knowledge to evaluate the organization soil-plant-water systems.

The scientists have made great efforts to identify the spatial distribution of soil temperature regimes. These geographic factors can be used to estimate soil temperature at other locations or to plot soil temperature regime distribution maps. Soil classes are defined in low categories based on soil temperature regimes, which are described a below.

3.15.1. Pergelic

Mean annual soil temperature is lower than 0°C.

3.15.2. Cyric

Average annual soil temperature ranges from 0 to 8°C with the mean summer soil temperature less than 15°C, if there is no O horizon and less than 8°C, if O horizon is present.

3.15.3. Frigid

Soils are hot in summer due to the frigid temperature regimes compared to the soil with a cryic regime, but it has less than 8°C of average yearly temperature. And it has greater than 6°C of average summer (June-August) and winter (December-February) temperature that exist at the depth of 50 cm or at lithic, para-lithic or densic contact.

3.15.4. Mesic

The average temperature of soil is 8°C or may be greater, but less than 15°C and at the depth of 50 cm or at lithic, para-lithic contact, the difference between average winter and summer temperature of soil is greater than 6°C.

3.15.5. Thermic

The average yearly temperature of soil is 15°C or even greater, but less than 22°C, and 6°C of average winter and summer temperature is recorded at the depth of 50 cm from soil surface or at a densic, lithic, or para-lithic contact, that is shallower.

3.15.6. Hyperthermic

The average annual temperature of soil is 22°C or greater and the temperature difference between average winter and summer is greater than 6°C either at a depth of 50 cm from the surface of soil or at a densic, lithic, or para-lithic contact, whichever is shallower. Prefix shows the soil temperature regime below 6°C at the depth of 50 cm or at para-lithic, lithic, or densic contact, that is shallower.

3.15.7. Isofrigid

The average yearly soil temperature is less than 8°C.

3.15.8. Isomesic

Annual temperature of soil is 8°C or greater than but less than 15°C.

3.15.9. Isothermic

The temperature of soil is 15°C or greater but not more than 22°C.

3.15.10. Isohyperthermic

The temperature regime of these soils is 22°C or greater.

3.16. Soil Moisture Regimes

These criteria are defined with respect to the absence or presence of groundwater or the water retained at lower than 1500 kPa or 15 bar tension in the control section of a soil between the fixed period of a year. Water is retained very tightly that many plants are unable to extract it. When a soil horizon is saturated, the water within is retained at tension near to 0 or sometimes at negative pressure. Thus, a horizon is recognized dry when water is retained at >1500 kPa tension and horizon is called as moist if<1500 kPa, but greater than 0 water tension is present. The soils which are saturated with salty or saline water is recognized as salty soils rather than dry soils. Different soil moisture regimes are given below:

3.16.1. Aqua moisture regime

This applies to reducing conditions in the soil indicated by low chroma mottles and colours. The soil in this regime is saturated by groundwater or water of capillary fringe and the dissolved oxygen is not present.

3.16.2. Aridic or torric moisture regime

Greater than half of the time, the moisture control section is dry and is never moist for 90 successive days when the soil temperature is above 8°C at 50 cm depth.

3.16.3. Udic moisture regime

Soil is not dry in any portion for 90 (cumulative) days. If there is thermic soil temperature regime, the soil does not remain dry for 45 successive days. There should be three-phase system (solid-liquid-gas) in soil moisture regime control section when the soil temperature is higher than 5°C.

3.16.4. Ustic moisture regime

The soil is dry for greater than 90 consecutive days but lower than 180 successive days or it is moist for greater than 90 consecutive days under a hyperthermic soil temperature regime. While the soil is dry for greater than 90 or more cumulative days, but it remains moist for greater than half of the time the temperature of soil is more than 5° C under thermic temperature regime.

3.16.5. Xeric moisture regime

This is characterized by typical Mediterranean climate in which winters remain cool and moist and summers are dry and warmer. The soil remains moist in many parts for greater than half of the time and the temperature of soil is above 8°C. Soils remain moist for more than 45 successive days during the four months after the winter solstice and remain dry for greater than 45 cumulative days during 4 months after the summer solstice under the thermic temperature regime.

3.16.6. Soil moisture control section

This system is 30 cm deep if soil is coarse silty, fine loamy, clayey or fine silty; if coarse loamy, then it is 20-60 cm deep and if sandy then 30-90 cm deep.

3.17. Land Capability Classification

The land capability classification categorizes soils according to their suitability for cultivation of various kinds of crops. In the soil capability classification, soils are usually grouped at these three levels — capability class, subclass, and unit. In land capability classification, the largest groups are selected by different number as 1 to 8. The statistics are indicator of gradually greater boundaries and narrower selections for practical work (Sanchez et al. 2003).

3.17.1. Class I

Soils in Class I have few restrictions that limit its use for Agriculture. These soils are suitable to a great variety of crop plants and can be used for growing crops, range, cultivated woodland, wildlife and pasture. These soils are drained, deep, and are simple to work with. Their moisture holding capacity. The local environment of soils of this Class I must support many of the field crop production. Soils of Class I may be in irrigated zones, if the constraints of arid environment have been removed by comparatively permanent irrigation mechanisms. Some of these soils may also require initial habituation including leaching of a minor accumulation of dissolved salts, leveling to the preferred grade.

Wet soils having slowly porous sub-soils are not employed in Class I. Soils present in Class I for growing crops of different varieties need regular strategic practices to sustain yield. These practices can include the use of following strategies: cover crops and green-manure, fertilizers and lime, preservation of crops residue and animal manure, and arrangements of modified crops (Rafiq 2001).

3.17.2. Class II

Soils in Class II require littlemanagement practices, including moderate conservation practices. These soils are used for growing crop plants, woodland pasture, wildlife feed, range land grasses or cover crops. The exact arrangements of practices differ from place to place, provisional to the native climate, the farming systems and features of soils.

3.17.3. Class III

Choice of growing crops is decreased in soils of Class III and these soils require different conservation practices. The soil conservation practices are more difficult when crops are cultivated. Soils can be used for growing crops and wood land. Clean farming, tillage, timing of embedding, harvesting and choice of crops or combination of all these are limited in Class III. The limitations may be due to: slightly sheer slopes, water or wind erosion susceptibility or unadorned results of erosion. The crop damage is attributed to frequent runoff, subsoil low penetrability, hardpan, bedrock depth, claypan or fragipan, that limit the water storage and rooting zone, low water-holding capacity, moderate salinity. To increase permeability or to stop puddling, usually organic solids are supplied to these soils to control wetness.

3.17.4. Class IV

Great limitations are present in soils of Class IV which may require careful management and choice of proper crop. The limits of soil are greater than Class III and crop selection is very narrow. These soils may be used for crop, pastures, range lands, woodlands or wildlife feed and shelter. Only two or three crops may be suitable for these soils. There is a restriction of growing crops given to high wind and water erosion susceptibility, steep slopes, severe effects of erosion, shallow soils, unembellished crop damage done by frequent runoffs, low water-holding capacity,

severe salinity or severe climate. Occasional cultivation is done in humid areas of these soils however, not fit for regular cultivation.

3.17.5. Class V

There is no or little threat to these soils by erosion. However, other restrictions exist which are impractical to eradicate. Therefore, the use of these soils is mostly restricted to woodlands, range lands, pasture or wildlife feed and shelter. Limited type of crops can be grown on these soils and usual tillage operations of growing crops are restricted as well. Soils are approximately leveled, but few of them are frequently flooded by streams, wet or stony and have climate limitations. Lower land soils are examples of Class V which are subjected to numerous overflow that lower the normal yield of cultivated crop. Due to such limitations, agronomy of the common crop is not practicable but pasturing can be enhanced and there is expectation to benefit from suitable management practices.

3.17.6. Class VI

The Class VI soils have many limitations and cultivation of crop plants is not suited in these soils. The wildlife feed, range land or pasture, shelter and woodland are decreased due to these limitations. Physical situation of these soils is such that they support range lands and pasture if required management practices such as liming seeding, contour drainage ditches of water regulator, and fertilizers, water spreaders, furrows and diversions are implemented. Continued limits are not modified in these soils, such as adverse erosion hazard, steep slope, past erosion effects, low rooting zone, stoniness, excessive humidity or overflow, salinity low moisture capacity or adverse climate. Crops are not suited for these soils however, these soils can be used for range lands, pasture, wildlife or woodland.

3.17.7. Class VII

The soils present in Class VII are not suitable to cultivate crops. There are restrictions for wildlife, woodland and grazing etc. Range land improvements and pasture are impractical to apply in Class VII. They have more unadorned soil limitations than Class VI, such as erosion, very steep slopes, stones, shallow soil, salinity or sodicity, wet soil, adverse climate, or other limitations which are unsuited to cultivate crops.

3.17.8. Class VIII

The soils and landforms present in Class VIII are used for commercial plant assembly, recreation, shelter, or wildlife feed or water source. Onsite benefits cannot be obtained by crop management, grasses and trees. The profits from watershed security, wildlife use or recreation might be possible. Sandy shores, badlands, river wash, rock ridge, barren lands and mine tailings are in Class VIII. It is important to manage the soils for plant growth in this class for aesthetic purposes or wildlife habitat.

3.18. Conclusions

The properties of each soil, as expressed by a soil profile through the different horizons, reflects the combined effect of the genetic factors responsible for the development of that soil. Thus, soil consists of horizons near the Earth’s surface that have been altered by physical, chemical and biochemical processes in contrast to the underlying parent material that remains unaltered. Classifying soils is vital to establish hierarchies of classes that clarify the relationship among soils and between soils and the factors responsible for their formation.

References

Sanchez, P.A., C.A. Palm and S.W. Buol (2003). Fertility capability soil classification: a tool to help assess soil quality in the tropics. Geoderma 114: 157−185.

Amin, R., A.H. Khan and M. Ikram (2010). Environment features and profile characteristics of official soil series of Pakistan reported upto June 30, 2009. In: M. Ahmad (Ed). Soil Compendium of Pakistan. Ministry of Food and Agriculture, Govt. of Pakistan. pp. 108−208.

Brady, N. C. and R. R. Weil (2002). The Nature and Properties of Soil, 13th Edition. Pearson Education Ltd., USA.

Casagrande, A. (1948). Classification and identification of soils. Transactions Am. Soc. Civil Eng. 113: 901−930.

Hogentogler, C. A., and C. Terzaghi (1929). Interrelationship of load, road and subgrade. Public Roads 10: 37−64.

Larson, W.E., S.C. Gupta, and R.A. Useche (1980). Compression of agricultural soils from eight soil orders. Soil Sci. Soc. Amer. J. 44: 450−457.

M. Rafiq (2001). Soil resources of Pakistan. In: A. Rashid and K.S. Memon (Eds.). Soil Science, National Book Foundation, Islamabad, Pakistan.

Razzaq, A and M. Rafiq (2001). Soil classification and survey. In: A. Rashid and K.S. Memon (Eds.) Soil Science, National Book Foundation, Islamabad, Pakistan. pp. 405−437.

Schaetzl, R.J. and S. Anderson (2005). Soils: Genesis and Geomorphology. Cambridge University Press, Cambridge, UK.

Soil Classification Working Group. (1998). The Canadian System of Soil Classification, 3rd Edition. Agriculture and Agri-Food Canada Publication, Canada.

Soil Survey Staff (1999). Soil Taxonomy, 2nd Edition. USDA, Government Printing Office, Washington DC, USA.

Soil Survey Staff (2003). Keys to Soil Taxonomy. 12th Edition. Department of Agriculture: Natural Resources Conservation Service. Government Printing Office, Washington DC, USA.

USDA, (2014). Introduction to soil classification. Available at https://sssnne.files.wordpress.com/2014/02/lecture-notes-deb.pdf

Windwolf (2016). Soil orders. Retrieved from http://sci.windwolf.org/soil/orders7.htm Accessed on November 16, 2016.

Chapter 4

Chemical Properties of Soils

Muhammad Sabir, Muhammad Zia-ur-Rehman, Saifullah and

Zia-ul-Hassan Shah†

Abstract

Soil is a complex system comprising of inorganic and organic constituents, air, water, microorganisms and plant roots. Soil develops from parent material through pedogenic processes which involves different physico-chemical reactions. Once the soil is developed, its different bio-geo-chemical properties are originated which greatly influence soil’s capacity to support plant growth and perform various environmental and ecological functions. These bio-geo-chemical properties of soils controls different reactions in soils which control adsorption-desorption, dissolution-precipitation and oxidation-reduction of different elements including essential plant nutrients in soil. Chemical properties of soils are the most important owing to their overwhelming role in chemical processes. Soil colloid is the most active fraction of the soil which controls all these chemical properties. Soil colloid may be of organic or inorganic nature but inorganic colloid dominates in most of the soils. Soil colloids carry negative charge which may be permanent or variable depending upon its mode of development. Inorganic colloids include layer silicates, oxides of iron and aluminum, allophones/amorphous clays. Chemical properties of soils include soil reaction, ion exchange, base saturation percentage, buffering capacity and soil redox potential. Conventionally, type and nature of soil colloid, origin of charges on soil colloids and soil chemical properties are dealt with Soil Chemistry. However, with

†Muhammad Sabir*, Muhammad Zia-ur-Rehman and Saifullah

Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: cmsuaf@gmail.com Zia-ul-Hassan Shah Department of Soil Science, Faculty of Crop Production, Sindh Agricultural University, Tando Jam, Pakistan. Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

76 M. Sabir, M. Zia-ur-Rehman, Saifullah and Zia-ul-Hassan Shah

increasing environmental issues, soil chemistry has now very multi-farious applications ranging from chemical properties controlling nutrient availability to the plants, to different environmental issues. In this chapter, soil colloid and its types, charge development, different chemical properties of soils and their application in agriculture and environmental studies have been discussed.

Keywords: Colloidal Particles, Charge Development, Soil Reaction, Diffuse Double Layer

4.1. Introduction

Soil is a porous media having multi-components which include solids, liquids and gases. Soil is developed from parent material (rocks) through various pedogenic processes occurring at the land surface. These pedogenic processes are derived from biological, geological and hydrological phenomena. Although, the soil is developed from the rocks and minerals but biogeochemical properties of the soil are different from those of parent material. Among different properties of soils, chemical properties are very important because these control solubility and bioavailability of different chemical compounds in soils. This implies the importance of chemical properties of the soil which establish strong relationship between soils and agricultural productivity by influencing bioavailability of nutrients to the plants. Chemical properties of soils are greatly influenced by the nature of parent material and different soil forming processes occurring during soil formation. Soil colloidal particles (clay and humus) are chemically the most active fraction of the soils that contribute to soil chemical properties. Soil colloidal particles are the sites for chemical reactions occurring in the soils. Several types of clay colloids are present in soil and most of clay colloids have layered structures (layer silicate clays). These particles carry net negative charge which could be of permanent or variable nature depending upon mode of charge development. In addition to these aspects, soil liquid phase (soil solution) is very important which is seat of all chemical reactions occurring in the soil. Soil solution contains dissolved ions, substances and oxygen which ultimately provide nourishment to the plants through water movement, dissolution, dissociation, oxidation-reduction, availability of the nutrients to the plants.

Soil chemistry is the branch of Soil Science that deals with bio-geo-chemical processes occurring in soils and their effect on the solubility, mobility, distribution, and chemical forms of essential plant nutrients and contaminants in soils. Better understanding all these processes would help to predict the fate and toxicity of elements in soils and their economical remediation through different strategies for sustainable environment and agriculture, nutrient retention and their release and ultimately availability to plants. Traditionally, soil chemistry focuses on chemical and biochemical reactions governing nutrient phyto-availability, organic and inorganic fertilization and their environmental consequences particularly related with nitrogen and phosphorus fertilization. But, recently focus of soil chemistry has shifted from its traditional role in agriculture to its emerging role in environmental quality of soil and water resources. Understanding the reactions and bio-geo-chemical processes of nutrients, potential pollutants and contaminants in soils will

Chemical Properties of Soils 77

enable a more accurate prediction of fate and toxicity of contaminants, and development of remediation strategies.

4.2. Chemical Principles

Mole is the basic unit used to indicate the amount of chemical substance and is abbreviated as mol. It is associated with chemical formula of the substance and contains 6.02 × 1023 (Avogadro number) chemical entities (atoms, molecules or ions). The millimole (mmol) is 1/1000th of mole and is used to measure the smaller amounts of chemical substances. Molar mass (also known as molecular weight) is the mass of 1 mole of that substance and is calculated by summing up atomic masses of all the atoms appearing in the chemical formula of a substance. For example, one mole of water (H2O) contains 6.02 × 1023 molecules which is equal to 18 g (2 g atoms of H and 16 g for one atom of oxygen). Similarly, one mole of Hydrogen gas (H2) contains 6.02 × 1023 H atoms which is equal to 2 g. Molarity (Molar concentration) is the unit of concentration and is defined as number of moles of a substance dissolved per liter of the solution (Skoog et al. 2006) and is abbreviated as M. For example, one molar solution (1 M) of Na means one mole of Na (23 g) dissolved in one liter of solution. Molality is the number of moles of a substance dissolved per kilogram of solvent. Molality depends upon the volume of solvent which changes with temperature and pressure and thus molality also changes but molarity remains constant. Mole fraction is the ratio of the number of moles of substance in solution with total number of moles of all constituent substances of that solution including number of moles of water in case of aqueous solution (Tan 2011, Skoog et al. 2006). Gram Equivalent (Equivalent weight) is the amount of a substance that can combine with or displace one gram of hydrogen (equal to Avogadro number of charges). Normality is the number of gram equivalent of solute dissolved per liter of the solution and it is abbreviated as N. Equivalents can be easily used to indicate concentration of ions or nutrients due to stoichiometric nature of exchange reactions and can be obtained by dividing molecular weight with valency of that ion (Tan 2011). Equivalents have been replaced with mole of ion charge. For example, one mole of Ca2+ equals 40 grams of Ca2+ while mole of ion charge would contain 20 grams of Ca2+. Another unit of concentration is mg L-1 or mg kg-1 which is used to express very low concentration and previously was termed as parts per million (ppm).

4.3. Soil Colloids

Clay and organic particles which are so small that they tend to remain suspended in standing water are called soil colloidal particles (soil colloids). The soil’s colloidal system is made up of the clay particles and highly decomposed organic matter or humus. The colloidal particles are also referred as micelles (microcells) (Weil and Brady 2016).

4.3.1. Properties of Soil Colloids

The colloids are chemically the most active fraction of soil and have distinct properties which are explained below.

4.3.1.1. Extremely small size

Colloids have extremely small size of about 1 µm and these cannot be seen with an ordinary (light) microscope. Colloidal particles can be seen only with the help of an electron microscope. Some soil scientists consider 2 µm as upper limit of size for colloidal particles to coincide with definition of clay particles (Weil and Brady 2016).

4.3.1.2. Surface area

The surface area and particle size are inversely related with each other. If the size of the particle is smaller, the surface area of the particle would be greater and vice versa. Due to extremely small size, colloids have large surface area which is more than 1000 times compared to the same mass of sand particles. Some silicate clays also possess large internal surface area due to plate like structure in addition to external surface area. The total surface area of soil colloid ranges between 10 m2 g-1 for clays with only external exposed surface to 800 m2 g-1 for those clays having internal surface exposed.

4.3.1.3. Charged particle

Colloid is a charged particle which may carry negative or positive charge on its internal or external surfaces. Most of the soil colloids carry negative charge with exceptions of highly acidic conditions where colloid may carry positive charge. Mode of origin of charge on soil colloids can vary among different types of colloids and some time prevailing chemical conditions e.g. pH could also influence charge. It attracts ions of an opposite charge towards its surface i.e., the negatively charged surfaces of a soil colloid attract cations (positively charged ions).

4.3.2. Types of soil colloids

Soil colloids are classified into organic (e.g., humus) and inorganic colloids (e.g., clay). Inorganic colloids are further classified into different groups based on their composition, structure and properties. The colloids present in soils are classified into four major classes: 1) crystalline silicate clays; 2) non-crystalline silicate clays; 3) iron and aluminium oxides; and 4) organic colloids (humus).

4.3.2.1. Crystalline silicate clays (Phyllosilicates)

Crystalline silicate clays are the dominant type of soil colloids present in most of the soils except the soils belonging to Andisols, Oxisols and Histosols. These clays have crystalline structure with an orderly internal arrangement. Crystalline structure of silicate clays consists of series of layers which are stacked on each other like the pages of a book. These layers are made up of sheets of silicon, aluminum, magnesium, and/or iron atoms surrounded by oxygen and hydroxy1 groups. In silicate clays, Silicon (Si4+) tetrahedron and Aluminium (Al3+) octahedron are basic structural units. Tetrahedron consists of a Si4+ atom (as a central cation) surrounded by 4 oxygen atoms (as ligands) leading to the formation of structure having 4 sides.

Numerous tetrahedra (plural of tetrahedron) are linked together horizontally to form a silica tetrahedral sheet. Octahedron is composed of Al3+ (as central cation) surround by six oxygen or hydroxyl ions (as ligands) resulting in the formation of structure having 8 sides (Fig. 4.1).

Fig. 4.1 Silicon tetrahedron and aluminium octahedron

Various octahedra link together horizontally to form an octahedral sheet. The tetrahedral (silica) and octahedral alumina sheets are bound together by shared oxygen atoms to form different layers (Fig. 4.2). The specific nature and combination of these sheets in the layers vary from one type of silicate clay to another and largely control the physical and chemical properties of silicate clays.

Fig. 4.2 The basic molecular and structural components of layer silicate clays

Based on number and arrangement of tetrahedral and octahedral sheets in the crystal units, silicate clays are classified into three different groups.

i. 1:1 type silicate clays

These are silicate clays in which one silica tetrahedral sheet is attached with one alumina octahedral sheet. Both types of sheets are tightly bound with each other due to shared oxygen atoms between two layers (Fig. 4.3). Oxygen is shared between two layers in such a way that that one apical oxygen of tetrahedron also serves as basal oxygen of upper-lying octahedral sheet. Alternating octahedral and tetrahedral sheets are stacked on each other in such a way that exposed hydroxyl atoms of one layer are adjacent to exposed oxygen atoms of the other layer. This type of arrangement leads to the formation of hydrogen bond between two layers which results in non-expansion of these silicate clays. Due to this, 1:1 type of silicate clays has low surface area thus leading to very low surface charge and low cation exchange capacity. This type of silicate clays includes kaolinite, halloysite, dickite, etc.

Fig.4.3 Kaolinite

ii. 2:1 Type silicate clays

In this type of silicate clays, one octahedral sheet is sandwiched between two tetrahedral sheets. The 2:1 type silicate clays are differentiated based on kinds and amounts of isomorphic substitution in tetrahedral and octahedral sheets. Isomorphic substitution leads to development of permanent charge (explained in section 3.4.1). The charge per unit formula balanced by cations other than the cations in crystal lattice is called as layer charge. Amount of layer charge determines the types and strength of interlayer bonds and thus plays important role in rendering expansion or non-expansion of silicate clays. If the layer charge is more, stronger will be the bond

between layers and vice versa. Layer silicates are sometime differentiated based on octahedral positions occupied by cations. When two of three octahedral positions are occupied by trivalent cations, the mineral is called as dioctahedral but when all three positions are occupied by divalent cations, then mineral is called as trioctahedral (Bohn et al. 2002). These types of silicate clays are further divided into expanding and non-expanding type of silicate clays.

Smectite (montmorillonite is a common member of this group) is an important group of expanding silicate clays. Expansion of such clays occurs due to the addition of water to interlayer space (Fig. 4.4). In contrast to kaolinite, top and bottom planes of adjacent sheets consist of exposed oxygen atoms and thus leading to weak oxygen-oxygen bond giving characteristic expansion property to these silicate clays. Due to expansion, C-spacing of these silicate clays varies between 9.6-21.4 Å Exchangeable cations and associated water molecules are strongly attracted towards interlayer spaces and thus layers separate apart from each other due to pushing water molecule in the interlayer space. Due to expansion of interlayer space, internal surfaces are exposed leading to very high specific surface area of these silicate clays. High surface area (600-800 m2 g-1) and high layer charge lead to high cation exchange capacity of montmorillonite which ranges between 80-120 cmolc kg-1. Expansion of these silicate clays due to interlayer water molecule gives very high plasticity, stickiness and cohesion to these clays. These clays make soils very hard on drying and very sticky and slippery on wetting thus causing cultivation of these soils very difficult. Such soils develop wide cracks on drying which make these soils undesirable for construction activities. Vermiculites is another group of silicate clays which are also classified as expanding type but their expansion is considered intermediate between kaolinite and smectite which ranges between 14-15 Å. Their expansion is larger than kaolinite but much lesser than smectites. This property is since unlike smectite group, these silicate clays contain strongly adsorbed water molecules, Al-hydroxy ions and cations such as magnesium which serve as bridge between two layers rather than simply separating apart.

Micas are major non-expanding 2:1 type silicate clays and are primary minerals in the soils. Among the micas, biotite and muscovite are commonly found in sand and silt fraction while illite and glauconite are found in clay fraction (Fig. 4.5). The main source of charge in micas is substitution of Al3+ in about 20% of the Si4+ sites in tetrahedral sheets and thus leads to development of strong negative charge even more than vermiculites. Due to strong negative charge, cations particularly K+ is strongly attracted towards interlayer spaces and strongly bind the adjacent layers due to formation of strong bond O-K-O. This type of interlayer bonding restricts expansion of mica type clays. Despite having high layer charge, micas have very low CEC (20-40 cmolc kg-1 and surface area (70-120 m2 kg-1). Behavior of these silicate clays is similar to kaolinite in terms of plasticity, stickiness and adsorption capacity for water and cations.

Fig. 4.4 Montmorillonite

iii. 2:1:1 Type silicate clays

In this type of silicate clays, one additional magnesium dominated trioctahedral sheet (brucite) is sandwiched between two adjacent 2:1 type layers and thus termed as 2:1:1 type silicate clays (Fig. 4.6). These are non-expanding silicate clays due to presence of magnesium-dominated trioctahedral sheet (which serves as a cation) between two 2:1 type layers. Colloidal properties of chlorites are almost similar to those of fine-grained micas. This type of silicate clay has cation exchange capacity of 10-40 cmolc kg-1 and surface area 70-150 m2 g-1. Such type of silicate clays is common in sedimentary rocks.

4.3.2.2. Non-crystalline silicate clays

In soils, there are significant quantities of colloidal matter that is not sufficiently ordered to be detected by X-ray Diffraction due to the lack of three-dimensional crystalline structure. These are more difficult to study than those minerals which have well-defined crystalline structure. These clays consist of tightly bonded silicon, aluminum and oxygen atoms but these clays lack ordered structures. These are formed from volcanic ash and are dominantly found in Andisols. These clays carry both positive and negative charges which are pH dependent. These minerals possess plasticity when wet and have very low degree of stickiness. These minerals have extremely high adsorption capacity for phosphate and other anions particularly under acidic conditions. These clays include allophane and imogolite. Allophanes have high CEC (10-150 cmolc kg-1) which is highly variable due to presence of pH dependent charges on these minerals.

Fig. 4.5 Structure of non-expanding 2:1 type silicate clay

4.3.2.3. Iron and aluminum oxide/hydroxide

These clays are present in a variety of soils but predominantly in the soils of warm and humid regions (highly weathered acid soils, i.e., Ultisols and Oxisols). These minerals have iron and aluminum atoms coordinated with oxygen atoms but Al is dominantly associated with H+ ions to make hydroxide. Some of these minerals like gibbsite (Al-oxide) and goethite (Fe-oxide) occur as crystalline sheets while other oxide minerals are non-crystalline and occur as amorphous coatings on soil particles. They have relatively low plasticity and stickiness and carry slightly negative to moderately positive charge depending upon the soil pH. Although, generally these minerals are termed as oxides for simplicity but these are hydroxides and oxyhydroxides due to the presence of hydrogen ions. Examples of these minerals include limonite (FeO3.H2O), goethite (FeOOH), hematite (Fe2O3), magnetite (Fe3O4), gibbsite (Al2O3.3H2O), boehmite (AlOOH) and alumina (Al2O3).

4.3.2.4. Organic soil colloids (humus)

This type of colloids is important in all types of the soils due to its very small size, large surface area and high CEC. These colloids are not well structured and are composed of carbon, hydrogen, oxygen and nitrogen rather than aluminum, silicon

and oxygen like the silicate clays. These colloids have no stickiness/plasticity and have very high water holding capacity. Due to non-cohesiveness, soils rich in organic colloids have low soil strength and are not suitable for engineering constructions like roads and buildings. The organic colloidal particles vary in size but they may be at least as small as the silicate clay particles. The humus colloids are amorphous and are not stable like clay and possess high CEC. These colloids carry both positive and negative charges but negative charge is dominant and pH dependent which is very high in neutral to alkaline soils. The humus colloid has functional groups containing covalently bonded hydrogen which dissociates with the increase in pH to produce negative charges on the humus colloid. These functional groups include enolic (-COH = CH), carboxylic (-COOH), phenolic (-C6H5OH) and amide (NH =).

Fig. 4.6 Structure of 2:1:1 type silicate clay

4.3.3. Role of clays in agriculture and environment quality

Clays greatly influence soil physical, chemical and biological properties. As discussed in previous sections, clays differ in surface area, surface charge density, shrinkage and expansion. Their role in agricultural productivity, environmental quality and engineering activities is dependent on these properties. For example, soils rich in 1:1 type silicate clays have high adsorption capacity for P due to ligand exchange of phosphate anion (PO4

2-) with functional groups OH- present on the exposed broken edges of these clays. Similarly, soils rich in 2:1 type silicate clays, like mica, have more K availability as compared to other soils as mica is the primary source of K in soils and provides K on sustainable basis due to weathering. Generally, soils having clay minerals with high CEC and more surface area, are termed as fertile

soils as such soils have high water-holding and nutrient holding capacities and less nutrient losses through leaching. In addition to the role of silicate clays in agricultural productivity, environmental quality is also greatly influenced by their presence in the soils. Soil clays control the movement of contaminants in the soils due to presence of negative charges on their surface. Most of the pollutants are cations and, thus, are attracted strongly by clays when present in soils and consequently prevent their movement and availability to plants. Clays play crucial role in purification of groundwater as most of the pollutants present in percolating water are retained by clays. Engineering activities are greatly affected by the type of soil clays as soil provides base to all type of engineering activities. Soil physical properties, like plasticity, cohesion, shrinkage-swelling and dispersion-flocculation, are important for engineering activities. Soils which are rich in swelling type of clays are not suitable for heavy engineering constructions like dams, buildings and highway constructions. Differential swelling/shrinkage of clays cause cracks in the buildings, roads and dams. However, swelling type of clay can be effectively used in environmental studies. Swelling types of clays are used to seal the ponds, sewage lagoons, industrial waste lagoons and landfills (Weil and Brady 2016) and prevent leaching/seepage from these constructions. A layer of smectite on the bottom or sides of constructions prevents the movement of water and contaminants due to swelling on wetting.

4.4. Development of Negative Charges

Soil reactivity depends upon surface area and surface charge. Surface area of the soil is inversely proportional to its particle size. Most of the surface area of the soils is due to presence of colloidal particles (< 2 µm) like clay and humus which are also charged particles and sources of charges for soils. In the soils, charges are originated through two major phenomena viz. isomorphic substitution and ionization of surface functional groups and thus give rise to permanent and variable or pH-dependent charges, respectively.

4.4.1. Isomorphic substitution

Isomorphic substitution is the replacement of central ion with another ion of similar size but having different charge in the crystal lattice of silicate clays during growth of crystals leading to the development of positive or negative charge depending upon the valence of substituting ions in the central position of octahedral or tetrahedral coordination (Fig. 4.7). This type of charge remains unchanged after its development and thus is termed as Permanent Charge. Net negative charge will be developed if higher valent cation (e.g., Al3+) is replaced with lower valent cation (e.g., Mg2+). This type of substitution is common in Al-dominated dioctahedral sheets of smectite, vermiculite, and chlorite. While in some other case, Si4+ is replaced by Al3+ in tetrahederal sheets of fine grained mica, vermiculite and some time in smectites. Positive charge will appear if lower valence cation (Mg2+) is replaced by higher valence cation (Al3+) in trioctahedral hydroxide sheet present in the interlayer spaces of chlorite. Isomorphic substitution is the major source of negative charges for 2:1

and 2:1:1 type silicate clays but is of minor importance for 1:1 type silicate clay (Bohn et al. 2002).

Fig. 4.7 Isomorphic substitution Al3+ in octahedral coordination by Mg2+ leading to the development of negative charge

4.4.2. Ionization of surface functional groups

Ionization of surface functional groups (OH- group) from the surfaces of inorganic and organic colloids, and broken edges of silicate clays leads to the development of variable charge. These functional groups include hydroxyl (-OH), carboxyl (-COOH), phenolic (-C6H4OH) and amine (NH2). This type of charge varies with pH of the soil and thus termed as pH-dependent or variable charge. Moderately acidic or neutral soils carry minor pH dependent charges. As pH increases, H+ ion from OH or COOH group dissociates leaving one negative charge un-satisfied. In some cases, removal of positively charged complex aluminum hydroxyl ions like Al(OH)2

+ lead to the development of negative charge. At low pH, this complex ion block the negative sites on silicate clays but as the pH increase this ion react with OH- in soil solution to form insoluble Al(OH)3. The silica layer develops a negative charge from the oxygen ions along the edge of the crystal. Only one of the two negative charges of oxygen is combined with a silicon ion. Therefore, oxygen ions at the end of the crystal have one negative charge unsatisfied. Depending on the activity of H+ in the soil solution, either hydrogen is added to the structure (protonation) or released from the structure (deprotonation). So, this charge is called a pH-dependent charge or a variable charge. It is the main source of charge in 1:1 type layer silicates, Fe and Al oxides and hydroxides, allophone and humus. Effect of pH on the cation exchange capacity or negative charges of soil colloids is illustrated in the following equation. At the exposed crystal edges and flat external surfaces of minerals, the covalently bonded hydrogen of hydroxyls dissociates at pH level of more than 7 leaving negative charges carried by oxygen. The loosely bonded hydrogen is readily exchangeable.

In some cases, the inorganic soil colloid may be responsible for -ve charge (high pH), no charge (intermediate pH) or +ve charge (low pH) due to fluctuation in pH as demonstrated below.

> Al-OH + OH - Al-O- + H2O Soil solid Soil Soil solid Soil Solution.

No charge Solution One -ve charge

So, the primary source of variable charge is due to loss or gain of H+ ions from functional groups on the surfaces of soil solids.

4.5. Soil Solution

The water present in soil pores is known as soil solution and it serves as interface between soil, atmosphere, biosphere and hydrosphere. Soil solution is the immediate source of nutrients for terrestrial organisms including plants. Soil solution is the most important medium by which chemical elements are transferred to life system via plant root absorption. Soil solution is not electrically neutral, and contains more cations compared to anions. This is due to the reason that net negative charge of soil colloids is balanced by excessive cations present in soil solution (Bohn et al. 2002). Among dissolved solids present in soil solution, those which are dissociated into ions are the most important which influence different chemical reactions occurring in soil solution. Composition of soil solution is influenced by soil parent material, precipitation and dissolution of minerals in soils, density and type of vegetation and soil drainage. Addition of chemical fertilizers and organic matter and absorption of nutrients by plant have significant effect on composition of soil solution (Khattak 1996). The ions present in soil solution are the predominant and immediate source of nutrients to plants required for their growth. Continuous supply of nutrients in soil solution is ensured by different types of interactions between ions and soil particles. These interactions include weathering of soil minerals, organic matter decomposition, rainwater and irrigation water containing soluble salts and desorption of ions from soil colloid (Bohn et al. 2002). All these sources replenish nutrients in soil solution continuously. Once the ions are released into soil solution, ions are subjected to different losses. Ions present in soil solution are retained by the soil through different surface mechanisms to interact with soil particles and the soil may retain these ions thereby preventing their leaching.

4.5.1. Chemistry of soil solution

Ions present in solution are seldom present in free state, rather, these ions are surrounded by water molecules or other ions. As concentration of solution increases, free movement of ions correspondingly decreases and their interaction with each other increases. In very dilute solutions, ions move freely in solutions while in case of concentrated solutions, ions and water molecules interact with each other and with other ions (Khattak 1996). Due to these interactions, the chemistry of soil solution is very complex. Water molecule, due to its bipolar nature, strongly reacts with ions present in soil solution. Ions interact with each other at longer distance (> 0.5 nm) in the diluted solution and this type of interaction is explained by the concept of chemical activity which is defined as:

α = γ M

where α is activity, γ is the activity co-efficient and M is molarity. Under ideal conditions, when concentration approaches zero, then γ approaches one and concentration and activity become equal. Interaction of ions in soil solution is influenced by amount of all the charges present in soil solution which is explained by ionic strength (I).

I = ½∑ MZ2

Where M is molarity and Z is charge. Effect of all ions present in soil solution on activity coefficient was explained by Debye and Huckel in 1924 by proposing an equation

log γ = -AZ2i I1/2

where γ is activity co-efficient, A is constant for aqueous solution at 25 oC. This equation is known as Debye-Huckel limiting law due to its application for very dilute solutions and works well upto ionic strength of 0.01 M. However, this equation does not work well at high concentrations and thus modified later to make this equation valid under conditions where ionic concentration approaches 0.1 M;

Where B = 0.33 for aqueous solution at 25 Co and “a” is individual ion parameter determined experimentally. Davies further modified this equation which gives fairly good results for ion activity coefficient over the range of ionic concentrations prevailing in soil solutions and fresh waters (Sposito 1989; Bohn et al. 2002)

Measuring ion activity is necessary to understand the availability of nutrients to the plants in soil. Ion or nutrient activity decreased in concentrated solution due to close range interaction (< 0.5 nm) that leads to the formation of Ion Pairs and Complex

Ions of ions with each other (Fig. 4.8). Ions in ion pairs and complex ions lose their separate entity and behave like a single entity in soil solution (Khattak 1996; Bohn et al. 2002).

Ion Pair Complex Ion

Fig. 4.8 Simple diagrams of ion pair and complex ions

In complex ions (also known as inner sphere complex), central cation is surrounded by one or more ligand. Ligand is any ion which is present in coordination sphere of central ion. In case of ion pairs (outer sphere complex), anions attached with central cation in second solvation sphere and thus weakly attached with central cation as compared to complex ion. Many of alkaline and transition metals are present in soil solution as ion pairs or complex ions. Complex ions/ion pairs are formed when attractive force between cation and anion supersede attractive forces between cations and H+ for ligands including water (Ghafoor et al. 2013). Formation of mono-fluoro-aluminium complex during Al3+ extraction from soil is the simple example of complex ion formation

Al (H2O)63+ + ==== (AlF) (H2O)5

2 + H2O

The strength of association between different ions in solution is explained by different equilibrium constants. Stability constant (formation constant or binding constant) is equilibrium constant for complex ions and indicates interaction between reagents that comes together for complex ions formation. Stability constant helps to calculate concentration of complexes in soil solution and has useful application in soil chemistry, pure chemistry biological studies and medicines (Ghafoor et al. 2013). Stability constant for above mentioned complex ion can be calculated as

The water of hydration is ignored due to excess of water in aqueous solution. Similarly, hydrolysis (deprotonation) constant is equilibrium constant of H+ dissociated from water ligand attached to the central cation. Ion pairs or complex ions are soluble in soil solution but their presence decreases the activity of individual ions and influence different chemical reactions in soil solution like ion exchange, nutrient availability and nutrient leaching.

4.6. Diffuse Double Layer

In dry soil, ions directly reside on soil colloids and are held tightly but in wet soils, ions are not tightly held by soil colloids (Fig. 4.9). Negatively charged soil colloids attract cations from soil solution and forms distinct layers of ions containing both positive and negative charges and this layer is known as diffuse double layer (DDL). Diffuse double layer is also known as Helmholtz double layer and describes the electrical phenomenon at liquid-solid interface. Diffuse double layer theory is based on the assumptions of Gouy-Chapman which states that: 1) exchangeable cations exist as point charges; 2) colloidal surfaces are infinite and planar; and 3) charge is uniformly distributed on the surface of colloid. In DDL, cations concentration decreases with increasing distance from the surface of soil colloid and become equal to anions concentration in bulk solution (point in soil solution where colloid has no influence on dissolved ions). Two forces are responsible for counter ions in DDL; electrostatic attractive force draws in the cation towards negatively charged surface while repulsive force between anions and negatively charged surface push the anions into bulk solution and maintain electrical balance. Later, Stern proposed model to

explain the retention of ions in double layer. He proposed that some ions are strongly adsorbed by charged surface and thus not subjected to changes. This layer of ions which strongly adsorbed on charged surface is called as Stern layer while other ions form diffuses double layer of ions which diffuse away into bulk solution. Diffuse double layer has role in adsorption and desorption of ions in soil, nutrients availability and dispersion/flocculation of soil colloidal particles.

Stern Layer Diffuse Double Layer Bulk Solution

Fig. 4.9 Diffuse double layer

4.7. Ion Retention

The most important property of soils is ion retention and their ultimate slow release into soil solution to match with plant absorption. The ion retention by soils has dual function of keeping ionic concentration in soil solution at optimum level. This phenomenon keeps the ion concentration in soil solution at adequate level but not at ideal level. The quest for maximum crop yields has put pressure on soils and thus may require boosting the ionic concentration in soil solution to the desired level through fertilization. Soils retain ions through process of ion exchange. Soil chemistry mainly deals with cation exchange process while ignoring anion exchange process due to dominance of negative charge on soil particles. Ion retention by soils can be divided into two categories; weaker electrostatic retention of alkali and alkaline earth metals and strong retention of trivalent and transition metals through chemical bonding of polyvalent cations with O2- and OH- of silicate clays, hydroxides and phosphates. Retention of ions through chemical bonding is known as precipitation or strong adsorption and is influenced due to different precipitation-dissolution reactions (Bohn et al. 2002).

4.7.1. Ion exchange

As already discussed, soil colloids carry positive or negative charges which may be permanent or variable. Irrespective of nature of charges, soil colloids attract oppositely charged ions which constantly vibrate in a swarm near the colloidal surface and are held by electrostatic force of attraction. Break away ions from this swarm are replaced by those ions present in soil solution. This process of ion replacement in the swarm by other ions in soil solution is referred to as ion exchange. This is the most important property of the soil, by which soil retains and releases the ions slowly into soil solution. If soil colloids carry positive charge, then it will attract and exchange negative ions (anions) with soil solution and process would be known as anion exchange. Conversely, if soil colloid is negatively charged, it will attract and exchange positive ions (cations) with soil solution and the process will be known as cation exchange. Due to dominance of negative charges on soil, cation exchange is widely discussed while ignoring the anion exchange process.

4.7.1.1. Principles of cation exchange reactions

i. Reversibility

Cation exchange reactions are reversible and can proceed in both directions depending upon the concentration of reactants or concentration of products. Consider simplest case of cation exchange in which sodium ion (Na+) on soil exchange complex is replaced by H+ ion in soil solution. If Na is added into the soil, then reaction will move to left and vice versa. In the following reaction “X” represents soil exchange complex

NaX + H+ ←→ HX + Na+

ii. Stoichiometry

Cation exchange reactions are always stoichiometric in nature emphasizing the replacement of ions with each other on charge equivalence basis. For example if soil exchange complex is occupied by one Ca2+ ion, then two monovalent cations (Na+) will be needed to replace adsorbed divalent cation (Ca2+) from exchange complex and vice versa.

CaX + 2 Na+ ←→ 2NaX + Ca2+

iii. Mass action

Cation exchange reactions are reversible reactions and their direction is controlled by law of mass action. According to law of mass action, rate and direction of reaction is determined by concentration of active ion. If concentration of active ion is increased, it would have more chances to replace the ion present on exchange complex.

CaX + 2Na+ ←→ 2NaX + Ca2+

High concentration of Na in this reaction will drive the reaction in forward direction.

iv. Anion effect

During the process of cation exchange, anions are always accompanying the exchanging cations and thus can greatly influence exchange reactions. As direction of exchange reaction is dependent upon concentration of active ion and will move in the right direction if the ion released from exchange site is removed from the system through precipitation, volatilization or strong association with anion. The role of anion is very important and thus will determine the direction of reaction. For example, if H+ ion from exchange site is replaced by Na from NaOH and end product will be H2O molecule which dissociates very weakly and thus reaction will move in right direction and exchange reaction will not be reversed. Consider another case where H+ is replaced by Na from NaCl and end product will be HCl which strongly dissociate and thus free H+ will reverse exchange reaction.

HX + Na2+ (from NaOH) ←→ NaX + H2O (poorly dissociated)

HX + Na2+ (from NaCl) ←→ NaX + HCl (strongly dissociated)

v. Valence dilution

Cations (irrespective of valence) present in soil solution are always in equilibrium with those present on soil colloids. Addition of the water into soil will disturb this equilibrium and favor the adsorption of multivalent cations compared to the monovalent cations.

vi. Cation selectivity

Cations are held by the soil colloid with different strength and thus arranged in the order of strength with which they are held by soil (lyotropic series). Generally, cations with smaller hydrated size and high charge density are strongly adsorbed and thus less likely to be exchanged. Different types of minerals present in the soil preferred particular cation for adsorption. For example, vermiculites and fine grained mica has the preference for potassium (K+). Potassium is strongly attracted towards the inter layer spaces and thus fixed in interlayer spaces strongly.

vii. Complementary cations

Soil is very complex system where cation exchange does not take place between two different ions present on soil colloid and soil solution only. Soil colloid is occupied by many different cations which are in equilibrium with similar type of diverse cations in soil solution. Cations present on soil colloid affect the replacement of neighboring cation and this phenomenon is known as complementary cation effect. For example, it would be easy to replace Ca2+ by NH4

+ from Ca2+-Al3+ system than from Ca2+-Na+ system.

viii. Speed

Exchange reactions are very rapid and instantaneous in nature which leads to the existence of dynamic equilibrium between soil solid and solution phases. Speed of the exchange reactions is only affected by movement of ions from or to exchange complex. This is important in case of field conditions where diffusion of ions is restricted by tortuous nature of soil pores.

4.7.2. Cation exchange capacity

Cation exchange capacity (CEC) is an amount of exchangeable cations that a soil can retain and exchange with soil solution at specific pH. Previously milliequivalent per 100 g soil (me 100 g-1) was used to express CEC but presently moles of charge per kg-1 is used to express CEC. For convenience and calculating whole number, centimoles of charge per kg-1 are generally used which is equivalent to me 100 g-1 soil.

1 me = 1 mmolc (millimole of charge)

1 me 100 g-1 = 1 mmolc 100 g-1

1 me 100 g-1 = 10 mmolc 1000 g-1 (multiply and divide by 10 on right side)

1 me 100 g-1 = 10 mmolc kg-1 (1000 g = 1 kg)

1 me 100 g-1 = 1 cmolc kg-1 (10 mmolc = 1 cmolc)

4.7.2.1. Factors affecting cation exchange

i. Soil pH

Cation exchange capacity of soils is directly related with soil pH. Cation exchange capacity of soils is the consequence of presence of negative charges on soil particles and it varies with the change in negative charge on soil particles. In soils, two types of charges are present viz. permanent and variable charges. Permanent charges remain constant while variable charges vary with a change in soil pH (explained in section 3.4.2). As the pH increases, negative charges on soil colloids, particularly organic colloids, allophones, oxides of Fe and Al and 1:1 type silicates, increase and thus cation exchange capacity is also increased.

ii. Type of soil colloids

The type of the colloids present in soil has significant effect on CEC of the soil. As already discussed in section 3.3.1.2., 2:1 types silicate clays (expanding type) have high charge density and surface area compared to the 1:1 type silicate clays. The soils which contain 2:1 silicate clay have high CEC than those having 1:1 type silicate clays provided that total amount of clay contents is equal.

iii. Organic matter

Organic matter is a source of variable charges in soils and thus contributes in CEC. More OM in soils more would be the CEC of soils and vice versa. One percent increase in soil organic matter results in 2 cmolc kg-1 increase in CEC of soil.

4.7.2.2. Significance of CEC

Cation exchange capacity is an important phenomenon in soil fertility, reclamation of salt-affected soils, alleviation of soil acidity/alkalinity, retention of pollutants and purification of percolating water. Significance of CEC is explained as below.

i. Soil fertility

Plants absorb nutrients mainly from soil solution and concentration of various nutrients in soil solution depends on the concentration of nutrients present on exchange sites. Most of the cations (Ca2+, Mg2+, K+) which are held by soils on its exchange sites are plant nutrients, except Al3+. Cation exchange is the process which drives cations from exchange sites into soil solution and thus plays a very important role in plant nutrition. For example, K is an important macronutrient and exchangeable K is the major and immediate source of K for plants which become easily available through the process of cation exchange. Similarly, exchangeable Ca and Mg serve as potentially available sources for plants. Cation exchange process slows down the leaching losses of some nutrients like Ca2+, Mg2+, K+ and NH4

+. After application of fertilizer, concentration of these nutrients become very high in soil and thus may be subjected to leaching losses but soils retain these nutrients due to cation exchange capacity. These nutrients are adsorbed on exchange sites and later on, are released slowly into soil solution.

ii. Correction of soil acidity

Soil acidity and alkalinity are undesirable soil conditions for plant growth and thus need to be managed properly. Under highly acidic soil conditions (i.e., pH below 5.8), exchange sites are pre-dominantly occupied by H+ and Al3+ ions which make soil conditions unfavourable for plant growth. To make the conditions suitable for plant growth, H+/Al3+ must be replaced from the soil’s exchange sites and rendered in-active. The excess H+ and Al3+ ions on the soil’s exchange sites are replaced with Ca2+ ions through liming, i.e., application of lime (CaCO3), increases the pH of the acid soil and pH moves towards neutrality. Rise of soil pH up to 5.8 with liming is considered adequate for the growth of most crops. Cation exchange capacity of soils helps to calculate the amount of lime required to ameliorate the soil acidity. Soils with high CEC require more lime and vice versa.

iii. Reclamation of sodic/saline-sodic soils

Sodic and saline-sodic soils are characterized by the presence of excessive amount of Na+ on exchange sites which deteriorate soil physical and chemical properties. Soluble source of Ca2+ (gypsum) is applied to reclaim the sodic/saline-sodic soils. Soluble Ca2+ obtained due to the dissolution of gypsum in soil solution replaces Na+ ions from soil’s exchange sites by mass action and thus reclaiming the sodic/saline-sodic soil. Application of gypsum to sodic and saline-sodic soils is accompanied by heavy irrigation which leaches down the desorbed Na into deeper layers of soils and improves soil physical and chemical properties.

2NaX + Ca2+ (from gypsum) ←→ CaX + 2Na+ (leached down)

iv. Purification of percolating water

Soil colloids serve as sorbent for many pollutants present in soil and water passing through soil due to the presence of negative charges. Soils purify the percolating water by adsorbing different pollutants present in the water and this is very important in case of waste water. Waste water contains variety of organic and inorganic pollutants (Fe2+, Cu2+, Mn2+, Pb2+, Ni2+, Cd2+) which are adsorbed by soil while

percolating through soil. Adsorption of these pollutants by soil colloids is due to cation exchange capacity and thus CEC plays very important role in purification of groundwater.

4.8. Soil Reaction (pH)

Soil pH is defined as negative log of hydrogen ion concentration [H+] (expressed in moles per liter) or hydrogen ion activity (H+) (Weil and Brady 2016; Ghafoor et al. 2013). Unit of pH is mole per liter if it is defined in terms of hydrogen ion concentration but it becomes unitless if it is defined in terms of hydrogen ion activity. Soil pH indicates the degree of soil acidity or soil alkalinity and greatly influences soil physical, chemical and biological properties. The pH scale varies from 0 to 14 with pH 7 as the neutral point (Ghafoor et al. 2013). As pH is inversely related with hydrogen ion concentration/activity, increasing H+ ions would decrease the soil pH causing more acidic conditions. As pH increases from 0 to 7, soil conditions remain acidic but between pH 7 to 14 the soil conditions are alkaline/basic (Bohn et al. 2002). Mathematically, pH can be expressed as (Khattak 1996; Ghafoor et al. 2013):

pH = log1

(H)= − log(H)

We can derive this equation as follows

HOH ==== H+ + OH-

�H��OH�

�HOH�= Kw

Concentrations of H+ and OH- are expressed in mol L-1 while Kw is equilibrium constant of water which is equal to 10-14

One mole of H2O is equal to 18 g at 25 oC and one liter of water is equal to 997 g. There will be 55.93 moles in one liter of water.

HOH = 55.39 mol L-1

[H+] = [OH-] = 10-7

(H+) = (OH-) = 10-7

By taking log on both sides

log (H+) = log (OH-) = log 10-7 or pH = pOH = 7

4.8.1. Significance of soil pH

Many of plant species and microbial populations in soils are pH sensitive. Common field crops like alfalfa, barely, beans, sugarbeet, clover and sugar cane grow well at pH range of 6-8 while rice, maize, tobacco, wheat, peas and peanuts grow well at pH range of 5.5 to 7.0. However, most of forest trees grow well at the pH range of 4-7

(Khattak 1996). Soil pH affects the availability of essential plant nutrients and other elements in the soil for root uptake by plants. For example, micronutrients, except for molybdenum (Mo), are more soluble in slightly acidic to moderately acidic soils, while macronutrients are more soluble at alkaline pH. Among the macronutrients, availability of P is more sensitive to pH changes. Phosphorus availability in soils is optimum in the range of pH 6.0 and 7.5. At low pH (i.e., < 6.0), P is precipitated with iron (Fe) and aluminium (Al) and at alkaline pH (i.e., > 7.5), P forms insoluble compounds with calcium (Ca). Most nutrient deficiencies can be avoided at soil pH 5.5 to 6.5, provided the soil minerals and soil organic matter contain adequate essential plant nutrients. The soil pH can also affect the types of micro-and macro-organisms present in the soil (Tisdale et al. 2006). Earthworms are usually absent from very acid soils but fungi prefer acidic conditions and bacteria prefer a neutral soil. This last point can be very important when growing peas and beans (legumes) because if the soil is too acid the rhizobium bacteria which fix nitrogen for the legumes will not be able to survive, so neither will the legumes, so that a vital part of the crop rotation may be lost. Another aspect of unfavourable soil pH affecting plant growth is through soil microorganisms inducing slime-mould which causes club root, a disease of crucifers; slime-mould thrives only in acid soils and raising the soil pH with liming can reduce or eliminate this problem.

4.9. Base Saturation Percentage

It is the amount of basic cations (Ca, Mg, Na, K) which a soil can adsorb on its surface (exchange complex) at some particular soil pH and is expressed in terms of percentage. Base saturation is positively related to soil pH because a high base saturation value would indicate that the exchange sites are dominated by basic cations. Indirectly, base saturation indicates the extent of leaching of exchangeable basic cations (Ca, Mg, K, Na) from soils. The soils of arid/semi-arid regions have high basic cation saturation as compared to soils of humid regions. Percent base saturation is calculated as:

Percent Base Saturation (%) =

Sum of exchangeable basic cations (Ca2+,Mg2+,K+,Na+)/CEC at a specific pH x 100

4.9.1. Significance of base saturation

Basic cation saturation percentage is considered an indicator of soil fertility. Soils having high base saturation, can easily release adsorbed cations into soils solution for plant absorption. For example, a soil with > 80% of base saturation is considered to be very fertile soil, a soil with base saturation between 50-80% is considered to be medium fertile soil and soil with base saturation of < 50% are considered to be non-fertile soil. Base saturation percentage is very important in soil classification to differentiate between mollic epipedon and umbric epipedon.

4.10. Buffering Capacity

Capacity of the soil to resist the change in pH on addition of acid or base is called buffering capacity of soil. In soils, soil acidity has three distinct pools, viz., active acidity, potential acidity and residual acidity. Residual acidity plays a crucial role in buffering of soil by maintaining equilibrium between these three pools of soil acidity. Potential acidity in soils maintains the equilibrium with active acidity. For example, if base is added into soil solution, this will disturb the equilibrium of H+ ions in soil solution. At this stage, potential acidity will release H+ ions into soil solution and this would restrict any change in pH due to disturbance of equilibrium of H+. Similarly, if some acid is added into soil solution, excess H+ in soil solution added due to acid would be adsorbed by potential acidity and thus any change in pH would be nullified. In soils clay and soil humus act as principal buffering agents because these both provides adsorbing sites for H+.

4.10.1. Significance of buffering capacity

Soil buffering capacity is very important due to agricultural and environmental reasons. Drastic fluctuations in soil pH in the absence of buffering capacity would have detrimental consequences on plants, soil microorganism and aquatic ecosystem. For example, plants absorb most of the essential nutrients at pH around 6-7 but if soil pH is not buffered, pH may be very high or very low thereby interfering in the availability of plant nutrients and ultimately plant growth. Well buffered-soils prevent acidifying effect of acid rains on soils and drainage water. Buffering capacity determines the amount of amendment (sulfur or lime) required to bring the desired change in soil pH.

4.11. Point of Zero Charge

This is pH of a soil where net charge on soil particles becomes zero. The point of zero charge is a fundamental description of a mineral surface which explains that the total concentration of surface anionic sites is equal to the total concentration of surface cationic sites. If the pH is more than PZC, the net charge on soil surface would be negative. Conversely if the pH is less than PZC, net charge on the soil surface would be positive (Appel et al. 2003).

4.12. Soil Redox Potential (Eh)

Redox potential is derived from reduction and oxidation. This indicates the capacity of some substance to accept or donate electrons. Soil redox potential represents the availability of electrons in the soil system. In redox reactions, there is simultaneous occurrence of oxidation and reduction of two substances involved in reaction where one substance is reduced while other substance is oxidized. This property of soils is related with soil aeration. In well aerated soils, oxidized states of elements dominate while in poorly aerated soils, reduced forms of elements dominate. For example, in well aerated soils, oxidized form of iron (Fe3+) and nitrogen (NO3

-) will dominate

while in poorly aerated soil reduced form of iron (Fe2+) and nitrogen (NH4+) will

dominate.

4.12.1. Significance of rodex potential

Redox potential is linearly related with soil pH and pH greatly influences nutrient availability to plants. Redox potential can be used to indicate aeration status of soils. Aeration of soil can greatly influence different physical, chemical and biological properties of soils. Low redox potential indicates that soil is submerged and has deficiency of oxygen. Deficiency of oxygen in soil greatly influences root respiration, and microbial activity. At low redox potential, iron is present as Fe2+, manganese as Mn2+, nitrogen as NH4

+ and sulfur as H2S while at high redox potential these elements are present as Fe3+, Mn4+, NO3

- and SO42-. Changes in redox potential

of soil drastically changes microbial population in soils. At low redox potential, anaerobic microbes will dominate while at high redox potential, aerobic microbes will dominate.

4.13. Exchange Equations

Irrigation water, liming, weathering, fertilizations and acid rains disturb the equilibrium between exchangeable cations and those present in soil solution thereby necessitating the prediction of exchangeable cations in soils (Bohn et al. 2002). The relationship between exchangeable cations and those present in soil solution is predicted by different exchange equations which provide valuable information about elemental deficiencies or imbalance, movement of toxic metals and soil dispersion (Khattak 1996; Bohn et al. 2002). Most of exchange equations have limited applicability under field conditions because these equations consider simple exchange reactions between ions against actual multi-element exchange process, constant exchange capacity, simple stoichiometery and complete reversibility of reactions (Ghafoor et al. 2013; Tan 2011; Bohn et al. 2002). Simplest exchange reaction is

CaX + 2Na+ → 2NaX + Ca2+

Reaction co-efficient would be

k = (NaX)2 (Ca2+)/(CaX) (Na+)

This equation is known as Kerr Equation which assumes that ionic concentration is directly proportional to ionic activities and this equation works well at low concentrations.

Another equation was proposed by a chemist Gapon in 1933 known as Gapon Equation. This equation has considerable application in soils particularly sodic/saline-sodic soils for Na-Ca exchange. This equation was based on the law of mass action and takes into ion concentration instead of ion activity. In this equation, exchanging cations are expressed on chemically equivalent basis. The Gapon equation cannot be applied satisfactorily on entire range of Na-Ca exchange, however its application is quite encouraging for range of ionic concentration suitable

for irrigated agriculture. Generally, range of Gapon constant is 0.010 to 0.015 (L mmol-1) in agricultural soils (Bohn et al. 2002). The Gapon Equation is written as under

Ca1/2X + Na → NaX + (Ca2+)1/2

kG = [NaX] [Ca2+]1/2 / [Ca1/2X] [Na]

whereas concentration of exchangeable cations are in mmoles of charge per gram (or kg) and soluble cations are expressed in mmoles of charge per liter.

4.14. Chemical Equilibria in soils

Generally chemical reactions are reversible and direction of chemical reaction is determined by concentration of reacting chemicals (reactants) or concentrations of emerging products of chemical reactions (products). Equilibrium is well explained by equilibrium constant (K) which is explained by the following equation

A + B ←→ C + D

Equilibrium constant can be calculated as

The condition or the point at which the forward and backward reactions occur simultaneously is known as chemical equilibrium. Concept of chemical equilibria is very important in different chemical reactions occurring in soils. Chemical equilibrium in soils is influenced by different processes which are explained in Fig. 4.10.

Soil comprises of solid phase (50%) and pore space (50%) which is jointly shared by water (liquid phase ≈ 25%) and air (gaseous phase ≈ 25%). Solid phase of soil provides foundation for the living organisms and tends to remain in dynamic equilibrium with solution phase of soil. Soil solution is important as plants absorb nutrients from soil solution. Upon the absorption of specific nutrient/ion by the plants from soil solution, deficiency of specific nutrient/ions emerges in soil solution. Deficiency of nutrient/ion in soil solution triggers the removal of ions from solid phases into solution phases and this process continues till the equilibrium between two phases is achieved. Solid phase of soil comprises of crystalline and non- crystalline minerals which controls chemical equilibrium in soil. For example, if soil solution becomes supersaturated with specific nutrient/ion, that specific nutrient/ion precipitates and become the part of solid phase and thus equilibrium is immediately achieved. Contrary to this, if soil solution becomes deficient of any nutrient/ion due to absorption by the plants or any other process, nutrient/ion on solid phase immediately dissolves into soil solution till the equilibrium is achieved.

Fig. 4.10 Chemical equiliria in soils

4.15. Conclusion

Chemical properties of soils are the most important in determining fate of different ions in soils. Ion retention, ion transformation, ion adsorption-desorption, ion solubility and bioavailability and ion leaching are important processes which ultimately affect the environmental quality, its sustainability and agricultural productivity. Ion exchange is the most important phenomenon after photosynthesis through which nutrients become available to the plants from solid phase. Soil reaction further affects the availability of these nutrients to the plants by Solubilizing or insolublizing nutrient elements in soil. An understanding of chemical properties of soils enables the students to better understand all the processes concerning environmental quality and agricultural productivity.

References

Appel, C., L.Q. Ma, R.D. Rhue and E. Kennelley (2003). Point of zero charge determination in soils and minerals via traditional methods and detection of electroacoustic mobility. Geoderma 113: 77–93

Bohn, H.l., B.L. McNeal and G.A. O’Connor (2002). Soil Chemistry, John Wiley and Sons, New York, USA.

Weil, R.R. and N.C. Brady (2016). The Nature and Properties of Soils, 15th Edition. Pearson Education Inc., Upper Saddle River, New Jersey, USA.

Ghafoor, A., G. Murtaza, Saifullah, M.Z. Rehman, M. Sabir and H.R. Ahmad (2013). Fundamentals of Soil Chemistry. Miraj Din Printers, Lahore, Pakistan.

Khattak, R. A. (1996) Chemical properties of soil. In: Bashir, E. and R. Bantel (Eds.) Soil Science, National Book Foundation, Islamabad, Pakistan. pp. 167–200.

Skoog, D.A., D.M. West, F.J. Holler, S.R. Crouch (2004). Fundamentals of Analytical Chemistry. 8th Edition, Brooks/Cole, Thomson, Singapore.

Sposito, G. (1989). The Chemistry of Soils. Oxford University Press, New York, USA.

Tan, K.H. (2011). Principles of Soil Chemistry, CRC Press, Taylor & Francis Group, Boca Raton, Florida, USA.

Chapter 5

Soil Microorganisms and Plant

Growth

Hafiz Naeem Asghar, Rizwan Ahmad and Muhammad Javed

Akhtar†

Abstract

Soil microbiology is concerned with microbes living in the soil, their interactions with each other and with plants in relation to soil management, agricultural productivity and environmental quality. The diversity of microbes in soil is the highest on Earth. These microbes are involved in different vital functions in soil. The most important function in soil is decomposition of organic matter, which is mediated through microbes, which has an enormous influence on soil structure, soil fertility, plant growth and carbon reserves. Soil microbes are involved in reduction of atmospheric gaseous nitrogen (N) to ammonia, which is said to be a second most important biochemical process on Earth after photosynthesis. Thus microbes regulate and maintain organic and inorganic pool of N in soil. Microbes transform different nutrients from unavailable form to plant available form. Microbes do much more than nourishing plants. Soil microbes drive carbon, nitrogen, phosphorus (P) and sulfur (S) cycles in soil, thus play critical roles in controlling composition, chemistry and physics of the atmosphere; therefore, have substantial impact on climate change. Soil microbes attribute in biological processes to degrade, transform, breakdown or remove contaminants from soil and water. Microbial activities are very important for bioremediation of xenobiotic and recalcitrant compounds in soil to improve soil

†Hafiz Naeem Asghar* and Muhammad Javed Akhtar

Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: naeemasghar@yahoo.com Rizwan Ahmad

Land Resources Research Institute, National Agricultural Research Centre, Islamabad, Pakistan. Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

104 H.N. Asghar, R. Ahmad and M.J. Akhtar

health and quality. Diversified and healthy soil microbial ecology is considered as a sensitive indicator of soil health and fertility.

Keywords: Rhizosphere, Microbes, Symbiosis, Decomposition, Remediation and Nutrients.

5.1. Introduction

Soil is a complex habitat for living organisms. These organisms are numerous and diverse. The branch of soil science which is concerned with microorganisms found in soils and their relationship to soil management, agricultural production and environmental quality, is called as soil microbiology. More simply, soil microbiology may be defined as “the study of microorganisms that live in soils, their metabolic activity, and their roles in energy flow and nutrient cycling” (Atlas and Bartha 1993). The Soil Science Society of America (1998) defines soil microbiology as “the branch of soil science concerned with the soil-inhabiting microorganisms, their functions and activities”. Soil microbiology takes into consideration the morphological and physiological characters of soil micro-organisms and their interactions with other organisms, plants, soil and environment, role of microbes in nutrient cycling, geochemical transformations and climate changes. The intense interactions between plants, microbes and soil are translated into plant health, and that is why the soil microbial diversity is said to be an important index of agricultural productivity.

Soil organisms are diverse, ranging from microscopic microorganisms to small mammals. The soil microbial diversity is estimated to range from several thousand to several million different genomes per gram of soil (Whitman et al. 1998; Torsvik and Øvreås 2002). Bacteria are the dominant inhabitants of soil, exceeding their population of 100 million in one gram of soil with approximately 104 to 106 different species. While population of actinomycetes is 106 to 107 per gram soil and of fungi 104 to 106 per gram soil. Collectively, all microorganisms (bacteria, actinomycetes, and fungi) are usually said to be soil microflora, while small microscopic animals are designated as soil microfauna (Sylvia et al. 2005).

The physical, chemical and even biological properties of the soil habitat and their interaction with the resident community of soil microorganisms have a significant impact on growth and activity of microorganisms. Understanding functions and activities of microbial community in soil can enable us to maintain soil health and to achieve the goal of sustainable agriculture.

Few examples of important aspects of soil microbiology are:

• Symbiotic N fixation.

• Decomposition of organic waste to make it a useful product like compost/biofertilizer.

• Nutrient transformations in soil; N, P and S cycles.

• Microbial mediated enzymes production in soil; phosphatases, ureases, cellulases, ACC-deaminases.

• Production of growth regulators; auxins cytokinins and gibberellins.

Soil Microorganisms and Plant Growth 105

• Bioremediation of organic and inorganic pollutants.

• Carbon cycling and emission of greenhouse gasses.

Along with soil microorganism, in soil there are many macroorganisms, which are also very important and cannot be ignored as a part of soil biology. Macrofauna includes earthworms, termites, ants, myriapoda, fly larvae and beetles. The soil macrofauna can modify soil chemical, physical and biological properties. They can mix the soil profile by burrowing action. Ants, termites, earthworms and ground beetles can bring soil form deeper layer to the surface, as well as enhance porosity and aeration. They can fragment litter, enhance decomposition of organic matter and formation of humus. They also ingest soil particles/organic matter with their food and thus contribute to aggregate formation and mineralization of organic matter and release of nutrients. Soil macrofauna disseminates bacteria and spores through excrement dispersion in soil or by on body transport (Ruiz 2008). The soil fauna contributes as much as 30% of the total mineralizable N. This contribution becomes more where C:N ration is high. Among macrofauna role of earthworms is well documented. They ingest organic debris, partly digesting and reducing the particle size of the substrates. Such activities enhance microbial ability to further decompose organic matter. In addition, worms excrete thick mucus containing polysaccharides, proteins and other compounds. Bacteria and fungi use these readily available substrates as additional nutrients (Sylvia et al. 2005). Along with these positive effects soil fauna has also some negative effects. Plant parasitic nematodes are best known for their negative effects on crop production, which can potentially reduce the amount of plant detritus deposited in the soil surface, such changes in litter inputs can decrease the number and activities of microbes in soil (Sylvia et al. 2005).

5.2. Historical Aspects of Soil Microbiology

Historically, humans have managed microorganisms consciously or unconsciously since time immemorial. First microbial process occurred over 100,000 years ago when fruit fermented and formed wine (Purser 1977). Also, cheese making and cheese consumption evidence dates back to 6000 years BC (Smith 1995). In 1800s, Edward Jenner inoculated people with cowpox germs as a preventive measure against smallpox (Wilson 1976). History revealed that Romans recognized relationship between legume crops and soil fertility. They knew that alfalfa and clover boost soil fertility but they did not know why? So this could be assumed as the initiation of soil microbiology but the main problem was that people could not actually see the microorganisms responsible for the changes they observed.

During the 17th century Antoni van Leeuwenhoek (1632-1723) made his own microscope and revealed the small creature as bacteria. He did not write stuffy academese, which was yet to be invented but he was the first human to see the new creature, the microbes (Anderson 2014). In the later half of 19th century, Ferdinand Cohn, Louis Pasteur, and Robert Koch were responsible for methodological innovations in aseptic technique and isolation of microorganisms (Madigan and Martinko 2006). Robert Hooke (1635-1703) from Royal Society of England (Coyne 1999) validated Leeuwenhoek's observations and published in his book

Micrographia in 1665, which may be considered as the first textbook of microbiology. Koch’s findings are summarized as Koch's postulates giving very important information about the isolation and growth of pure culture. Koch was basically concerned with disease causing organisms, which are mostly culturable, while in case of soil microorganisms it has been observed that some microbes are viable but not culturable. Microbial activities directly related to soil were elaborated by a Russian scientist, Sergei Winogradsky (1856-1953), who is often called the "Father of Soil Microbiology". His investigations are worth to mention regarding the followings:

• Winogradsky column, a self-contained ecosystem for studying the S cycle.

• Nitrification process in which ammonium (NH4+) is converted to Nitrate

(NO3-).

• Microbial oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+).

• Isolation of anaerobic, spore forming, N-fixing Bacilli.

Another scientist, Martinus Beijerinck (1851-1931), cultured the first N-fixing bacteria that grew symbiotically in association with legumes and the first aerobic N-fixing bacteria that grew asymbiotically as a free-living soil organism. These were Rhizobium and Azotobacter, respectively.

Near the end of the 19th century at the University of Delft (the Netherlands), Beijerinck developed enrichment techniques that allowed Beijerinck’s crucial discoveries including microbiological transformations of nitrogen and carbon, and also other elements such as manganese (Madigan and Martinko 2006).

Jacob Lipman was the founder of American soil microbiology. He considered soil as a complex and living entity which needed to be understood and studied from the standpoint of soil fertility and crop production. This revolutionary concept stands as a milestone in soil microbiology. Two prominent individuals who shared Lipman’s revolutionary concept of soil were Selman Waksman and Robert Starkey. Besides promoting the “Lipman Philosophy,” Waksman’s book, Principles of Soil Microbiology, and a later work by Waksman and Starkey, The Soil and the Microbe, were considered as the standard soil microbiology texts for most of the period between 1925 and 1950. However, in many circles Waksman is remembered less for his contributions to soil microbiology than for his discovery of the antibiotic streptomycin, for which he was awarded the Nobel prize in physiology and medicine in1952.

5.3. Microbial Diversity in Soils

Plants, animals, human beings, microbes, and every individual are fairly different from another one, even within the same genera and species. Variation is beauty of nature, its law of nature to maintain sustainability of the fittest in the community. The variation may be linear or cyclic. The variety and variability of organisms and ecosystem is referred to as diversity or biodiversity (Singh et al. 2012). Biodiversity comprises different aspect of ecosystem in addition of being concerned with only number of species present in certain ecosystem. It is actually the degree of variation

of life; this may be linked with genetic variation, species variation or ecosystem variation. Biodiversity can be assessed by focusing on the clusters of soil organisms that play key roles in soil ecosystem (Coleman et al. 2004). Microorganisms in soil play different roles like decomposition of organic matter, formation of soil structure, removal/degradation of pollutants in soil, transformation of nutrient in soil to regulate the consistent supply of nutrient to the plants. Microbes are also responsible for production of growth regulators like auxins, cytokinins, gibberellins, suppressing soil borne diseases and to regulate plant growth in biotic and abiotic stress through production of different enzymes. Soil biodiversity has also very important role in soil and atmospheric changes thorough carbon dioxide flux and carbon sequestration. Soil biodiversity helps to maintain soil quality. Soil quality reflects the ability of soil to support plants, to sustain or improve soil, air and water quality and functioning to support human health. Soil quality is directly linked with soil microbial community, morphologically and phenotypically diverse microbial community is required to mediate different processes carried out in soil (Sylvia et al. 2005). To gain an insight in biodiversity of microbial community composition and its dynamics at both taxonomic and functional levels, it needs to be fully elucidated. Changes in microbial community composition may be monitored by substrate utilization pattern (Biolog), by using fatty acids as markers of microbial diversity and DNA sequencing.

Bacterial diversity has been arranged into different groups (Singh et al. 2012) as under:

1) Spirochetes (e.g., Triponema palidum and Borrelia recurrentis);

2) Aerobic/microaerophilic, motile, helical/vibroid Gram negative bacteria (e.g., Spirillum volutans, Aquaspirillum sp., Campylobacter sp., Helicobacter pylori, Bdellovibri obacteriovorus);

3) Non-motile or rarely motile Gram negative curved bacteria (e.g., Cyclobacterium, Ancylobacter and Brachyarchus);

4) Gram negative aerobic/microaerophilic rods and cocci (e.g., Pseudomonas, acetic acid producing Acetobacter,Gluconobacter and Frateuria; N fixing Azotobacter chroococcum, Agrobacterium tumefaciens and Rhizobium; methylotrophic bacteria in water bodies, Legionella and intracellular Neisseria gonorrhoeae);

5) Facultative anaerobic Gram negative rods (e.g., enteric bacteria Escherichia

coli, Salmonella, Serratia marcescens, cholera causing Vibrio cholera,

luminescent Photobacterium);

6) Gram negative anaerobic, straight curved and helical bacteria (e.g., Haloanaerobium that prefers 13% NaCl, Halobacterioides that prefers 8.5 to 14% NaCl, Thermosipho that prefer 1-3% and Thermotoga with preference of 3-6% NaCl;

7) Dissimulatory sulphate reducing and sulphur reducing bacteria (e.g., Desulfotomaculum acetoxidans);

8) Anaerobic Gram negative cocci (e.g., Veillonella found in saliva, tongue, cheek, mucosa and gingival crevices of human oral cavity);

9) Rickettsia and Chlamydias;

10) Phototrophic bacteria that are of two types: Anoxygenic phototrophic bacteria (e.g., purple Chromatium, or green sulphur bacterium Chlorobium) and Oxygenic phototrophic bacteria (e.g., Cyanobacteria, Chroococcus,

Spirulina, Lyngbya, Nostoc and Anabaena);

11) Aerobic chemolithotrophic bacteria, which are:

Hydrogen oxidizing bacteria (e.g., Alcaligenes eutrophus), colorless sulphur oxidizing bacteria (e.g., Achromatium, Thiobacterium, Thiospira and Thiobacillus), iron oxidized and manganese oxidizing bacteria (e.g., Gallionella) magnetotactic bacteria and nitrifying bacteria (e.g., Nitrococcus and Nitrobacter);

12) Budding and appendaged bacteria (e.g., Caulobacter crescentus,

Hypomicrobium and Planctomyces maris);

13) Sheathed bacteria (e.g., Leptothrix discophora);

14) Bacteria with gliding mobility, which may be non-photosynthetic non-fruiting gliding bacteria (e.g., Cytophaga and Simonsiella) fruiting gliding bacteria (e.g., Myxobacterium, Stigmatella);

15) Gram Positive cocci (e.g., Deinococci, Staphylococcus aureus and Streptococcus pneumoniae) ;

16) Endospore forming Gram positive rods and cocci (e.g., Bacillus sphaericus,

Bacillus subtilis and Clostridium perfringens);

17) Asporogenous Gram positive rods, which are non-spore forming Gram positive rods (e.g., Lactobacillus, Listeria and Renibacterium) irregular non-spore forming Gram positive rods (e.g., Arthrobacter globiformis);

18) Mycobacteria (e.g., Mycobacterium tuberculosis, M. bovis and Arthrobacter globiformis); and

19) Actinomycetes (e.g., Streptomyces and Planomonospora); mycoplasmas or the cell wall-less bacteria (e.g., Mycoplasma pneumonia, M. hominis and Spiroplasma).

5.3.1. Factors affecting microbial diversity in soils

The microbial diversity has impact on the functional stability of an ecosystem and especially on its resilience towards any disturbance (natural or man-made). The diversity of microbial community has impact on stability of ecosystem functions and resilience to disturbance in soil ecosystem. Microbial community also affects the soil and plant quality and ecosystem sustainability. There are different factors which affect microbial diversity in soil.

• Soil management practices

• Application of pesticides

• Introduction of genetically modified microorganisms

• Physicochemical properties of soil (i.e. pH, cation exchange capacity, aeration, porosity, soil structure, organic matter, soil water and soil temperature).

USDA, Natural Resources Conservation Service (1998) has recommended following management practices to maintain soil biodiversity.

5.3.1.1. Cultivation

Deep and frequent tillage has deleterious effects on microbial diversity while minimum tillage or no till maintains soil physical properties, hence improves biological habitat and diversity of microbes in soil.

5.3.1.2. Compaction

Compaction suppresses pore spaces in soil and reduces the soil aeration for suitable habitat of soil microorganisms. Compaction can create anaerobic conditions in soil and ultimately can affect type and distribution of soil organisms.

5.3.1.3. Pest control

Pesticides, herbicides, fungicides and insecticides can destroy soil microbial diversity. Recommended dose of herbicides and insecticides has comparatively a minor impact on soil organisms, while fungicides and fumigants have more deleterious effect (USDA Natural Resources Conservation Service 1998).

5.3.1.4. Fertility

Availability of limiting factors like balanced nutrients enhances biological diversity. Plenty of carbon (green manures, compost, organic residues) improves biological activity in soil.

5.4. Soil Microbes and Soil Fertility

Microorganisms play a major role in the biogeochemical cycling of different nutrients required for plant growth. Most important nutrients affected by soil microorganism are C, N and P. Some free living or symbiotic microorganisms have the mechanisms to fix the atmospheric N and make it available for plant use. Similarly certain groups of bacteria and fungi have the ability to solubilize fixed nutrients particularly P and refresh the soil fertility status. Heterotrophic microbes are the consumers and their activity plays a key role in cycling the fixed carbon and other nutrients through decomposition of the organic fraction of soil, i.e. plant and animal residues. Initially, easiest compounds are broken down and eventually resistant organic residues along with microbial waste product combine to form soil humus. Plant growth promoting rhizobacteria are the bacteria residing near the plant roots which help the plant in a number of ways, like alteration in soil pH, production of certain enzymes and plant hormones which stimulate growth, physiology, as well as mitigate certain stress factors or protect plant against diseases. Basic understanding about the role of microbes in soil fertility and crop production is discussed in the following section.

5.4.1. Biological nitrogen fixation

Nitrogen is the foremost nutrient required by the plants in large quantity, and most often its deficiency can limit plant growth. Although elemental N constitute nearly

79% of the atmosphere as molecular N gas (N2), but it is completely un-available to majority of green plants in this form. The stability of the N≡N triple bond makes N2 extremely inert and demands high activation energy. There are two most common pathways that make the inert N available to the plants.

i) Through the formation of industrial fertilizers; and

ii) N fixation by microbes, i.e., the biological N fixation.

In biological N fixation, N is drawn from atmosphere through N fixation activity by some prokaryotes. Atmospheric N is reduced by prokaryotic bacteria to ammonia, thus, utilization of atmospheric di-N gas (N2) by certain microbes, through its reduction to ammonia is called biological N fixation.

The reduction of N2 to ammonia is catalyzed by nitrogenase, a key enzyme of biological N fixation system. The genes that code for the enzyme nitrogenase are collectively called nif gene.

5.4.1.1. Modes of nitrogen fixation

There are three principal modes adopted by microbes to fix atmospheric N to ammonia. These are: non-symbiotic, associative symbiotic, and symbiotic.

i) Non symbiotic nitrogen fixation

This type of biological N fixation is processed by the microorganisms which live freely and independently in the soil. A large number of bacteria and some cyanobacteria are capable to fix N2 non-symbiotically.

ii) Associative symbiotic nitrogen fixation

Associative symbiotic N fixation mostly takes place in association with the roots of grasses and cereal plants. In this type of association no nodules are formed like symbiotic bacteria. The population of bacteria near the roots increases and fixes the atmospheric N. Representative genera involved in associative symbiotic N fixation are Azospirillum, Enterobacter, Azotobacter, Pseudomonas, Klebsiella and Bacillus,

iii) Symbiotic nitrogen fixation

(a) Through nodule formation in legumes

Symbiotic association is the most interesting and important plant-bacteria interaction; the plants being the legumes and the bacteria being Rhizobium,

Bradyrhizobium, Sinrhizobium, Mesorhizobium and Azorhizobium. Infection of the roots of a leguminous plant (e.g. soybean, clover, alfalfa, beans, and peas) with respective rhizobium results in the formation of nodule where atmospheric N is converted to ammonia. Legume-rhizobium N fixation is of considerable agricultural significance as it leads to greater quantitative enhancement of combined N in the soil. Nitrogen fixation rates vary enormously in different legumes, up to 600 kg ha-1 yr-1

N2 + 6H + 6e- 2 NH3 (Nitrogenase)

(Fe, Mo)

have been reported in forage legumes (Coyne 1999), most rates are much less, in general grain legumes fix less N than do the forage legumes. Rhizobium strains are highly specific to legume species. A single rhizobium strain can generally infect only certain species of legumes and not others. A group of rhizobium strains capable of infecting a group of related legumes is referred to as cross-inoculation group (Table 5.1). Before these bacteria can fix N, they must establish themselves in the root cortical cell of the host plant, to form ‘root nodules’.

(b) Through nodule formation in non-legumes

Recently it has been observed that certain non-leguminous plants form nodules to fix N. The best known plant, in temperate regions, is alder (Alnus spp.); the microbes involved in nodule formation are actinomycete of genus Frankia (Wall 2000).

Table 5.1 Cross-inoculation groups of Rhizobia bacteria and associated legumes

Bacteria (Genus) Species/subgroups Host legume

Rhizobium R. leguminosarum bv. Viceae Vicia (vetch), Pisum (pea), Lens (lentils),

Lathyrus (sweet pea) bv. Trifolii Trifolium spp. (most clovers) bv. Phaseoli Phaseolus spp. (dry bean, runner bean,

etc.) R. Meliloti Melilotus (sweet clover, etc.), Medicago

(alfalfa), Trigonella, (fenugreek) R. loti Lotus (trefoils), Lupinus (lupins), Cicer

(chickpea), Anthyllis, Leucaena, and many other tropical trees

R. Fredii Glycine spp. (e.g., soybean) Bradyrhizobium B. japonicum Glycine spp. (e.g., soybean) B. sp. Vigna (cowpeas), Arachis (peanut),

Cajanus (pigeon pea), Pueraria (kudzu), crotolaria (crotolaria), and many other tropical legumes

Source: Brady and Weil (2007)

5.4.2. Decomposition of organic matter and nutrient release

Soil organic matter includes all materials of plant, animal or microbial origin regardless their degree of decomposition. As soon as plant and animal residues are added to the soil, soil microorganisms immediately act on them to gain food and energy. Organic matter does not decompose at once as a whole. Degree and rate of decomposition depends on many factors, i.e., environmental conditions, nature of the material, nutrient status of soil and micro fauna present in the soil. Simple sugars, amino acids, most proteins and certain polysaccharides decompose very quickly. Large macromolecules such as cellulose are cleaved into oligosaccharides first and then into simple sugars. Lignins decompose very slowly, a less lignified plant residue such as green manure of leguminous plants will decompose faster than more lignified material like wheat straw.

Besides microbial synthesis, during decomposition a heterogenetic mixture of complex nature; ‘humus’ is also formed and the process is called humification. Humus is a brown to dark brown amorphous and colloidal substance of complex nature rather resistant to further break down. The process of decomposition is carried out through microbial enzymes and greatly influenced by factors like temperature, moisture, pH and C: N, C: P and C: S ratios of the decomposable organic residues. Thus, nutrients released by organic matter decomposition help in maintaining soil productivity and increasing its fertility. During decomposition, plant and animal residues release N, P, S and other essential nutrients for plant growth. Soil microbes play a significant role in converting these elements into ionic species which can be utilized by plants. This conversion is referred as mineralization. The biotransformation of mineral forms of nutrients into organic forms is called immobilization. This inter-conversion of nutrients and humus formation has a great role in slow releasing of plant nutrients which ultimately maintains soil fertility and productivity.

5.4.3. Biotransformation of nutrients in soil

5.4.3.1. Carbon cycle

Carbon dioxide is fixed in different organic forms by photosynthetic organisms in the presence of sunlight. Fixed carbon dioxide is consumed by animals and heterotrophic microorganisms. These organisms release carbon dioxide into the atmosphere during respiration. Respiration processes can be aerobic or anaerobic releasing CO2 or reduced products such as CH4.

To optimize soil fertility by way of desirable soil physical properties, like structure, and moisture retention to gain slow release of nutrient for plants, it is necessary to optimize carbon pool in the soil. Crop residues, farm yard manure, composts and other carbon sources can be incorporated into the soil to keep the soil micro biota alive and active, improve soil physical properties and attain release of plant nutrients.

5.4.3.2. Nitrogen cycle

The transformations of Nitrogenous substances in soil are largely function of microorganisms. Three fourth of the soil N contents come from biological N fixation. The release of N as ammonium during the process of decomposition is called ammonification.

R-NH2 + H2O → NH4 + Energy + CO2 + other products

Ammonium in the presence of oxygen is oxidized to nitrate by certain bacteria. The conversion of ammonium to nitrate is called nitrification

2NH4 + 3O2 Nitrosomonas 2 NO2- + 4H+ + 2H2O

(Ammonium) Bacteria (Nitrite)

2NO2 + O2

Nitrobacter 2NO3-

Bacteria (Nitrate)

The bacteria contributing in the process are called nitrifiers. In first step ammonium is oxidized to nitrite by Nitrosomonas and oxygen is used as electron acceptor. Nitrite is toxic substance and quickly oxidized to nitrate by another bacterium called Nitrobacter.

Under anaerobic conditions microbes use nitrate as terminal electron acceptor and N is lost into the atmosphere as molecular N, this process is called as denitrification.

2NO3 → 2NO2 → 2NO → N2 → N2O -O2 -O2 -O2 -O2

The nitrification process is very important as all aerobic plants take up N predominantly in the form of nitrate. Beside microbial immobilization small quantities of N is also lost by leaching to lower layers of the soil, beyond the root zone. So proper soil management practices to maintain healthy microbial community in soil could be a good strategy to maintain soil fertility.

5.4.3.3. Phosphorus cycle

In soil P exists in various forms like phosphates of iron, aluminum, calcium, etc. Due to their low solubility, only a small fraction of total P present in soil is available for plant growth. Several microorganisms cause solubilization of P by producing organic acids that lower the soil pH. Phosphorus Solubilizing Bacteria (BSP), particularly Pseudomonas and Bacillus, possess the ability to transform insoluble forms of P to plant available forms by secretion of organic acids and phosphatase enzyme (Hussain et al. 2013)

5.4.3.4. Sulphur cycle

Sulphur generally is not a limiting nutrient for plant growth; however, it has a role in producing high crop yield under intensive agricultural system. In soils, it mainly occurs in organic form but small quantity of inorganic sulphur also exists as sulphate. Plants and microorganism consumes sulphur as sulphate and it makes approximately 1% of the dry weight of soil bacteria. Soil bacteria are involved both in oxidation and reduction of S.

5.4.4. Bio-fertilizers

Biofertilizers comprise of living cells of certain types of microorganisms and/or their products which have the ability to transform/mobilize nutritionally important plant elements from non-available forms to available forms through biological processes. Biofertilizers do not directly increase soil fertility but they initiate, stimulate or accelerate the processes which could contribute to improve soil fertility. Microbes in biofertilizers can fix N symbiotically in the roots of leguminous crops or non-symbiotically by free living microorganisms or transform fixed soil nutrients such as P, Fe , S , Cu and Zn, to usable forms or may carry those microbes which are able to produce growth regulators and enzymes, which can help plants to cope with water and salinity stress.

Biofertilizers are cost effective and environment friendly agricultural input which play a significant role in sustainable agriculture. Effective quality control measures

are very important in production of good quality biofertilizer. Efficacy of microbial strains present in commercial biofertilizer must be ensured. Viability of inoculants with desired population of approximately above one hundred million per gram can indicate a good quality biofertilizer. Packing and selection of good carrier material can make biofertilizer really a good supplementary option to fulfill the crop nutritional requirements to improve crop production and food safety.

5.5. Plant-microbe Interactions

It mainly constitutes the association of microorganism with plants either in a positive or negative way. The positive approach is mainly the symbiotic relationships and the negative approach constituents mainly pathogen plant interactions.

5.5.1. The rhizosphere

The rhizosphere is the portion of soil around the vicinity of plant roots that is directly influenced biologically, physically and chemically by plant roots, leading to a favorable conditions/habitat for microorganisms. Typically, rhizosphere contains almost 109 microbes per g of soil. Rhizosphere is rich with root exudates and root deposits, which are important sources of substrates for microbial communities present in the rhizosphere. It has been investigated that composition of root exudates and root deposits may vary from species to species, cultivars to cultivars and also depend on plant developmental stage. In response to the impact of root exudates in rhizosphere, the microorganisms in turn influence the plant growth. Environmental and soil conditions have known effects on both plant and microbe. However, it is difficult to tear apart complex interactions between these organisms (Elsas et al. 2007).

5.5.2. Plant’s influence on microbes

Plants influence the microbes through secretion of fixed carbon, organic acids, sugar and amino acids in the rhizosphere. These root exudates provide nutrients for proliferation of the microbes in vicinity of roots and indirectly influence on microbial community composition and control over selection or rejection of species that are the most suitable to utilize those nutrients. Plants sense and react to physical stimuli from harmful and beneficial microorganism (Jayaraman et al. 2014). Some exudates can specifically recruit beneficial microbes. Release of malic acid from plant attract favorable microbes while release of proteins and defense chemicals suppress the growth of plant pathogens like Rhizoctonia solani, Pythium aphanidermatum, P.

ultimum and Pseudomonas solanacearum (Flores et al. 1999: Lugtenberg et al. 1999: Simons et al. 1997).

5.5.3. Microbial influence on plants

In rhizosphere the microbes have both, beneficial and deleterious effects on plants. Some microbes are known plant pathogens, such as Phytophthora cinnamomi and some can secret phytotoxic metabolites, such as Fusarium moniliforme suppresses

the seed germination. Some toxic metals become more available to the plants, additionally microbes compete with plants for essential nutrient. Compare to deleterious effects of soil microbes, beneficial effects are more prominent. Many microbes in the vicinity of plant roots are considered as plant growth-promoting rhizobacteria; these bacteria live in rhizosphere and regulate plant growth and development through N fixation, production of growth regulators, enzymes, siderophores and formation of biofilms to exclude toxic metals (Elsas et al. 2007; Glick et al. 1998).

5.6. Environmental Implications of Soil

Microbiology

In modern society, daily life starts with the use of different organic chemicals and use of such chemicals is increasing day by day. Among these chemicals some are xenobiotic compounds which are “a stranger to life” because these are not produced naturally in biosphere. The biodegradation of such compounds is very difficult. Chemical approaches to degrade/transform these chemicals to environment safer form are usually costly, demand chemical expertise and sometimes byproducts are more toxic than the parents.

Biological processes may lead towards complete transformation of such organic molecules to inorganic products or comparatively less toxic molecules. The major agents affecting the fate of chemical are naturally occurring microorganisms (e.g., fungi and bacteria). The application of microbes to handle toxic materials is generally termed as bioremediation. Along with microorganisms, certain plants also play important role to remove, contain or transform contaminants; this is called phytoremediation. This approach of remediation is effective for organic as well as inorganic pollutants. Through thermophilic decomposition waste materials can be converted into useful products, called compost. This section explains bioremediation, microbial-assisted phytoremediation and composting.

5.6.1. Bioremediation

Bioremediation is the use of living organisms to clean the environment from hazardous compounds. It involves microorganisms, plants or their enzymes to remove the contaminants for the purpose of cleaning the environment (Hillel 2008). Increase in industrialization, modernization and urbanization caused the production and release of toxic compounds into the soil and environment. These contaminants are mainly divided into two types, organic and inorganic.

Organic contaminants can stay for longer period of time in the environment and bring great threat to soil fauna, flora and to human health. Organic contaminant includes polycyclic aromatic hydrocarbons (PAHs), total petroleum hydrocarbons (TPHs) and polychlorinated biphenyls (PCBs), other chlorinated aromatics such as halogenated compounds, polychlorinated terphenyls (PCTs), and pesticides like bentazon and atrazine (Saleh et al. 2004).

Inorganic contaminants are heavy metals which include chromium, lead, cadmium, copper, nickel and mercury etc. Many of the recent studies have reported the success of bioremediation technique to clean polluted soil (Khan et al. 2013).

In order to control or eliminate these contaminants from soils, chemical, physical and biological techniques can be used. Biological methods have advantages over physical and chemical techniques, but comparatively they are slow. Different bacteria/rhizobacteria have been reported to have their important role to combat organic and inorganic pollutants. Among these bacteria some colonize the plant root and have ability to improve plant growth as well.

There are different strategies for bioremediation, like passive bioremediation (by indigenous microorganism), biostimulation (addition of nutrients), bioventing

(addition of gaseous stimulants, such as oxygen and methane) and bioaugmentation (inoculation of a contaminated site with microorganisms).

5.6.1.1. Remediation of inorganic pollutants

Rhizosphere bacterial communities withstand high concentration of heavy metals and can remove the heavy metals from contaminated sites through bioreduction, biotransformation, biosorption and bioaccumulation. Microbes improve plant growth in metal contaminated soil thus increase plant biomass, which is basic requirement of plant to act as hyper accumulator. Bacteria can stimulate growth of plant through production of ACC deaminase, indole-3-acetic acid (IAA), siderophores, and antibiotics or though stimulation of several metabolic pathway and enhance metal availability and uptake to the plant through redox changes, acidification or by producing iron chelates.

5.6.1.2. Remediation of organic contaminants

In contrast to the inorganic contaminants, microorganisms play significant role in biodegradation of organic contaminants. Bacteria having capacity to degrade the organic compounds like polychlorinated biphenyls have been obtained from the contaminated sites. Several techniques have been developed to promote the degradation and tolerance of bacteria to soils contaminants. Due to variety of contaminants, microbes and plants separately become less efficient to remediate contaminants. Alternatively, the combined use of plants and degrading microbes was found to be more efficient. Both plants and microbes assist each other and synergistic use of both multiplies the efficiency of bioremediation process.

5.6.2. Phytoremediation

Phytoremediation is a combination of the Greek word phyto which means plant and the Latin word remedium which means to correct or remove an evil. Plants are typically used when environment is contaminated with heavy metals like lead, chromium, mercury and selenium. This technique encompasses use of plants to remove, detoxify, transform or contain the contaminants. It is a technology that involves the application or use of higher plants to remove or degrade contaminants directly or indirectly. Directly, by releasing the enzymes that degrade the contaminants or indirectly by secreting organic acids that promote the efficiency of

microorganisms to biodegrade the contaminants. It involves the accumulation of contaminants by plants or degradation of contaminants through root colonizing microorganisms (Fig 5.1). This technique is less expensive, natural process and has high public acceptance.

5.6.2.1. Techniques of phytoremediation

Phytoremediation includes phytoextraction, phytodegradation, phytovolatilization, phytostabilization and phytodetoxification (Fig. 5.1) (Sylvia et al. 2005).

Fig. 5.1 Different approaches of phytoremediation/bioremediation

i) Phytoextraction

Uptake and accumulation of contaminants by plants is known as phytoextraction. In this case hyper accumulator plants are used. Hyper accumulator plants uptake significant amount of contaminants in their bodies. Plants contained heavy metals are harvested or disposed off.

ii) Phytodegradation

It is also called as phytotransformation, it is use of plant secreted organic acid or enzymes to degrade the contaminants around the plants or breakdown of the contaminants which are taken up by the plants and then in the metabolic process plant transform these contaminants.

iii) Phytovolatilization

Volatilization of accumulated contaminants in plants from plant tissues is known as phytovolatilization. Water soluble contaminants taken up by the plants, may become modified and then released in to the environment

iv) Phytostabilization

In this technique of phytoremediation, organic acids secreted by plants make complex with contaminants and stabilize them in soil. Phytostabilization is an in situ approach where interactions of metal tolerant plants with different soil amendments reduce the mobility of contaminants to plant, soil, air, water and ground water.

5.6.3. Composting

Composting is a process of converting raw organic waste to a beneficial end product called compost which is a humus-rich soil amendment; this process is carried out by the sequential action of microbes. Composting is different from natural decomposition as it can be accelerated by monitoring, modifying and controlling various factors involved in the process. This process leads to decomposition and stabilization of raw organic waste into a product that can be used as soil conditioner and/or organic fertilizer which is stable having dark-brown or black color with earthy smell.

5.6.3.1. Composting process

A variety of organic materials, like leaves, grass clipping, kitchen waste, animal or poultry waste and municipal waste, can be composted. Composting is a dynamic process in which three groups of microorganisms namely bacteria, actinomycetes and fungi are involved. This mixed microbial population acts in a rapid succession. Most organisms required for composting are aerobic, as they are needed for rapid and complete composting. As the process continues bulky volumes of decomposable organic substances reduces to small volumes (Cambardella et al. 2003).

At the initial stage, mesophilic microorganisms take soluble sugars and amino acids from the plant materials followed by starch. As temperature increases due to oxidation of carbon compounds thermophilic take over. Temperature rises from 45-70 °C that is enough to kill pathogens, weed seeds and phytotoxins. Oxidation of organic compounds releases essential plant nutrients like N, P, S and micronutrients in small amounts. Depletion of readily available food resources leads toward decreased microbial activities. Temperature falls gradually and second mesophilic stage prevails. Finally compost enters the maturation or curing stage, time required may be different in different cases, depending upon the nature of the material and microbes involved. If temperature, moisture, nutrients, aeration and microbial activities are managed properly, the time span for maturation of compost may be reduced. Final product is nutrient rich, free of viable weed seeds, pathogens and easy to apply for potted plants or field crops. Along with providing different nutrients to the plants, compost also acts as good soil conditioner and improves soil physical, chemical and biological properties.

i) Physical properties

Compost is more stable form of organic matter than raw waste and its application in soil improves soil structure, aeration, porosity, soil aggregation, water holding and filtration. Compost stop rapid changes in soil temperature, hence, provide better conditions for growth of root. Compost decreases soil crusting, bulk density, run off, erosion and maintain the soil quality and productivity.

ii) Chemical properties

Application of compost influences chemical properties of soil. Cation exchange capacity (CEC) is increased while pH is slightly decreased with application of compost. Availability of nutrients is increased in composted soil. Compost buffers the soil against rapid changes due to salinity, alkalinity, acidity, pesticides and heavy metals.

iii) Biological properties

Addition of organic matter through compost enhances microbial activity and diversity in soil. Overall improvement in microbial activities in soil enhances decomposition of organic matter, improves nutrient availability and strengthen the symbiotic relationship with plants for N fixation and P accusation, thus improvers the soil quality.

5.7. Conclusion

Microbes are the most diverse group of soil organisms, yet very little is known about them. Until recently the research is only focused on those microbes which are cultureable, identification and exploration of many viable but un-cultureable microbes is yet a challenge. New research methods involving molecular techniques are required to understand taxonomic and functional diversity in soil system. Various kinds of organic wastes are being converted into useful soil amendments through recycling by microorganisms, however, in future, various kinds of organic wastes may accumulate in huge quantities due to population outburst, urbanization, economic growth and industrialization. So to keep our environment healthy on sustainable basis, there is dire need to use the microorganisms which are efficient decomposer but can also add beneficial metabolites to final soil amendment to reduce waste burden on the environment and for sustainable agriculture. Use of new organic compounds and heavy metals in industrial and commercial processes is posing serious threat to soil, water and environment. A better understanding of the interactions between bacteria and host plants can play a key role to enhance remediation efficiency; and genetically modified microbes may help in this regard. To address the threat of environmental degradation, the challenge is to have better understating of the carbon sequestration process and different geochemical cycles to control atmospheric gasses and climate change. Biofertilizers are very good supplement of chemical fertilizers but to maintain their quality up to the farmer’s field is a challenge to the soil microbiologists. The life span of biofertilizer is short, as they are living products and can be used with full benefits provided the material is not subjected to high temperature or other un-favorable conditions. The

maintenance of the same during transportation and storage is real task. Moreover, the availability of cheap, easily accessible and efficient carrier material is a major hurdle for production at large scale. However, on the part of soil microbiologists the real future challenge is to explore the strains which can tolerate the diversified agro-climatic conditions and selection of the suitable carrier which store and sustain the living strains of biofertilizers for longer time under variable environmental conditions.

No doubt soil fertility is a complex property of soil ecosystem but it could be more understood and managed by studying soil microbial ecology and managing beneficial microbial population in soil. Management of microorganisms in soil helps to ensure soil and environmental health and ultimately more crop growth.

References

Anderson, D. (2014). Still going strong: Leeuwenhoek at eighty. A. Van. Leeuw. J. Microbiol. 106: 3–26.

Atlas, R.M. and R. Bartha (1993). Microbial Ecology: Fundamentals and Applications, 4th edition, Benjamin Cummings, Menlo Park, CA, USA.

Brady, N.C. and R.R. Weil (2007). The Nature and Properties of Soils, 14th edition, Pearson Education, NJ, USA.

Cambardella, C.A., T.L. Richard and A. Russell (2003). Compost mineralization in soil as a function of composting process conditions. Eur. J. Soil Biol. 39: 117–127.

Coleman, D.C., D.A. Crossley, Jr. and P.F. Hendrix (2004). Fundamentals of Soil Ecology, 2nd edition, Elsevier Academic Press, San Diego, CA, USA.

Coyne, M. (1999). Soil Microbiology: An Explanatory Approach. Delmar Publishers, NY, USA.

Elsas, J.D.V., J.K. Jansson and J.T. Trevors (2007). Modern Soil Microbiology, 2nd edition, CRC Press, Taylor and Francis, NY, USA.

Flores, H.E., J.M. Vivanco and V.M. Loyola-Vargas (1999). Radicle biochemistry: the biology of root-specific metabolism. Trends Plant Sci. 4: 220–226.

Glick, B.R., D.M. Penrose and J. Li (1998). A model for lowering of plant of plant ethylene concentrations by plant growth promoting bacteria. J. Theor. Biol. 190: 63–68.

Hillel, D. (2008) Soil in the Environment. Academic Press, London, UK. Hussain M.I., H.N. Asghar, M. Arshad and M. Shahbaz (2013). Screening of

multitrait rhizobacteria to improve maize growth under axenic conditions. J. Anim. Plant Sci. 23: 514–520.

Jayaraman, D., S. Gilroy and J.M. Ane (2014). Staying in touch: mechanical signals in plant-microbe interactions. Curr. Opin. Plant Biol. 20: 104–109.

Khan M.Y., H.N. Asghar, M.U. Jamshaid, M.J. Akhtar and Z.A. Zahir (2013). Effect of microbial inoculation on wheat growth and phyto-stabilization of chromium contaminated soil. Pak. J. Bot. 45: 27–34.

Lugtenberg, B.J.J., L.V. Kravchenko and M. Simons (1999). Tomato seed and root exudate sugars: composition, utilization by Pseudomonas biocontrol strains and role in rhizosphere colonization. Environ. Microbiol. 1: 439–446.

Madigan, M.T. and J.M. Martinko (2006). Brock Biology of Microorganisms, 11th edition, Prentice Hall, Englewood Cliffs, NJ, USA.

Purser, J.E. (1977). The winemakers of the Pacific Northwest. Harbor House, Vashon Island, WA, USA.

Ruiz, N., P. Lavelle and J. Jimenez (2008). Soil macrofauna field manual. Food and Agriculture Organization of the United Nations, Rome.

Saleh, S., X.D. Huang, B.M. Greenberg and B.R. Glick (2004). Phytoremediation of persistent organic contaminants in the environment. In: Singh, A. and O. Ward (Eds.). Applied Bioremediation and Phytoremediation. Springer, Berlin Heidelberg. pp. 115–134.

Simons, M, H.P. Permentier., L.A. de Weger, C.A. Wijffelman and B.J.J. Lugtenberg (1997). Amino acid synthesis is necessary for tomato root colonization by Pseudomonas fluorescents strain WCS365. Mol. Plant-Microbe Int. 10: 102–106.

Singh, T., S.S. Purohit and P. Parihar (2012). Soil Microbiology. Agrobios, Jodhpur, India.

Smith, H.J. (1995) Cheese making in Scotland-a History. Scottish Dairy Association. Glasgow.

Soil Science Society of America (1998). Glossary of soil science terms. Soil Science Society of America, Madison, WI, USA.

Sylvia, D. M., J.F. Fuhrmann, P.G. Hartel and D. A. Zuberer (2005). Principles and Applications of Soil Microbiology. Pearson Education, NJ, USA.

Torsvik V. and L. Øvreås (2002) Microbial diversity and function in soil: from genes to ecosystems. Curr. Opin. Microbiol. 5: 240–245

USDA Natural Resources Conservation Service (1998). Soil Quality Information Sheet, Soil Quality Resource Concerns.www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_050947.pdf

Wall, L.G. (2000). The actinorhizal symbiosis. J. Plant Growth Regul.19:167–182. Whitman, W. B., D.C. Coleman, and W.J. Wiebe (1998). Prokaryotes: the unseen

majority. Proc. Natl. Acad. Sci. USA. 95: 6578–6583. Wilson, D. (1976). In search of Penicillin. Random House, New York, USA.

Chapter 6

Soil Physical Quality Indicators

and Plant Growth

Anwar-ul-Hassan and Muhammad Iqbal†

Abstract

Soil physical quality parameters are playing key role in understanding the soil performance regarding plant growth and development. Some of these health indicators directly carry their impact on crops and plants, while others effect indirectly by affecting other chemical and biological soil quality parameters. Now a day’s soil health and quality are gaining more impotence worldwide. Actually these two words are considered differently but are synonymous. The inherent soil quality addresses soil aspects which are related to the soils natural composition and properties affected by soil formation factors and processes. In this chapter we have covered the soil quality parameters which will furnish the reader about the soil texture, structure and many others properties which in turn will determine soil water holding capacity, nutrients holding capacity and its cycling, soil infiltration and ultimately selection of crops for a particular area in a region, soil carbon sequestration and NO3 leaching. Soil aeration and temperature are key factors deciding the functioning of microbial community which will be determined according to soil redox potential and biochemical oxygen demand. We have emphasized not only on soil physical quality parameters but also on chemical and biological soil quality indicators. The soil health is the integrated approach to integrate all the parameters to sustain our crop productivity which has become stagnant at one point for many years.

†Anwar-ul-Hassan* and Muhammad Iqbal Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: iqbal@uaf.edu.pk Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

124 Anwar-ul-Hassan and M. Iqbal

Keywords: Biochemical Oxygen Demand, Soil genesis, Soil quality, Soil structure, Soil texture, Soil Porosity

6.1. Introduction

Soils are complex medium, made up of a heterogeneous mixture of solid, liquid, and gaseous phases, as well as a diverse community of living organisms. Soil quality is a measure of the ability of a specific kind of soil to function, within an ecosystem and land use boundaries, to sustain animal and plant productivity, maintain environmental quality and promote human health and habitation. The concept of soil quality deals with integrating and optimizing of physical, chemical and biological components of soil for improved productivity and environmental quality. The potential soil physical indicators used in soil quality assessment comprised of bulk density, pore-size distribution, available water capacity, aggregate stability, surface and sub-surface penetration resistance, saturated hydraulic conductivity and water infiltration. In addition to soil physical quality indicators, other physical properties of soil are also discussed as these are not included in other chapters of this book. The soil properties that we can see or feel are physical and include texture, structure, particle and bulk density, pore space, aeration, water, consistence, color and temperature.

6.2. Soil Texture

Soil texture refers to the relative proportion or percentage distribution of sand, silt and clay in a less than or equal to 2 mm soil called fine-earth fraction. It is relatively a static property and is difficult to change through soil use and management. Soil inorganic fractions that are greater than 2 mm in diameter are referred as coarse fragments. Soil particles larger than 2 mm but less than 250 mm, i.e. between gravels and cobbles, are considered as part of soil as they greatly influence its behavior or use. If they are more than 15 % by volume in whole of the soil, they are used as modifier of textural class names, e.g., gravelly loam and cobbly sandy loam.

6.2.1. Classification of soil separates

The mineral soil particle-size groups are referred as soil separates. Several systems of soil separate classification have evolved over a period of time. The two classification systems, the International Society of Soil Science (ISSS) system and the U.S. Department of Agriculture (USDA) system are used by soil scientists. Coarse sand (2000 – 200 µm), fine sand (200 – 20 µm), silt (20 – 2 µm) and clay (< 2 µm diameter) are the four particle-size fractions that define the soil separate classes according to ISSS system. The size ranges of soil separates followed by USDA system are: Very coarse sand (2000 – 1000 µm), coarse sand (1000 – 500 µm), medium sand (500 – 250 µm), fine sand (250 – 100 µm), very fine sand (100 – 50 µm), silt (50 – 2 µm) and clay (< 2 µm). The classes of coarse fragments are gravels

Soil Physical Quality Indicators and Plant Growth 125

(2 – 75 mm), cobbles (75 – 250 mm), stones (250 – 600 mm) and boulders (> 600 mm diameter).

6.2.2. Nature of soil separates

The fine-earth fraction is the most active fraction of the soil as it has the greatest specific surface area. It controls important properties of most soils. The clay fraction has a significant effect on many soil physical and chemical properties, largely since the small-sized particles have large and reactive surface area. On the contrary, the sand and silt fractions usually do not have much influence on chemical processes, and their relatively small surface areas do not adsorb or hold water as much as clay fraction does. As a result, the sand and silt fraction of the soil matrix may be regarded as an inert entity whose impact on soil water is expressed primarily by the geometric arrangement of the soil particles. The coarse fragments can cause hindrance in cultivation, may damage tillage implements and influence land use greatly.

As clay fraction has a definitely different effect on soil water and solutes than sand or silt, one could make some generalizations about various characteristics of field soils that contain dominant quantities of one or the other of the three fractions. For instance, sandy soils do not hold considerable quantity of water, thus need more frequent irrigations to avoid plant water stress than do soils in the same type of weather that contain large quantity of clay.

Soils are given different textural class names based on the quantities of sand, silt, and clay particles present in them. The USDA textural triangle is an appropriate method of presenting the relationship between the soil textural class name and soil separates (Fig. 6.1; Gee and Or 2002).

Two procedures, mechanical sieving (if particle size > 50 µm) and sedimentation (if particle size < 50 µm), are used to measure the size of individual soil particles. A soil sample whose mineral phase is to be characterized is first pretreated to remove organic material and other cementing agents to disperse soil aggregates. Then the completely dispersed soil particles are passed through a series of coarse screens of specified opening sizes with the smallest screen having an opening of about 50 µm. The sizes of the remaining dispersed particles (silt and clay) are determined by sedimentation procedures using either the hydrometer method or the pipette method. Both methods are based on Stokes’ law, which is used to establish a relationship between the settling velocity (V) of particles and the particle sizes.

In the hydrometer method, density of soil suspension is measured after various time intervals because density decreases with time due to settling of particles. The hydrometer method is simple, rapid and more widely used but is less accurate than pipette method (Gee and Or 2002). The pipette method consists of direct sampling of the soil suspension with a pipette at a certain depth (usually 0.1 m) after various time intervals depending on temperature of the suspension. The pipette method is accurate but is laborious and time consuming.

Fig. 6.1 USDA textural triangle for soil textural class determination

6.3. Soil Structure

Soil structure refers to the grouping or combination of primary soil particles into secondary particles called aggregates or peds, which are separated from each other by natural lines of weakness. Soil structure is better studied in undisturbed soil under field conditions than in the laboratory. Soil structure may be classified into three main structural groups: single-grained, massive, and aggregated. The single-grained refers to the geometric arrangement of soil particles into a porous formation, with little or no cementation or flocculation among its constituents. Coarse-textured sands frequently orient in this manner and often are called structure-less. A massive structure represents the opposite extreme of a complete consolidation of soil with no natural lines of weakness and generally occurs in fine textured soils. In between these two extremes are aggregated soils, in which soil separates stick together to form larger units or aggregates. Natural soil aggregates are sub-classified into three categories on the basis of shape, size, and degree of distinctness and durability, referred as types, classes and grades of soil structure, respectively.

6.3.1. Types of soil structure

The four principal types of soil structure based on shape of the aggregates and their arrangement in the soil profile are platy, block-like, prism-like and spheroidal. Plate-like structure has relatively thin, flat horizontal aggregates or plates or leaflets. These plates are generally formed as a result of soil forming processes, may be inherited from parent material or are formed due to compaction of fine-textured soil by heavy

machinery. They may be found in both surface and subsurface horizons. Blocky aggregates are irregular in shape, six-faced with their three dimensions more or less equal in size. Blocky aggregates are of two sub-types, i.e., angular blocky (when blocks’ corners are not rounded but have sharp edges) and sub-angular blocky (if aggregates have rounded corners). These are usually found in B horizon.

Prism-like structures are characterized by vertically oriented aggregates or pillars which are longer vertically compared to their width. Two sub-types of prism-like structure are prismatic and columnar. When prisms have angular edges and flat clean cut tops, these are called as prismatic. When tops and sides of prisms are rounded like columns, these are designated as columnar. Both prism like aggregates are associated with swelling types of clay minerals and are most common in sub-surface horizons of arid and semi-arid region soils. The soils with spherical or rounded aggregates are placed in spheroidal category and are further sub-divided into granular (with relatively less porous peds) and crumb (when granules are porous) sub-types. These are generally common in surface soils or in A-horizons, mostly those high in organic matter.

6.3.2. Genesis of soil structure

The causes and mechanisms of formation of the aggregates are known as the genesis of soil structure. The development of stable aggregates or peds in soil takes place due to flocculation of fine soil particles by lowering of negative charge by polyvalent cations and stabilization of these by cementation with adhesive substances. Soil aggregates are formed during soil formation under the influence of alternate wetting and drying, freezing and thawing, vegetation, burrowing animals, earthworms, fungi, organic matter, nature of adsorbed cations or other disruptive forces. Aggregates are formed as a result of swelling and shrinkage due to wetting and drying of soil clod. For instance a soil mass swells due to wetting or freezing and then shrinks as a result of drying or thawing, cracks or lines of weakness are formed due to uneven expansion and contraction. The soil mass between cracks is stabilized by cementing agents.

Flocculation is a pre-requisite but not a sufficient condition for aggregation. Flocculation occurs by the adsorption of divalent (e.g., Ca++ and Mg++) and trivalent (e.g., Al+++) cations and / or by high solute concentrations. The polyvalent cations effectively neutralize negative charges on the soil particles and form bridges between clay particles that bring particles close together in small floccules. Double layer is compressed at high solute concentration and its repulsive effect is lessened and individual clay particles join each other by short-range attractive forces, called van der Waals forces, to form floccules. Then floccules are stabilized by cementing agents to form peds or aggregates.

Plants, animals, and the microbial community produce cementing agents that help bind the aggregates together. Bacteria decompose plant residues and produce polysaccharides and other organic glues. Plant roots, particularly root hairs, and fungal hyphae secrete sticky polysaccharides and other organic compounds that bind together soil particles and very small-sized aggregates. The mycorrhizae exude a sticky protein, called glomalin, which is very effective as a stabilizing agent. In many cases, raindrops are the major cause of the dispersion of soil aggregates. The

aggregates cemented with iron and aluminum hydroxides are very stable and their structural degradation is least common even after intense rains in these soils.

6.3.3. Soil structure and its relationship with soil tillage and

Good soil structure is an important feature of soil tilth. Tilth refers to the physical condition of the soil in relation to plant growth. Tillage has both beneficial and harmful effects on aggregation. If tillage is performed at optimum soil wetness level, its short-term effect is positive. Tillage implements break the clod, loosen the soil, increase water infiltration and reduce water run-off. When surface soil loses its structure and nutrients, inversion by tillage may bring aggregated and relatively more fertile subsoil to the surface which improves the soil tilth.

Over longer periods of time, tillage operations may have negative effects on soil structure. Plowing at very high or low soil water levels may be detrimental to soil tilth. Frequent use of heavy equipment may break aggregates and compact the soil surface. Continued cultivation and other tillage operations generally decrease stability of soil aggregates due to increase in decomposition of organic matter by exposing it to environmental factors. Tillage operations may also produce plow pan, if plowing is carried-out continuously at same depth with the same implement.

6.3.4. Importance of soil structure

Aggregation is a highly desirable state for soils because it greatly influences aeration, porosity and pore-size distribution, bulk density, heat transfer, transport and retention of water, root growth and penetration, microbial activity, and susceptibility of the soil to wind and water erosion. The formation and maintenance of stable aggregates are the most desirable characteristics for good soil tilth. Soil structure affects all plant growth factors, thus has a great agricultural significance, particularly at critical stages of seed germination and seedling establishment.

Soil structure is an important criterion used in studying soil profiles and classifying soil. It also influences infiltration of water in soil. For example, granular and single-grain soils have rapid infiltration, blocky and prismatic type soils have moderate, and platy and massive soils have slow infiltration. Erodibility of soil decreases with the increase in stability of the soil structure. Assessment of size distribution of peds, proportion aggregated and strength of soil aggregates is important for determining both the porosity and pore-size distribution of a soil. The extent and nature of aggregation modifies the influence of soil texture on crop growth due to change in pore-size distribution.

6.4. Soil Density

6.4.1. Particle density

Particle density is defined as the mass of oven-dry soil per unit volume of solid fraction or total soil volume excluding pore-spaces. It is an intrinsic property of soil

and is same as the specific gravity of a solid substance. Particle density (ρp) is expressed as kilograms per cubic meter (kg m-3) or mega grams per cubic meter (Mg m-3) which is equal to grams per cubic centimeters (g cm-3). Particle density is determined by the proportion of organic and mineral fractions and specific gravity of these particles. Most mineral soils have a particle density usually equal to about 2.65 Mg m-3, which is an average density of both inorganic and organic particles. Factors affecting particle density are chemical composition and crystal structure of soil minerals and proportion of soil organic matter in the soil. It is a static property and is not affected by texture, structure, pore-space or any other property of soil. The ρp has no significance in plant growth. It is determined by pycnometer method.

6.4.2. Bulk density

Bulk density is defined as mass of oven-dry soil per unit bulk volume of soil including pore-spaces. It is expressed as kg m-3 (SI unit) or Mg m-3 (derived SI unit) or g cm-3 (old units). Bulk density (ρb) is generally determined by core-method. Core sampler is used to obtain an undisturbed soil sample of known volume and mass is determined after drying the sample in an oven at 105 °C and cooling in a desiccator. If soil is sandy or stony, then the simplest and best procedure is excavation method. A smoothed hole of 10 cm diameter and 10 cm depth is dug with a spatula through a 10 cm diameter hole of a metallic template placed on the soil surface. Total soil volume is determined by lining the hole with very thin plastic sheet and filling it with a measured volume of water up to base of the template. The ρb can also be determined by clod method. The volume of the wax coated clod is determined by weighing it first in air, and then while immersed it in water, making use of Archimedes’ principle (Blake and Hartge 1986).

6.4.3. Factors affecting bulk density

Soil bulk density decreases with an increase in pore-space due to increase in the value of denominator, i.e. volume of bulk soil. Thus, any factor that influences pore-space, will also affects bulk density of soil. Factors affecting bulk density are soil texture, structure and its type, organic matter content, soil compaction, depth in the soil profile, nature of crops and soil management practices. Bulk density generally decreases with fineness of texture, increase in aggregation of soil and spheroidal type of structure, increase in organic matter content, addition of crop residue and farm manures to the soil. It increases with increase in soil profile depth and compaction, intensive cultivation, long-term tillage, repeated trips by heavy machinery and uncontrolled traffic.

6.4.4. Relationship of soil bulk density and root growth

Bulk density is a dynamic soil property varying with space and time. It is used to determine volumetric water contents, porosity and total mass of a volume of soil too large to weigh. It indicates the looseness or compactness of a given soil. Roots of field crops are hindered in soils with very high bulk density due to increase in soil resistance to root penetration and proliferation, slow movement of water and air, poor aeration and accumulation of toxic gases. Root growth is inhibited by soil

compaction or higher soil strength and decrease in pore-size; all of these are affected by bulk density. Hence, these factors are taken into account to determine the influence of bulk density on root growth and it penetration in soil.

6.5. Pore-Space

Pore-space is the volume percentage of the soil volume not occupied by solid particles but is occupied by water and/or air. It is also called porosity which is defined as a ratio of volume of pores or voids per unit bulk volume of soil (volume of solid + volume of pores). Equation used to compute porosity or pore-space is based on the definitions for bulk density and particle density, is given below:

Total porosity = 1 - (bulk density / particle density)

Bulk density is the main soil property that influences pore-space because denominator (particle density) value commonly used is 2.65 Mg m-3. Nonetheless, it does not give any idea about pore-size distribution which is more important from plant growth point of view.

Factors influencing porosity are essentially the same as that affecting soil bulk density. Total pore-space decreases with increase in soil bulk density, compaction, soil profile depth and cultivation. It generally decreases in surface soil due to more aggregation as a result of relatively higher organic matter and lack of overburden than subsoil.

6.5.1. Pore-size distribution

The volume-based percentage of various sizes of pores of the bulk volume in a soil is called pore-size distribution. Classification of pores on the basis of size is somewhat arbitrary. Based on their effective diameter, soil pores are generally grouped by size into micropores (< 0.03 mm), mesopores (0.03 to 0.08 mm) and macropores (> 0.08 mm). Micropores are generally present as intra-aggregate spaces, in which water and air movement is slow and are dominant in fine-textured soils. Larger micropores (effective diameter 0.005 to 0.03 mm) can accommodate most bacteria and root hairs, and plants can use water held in these pores. Mesopores can accommodate root hairs and fungi, retain water after drainage and transmit water by capillary rise. Water and air movement, water and nutrient retention in these pores lie in between macropores and micropores.

Macropores are commonly present as inter-aggregate spaces and voids between sand grains in coarse-textured soils. These can accommodate plant roots and certain tiny animals, and have rapid air and water movements. Macropores formed by plant roots, earthworms and other organisms are called biopores. They are more important in clayey soils and may be continuous up to one meter length or more.

6.5.2. Relationship of pore-space and bulk density

An inverse relationship exists between porosity and soil bulk density. Dry mass and volume of soil solids do not vary in the method used to calculate bulk density. Any

factor that influences total porosity also effect bulk density of soil. The volume of pore-space is included in the denominator of the bulk density formula, thus as the value of total porosity decreases, soil bulk density increases.

Pore-size distribution is very important in determining water holding capacity, drainage, aeration and root penetration. Thus, knowledge about pore-size distribution has great agricultural significance than just knowing total porosity of the soil. The balance between various sizes of the pores is affected by soil texture, aggregation, organic matter contents, compaction and soil depth. Pore-size is enhanced from micropores to macropores by increase in sand, organic matter and aggregation in soil.

6.6. Soil Consistence

Soil consistence may be defined as the resistance of soil material to rupture and deformation or its degree of cohesion and adhesion at various levels of water contents. Soil consistence is the only soil physical property which has significance in agriculture and relevant to engineering uses. The term soil consistency is used by soil scientists for determining its suitability for tillage and traffic by estimating resistance to rupture. It is determined by crushing the soil clod between thumb and fore-finger or between hands or crushing under foot. The consistence is described at dry, moist and wet soil water content levels. The friable soils are easily tilled or excavated and term is more important from crop growth point of view. When moist soil clod breaks into aggregates under slight force between thumb and fore-finger without difficulty it is called friable. To puddle the soil for rice planting, plastic consistency is the optimum condition.

Factors affecting consistence are soil water content, soil texture, type of clay, specific surface area, soil structure and organic matter content. Soil consistency increases with decrease in water level, increase in clay content along with expanding type of clay and by puddling of soil.

Soil consistency is a term used by soil engineers to describe the interaction of forces of cohesion and adhesion within a soil material as expressed by the relative ease with which soil can be crushed or deformed at various soil water levels. It is determined by the soil’s resistance to penetration of blunt end of a lead pencil or a thumbnail. Consistence and consistency provide valuable facts to guide decisions regarding loading and manipulation of soils.

6.7. Soil Color

Soil color is most obvious feature of soil and is an important criterion in soil classification and interpretation. It gives valuable information about other properties and conditions of the soil. It is determined by comparing the soil color to Munsell color charts and soil color description or nomenclature is standardized. Soil color notation is divided into three parts, namely hue, value and chroma. Hue is the portion of visible spectrum or ‘rainbow’ color and is related to the dominant wave-length of the light. Hue changes in units of 2.5 from 2.5 to 10 with 5.0 used as midpoint of each hue. As numerical number increases, wavelength decreases, i.e., less red. Hues

commonly used for soils are: 5.0R, 7.5R, 10R, 2.5YR, 5.0YR, 7.5YR, 10.0YR, 2.5Y and 5.0Y.

Value is a measure of the relative darkness or whiteness of the color and is related to the amount of reflected light. It ranges between 0 for absolute black (all light absorbed) to 10 for absolute white (all light reflected).

Chroma is a measure of the relative purity or strength of spectral color and is related to the range of wavelengths reflected. It increases as greyness decreases and color is more brilliant as grayness decreases. Chroma subdivision ranges from zero (neutral gray) to 10 (brilliance) to 20 for pure colors which do not exist in soil.

The color designation for soil horizon is written in the order of hue, space, value, virgule and chroma, e.g., 10YR 5/4. In this example, color is dark red, 5 is value and 4 is chroma. Major factors that influence the soil color are its carbonates contents such as calcite, its water content, its organic matter content and the presence and oxidation states of iron and manganese oxides.

6.7.1. Effect of color on plant growth

Color has indirect influence on plant growth. Dark colored soils are generally warmer than light colored soils because they absorb more heat than do light colored ones. Due to greater amount of heat energy available, dark colored dry faster than light colored soils. Thus, colored surface affects soil temperature and water and indirectly soil structure, microbial activity and plant growth. However, dark colored soils with high humus contents may not be warmer because they also mostly hold more water and wetter soils require relatively larger quantities of heat than drier soils to raise their temperature. Productivity potential of these soils is higher due to more organic matter contents even though they are not warmer.

6.8. Soil Temperature

Soil temperature greatly influences the quality of soil as habitat for plants and microorganisms. Soil temperature influences soil aeration generally, through its stimulation effect on plant growth and on the rates of biochemical reactions. These interrelations are more important in water saturated or poorly aerated soils.

6.8.1. Soil temperature

Heat is a form of kinetic energy and level or index of heat is called temperature which is measured in degree Centigrade or degree Fahrenheit or Kelvin. Major factors affecting soil surface temperature are angle at which sun rays strike soil surface, latitude, altitude, season, time of the day, direction and aspect of slope of land, clouds, fog, soil color, its water content and soil surface covered by vegetation, snow or crop residues. As most of the solar radiations striking perpendicular or 900 to soil surface are absorbed by the soil, thus soil temperature is higher near equator, at sea level, during summer season and at noon. Soil temperature is also high when sky is clear; soil surface has no cover and dark colored, and has less water contents. Increase

in water contents of dry soil increases its specific heat capacity, thus a wet soil will warm up much more slowly than a dry soil.

Solar radiation, convection, conduction and latent heat transfer are four mechanisms responsible for heat transport in soil. Out of these four mechanisms, conduction and latent heat convection are most important processes of heat transport into soil. Solar radiation is the short-wave radiation emitted from the sun in the form of electromagnetic waves through empty space and is the main source of energy for heating at or near the soil surface. Convection or the mass flow of heat is the process in which heat carrying particles actually moves from one place to another. It is associated with liquid and gas flow, for example with ocean currents and sensible heat, respectively. The sensible heat or convective heat flux represents the vertical transport of warm air from the soil surface to the atmosphere during day time and towards soil surface at night time.

Heat transmitted generally through solids by internal molecular motion without transfer of material particles is called conduction and is the most important mechanism for subsurface heat transport in soils. Latent heat transfer is convective heat transport associated with vapor flow in soil through heat absorption during evaporation at one place and release of heat during condensation at another place. Soil water resists significant changes in soil temperature due to its high specific heat capacity and evaporation in which high heat energy is required. The amount of heat required to raise temperature of one kg of water by one Kelvin is called specific heat and is expressed in J kg-1.

6.8.2. Soil temperature management

Human ability to modify soil temperature is limited. Soil temperature must be between 4 – 10 °C for wheat, 10 – 29 °C for maize and 27 – 33 °C for cotton to get maximum seed germination. Even relatively small changes in soil temperature may have significant effects on seed germination and plant growth. Temperature of the soil surface can be modified by keeping some cover or mulch on the soil surface, controlling soil water contents through irrigation and drainage, sprinkling water on high-value plants to prevent frost damage and heat stress, growing vegetation, contoured and ridge planting, and by tillage practices.

The use of clear plastic mulch in cold season increases soil temperature by producing greenhouse effect. It permits production of high-value crops (like vegetables and strawberry) to take benefit of the high prices in early season markets. Vegetation lowers the soil temperature by means of intercepting considerable portion of incoming radiation and using the heat energy for evapo-transpiration and photosynthesis. Planting seeds on ridges increase temperature in cold season due to positioning them to receive more direct sunlight along with rapid drying of the soil. Organic and plant residues left on the soil surface decrease conduction into and out of soil, causing warmer temperature in winter season and cooler temperature during summer season. These plant residues tend to minimize day and night fluctuations by working as insulating agents and do not allow the soil to become either too hot during summer and too cold during winter season.

6.8.3. Significance of soil temperature in plant growth

Soil temperature is one of the most critical factors which affect important physical, chemical and biological processes in soils and plants. If soil temperature is too low or too high for optimal plant growth, productivity of crops and other vegetation is often decreased, resulting in temperature dependent growth and yield patterns.

Soil temperature greatly influences seed germination, seedling emergence and development, root growth, plant growth and maturity of crops. It also affects microbial activity, decomposition and mineralization of organic matter, and nutrient and water uptake by roots. Water and nutrient uptake is decreased below optimum rates at both very high and low temperatures. All these factors are very important in plant growth and yield of crops.

6.9. Soil Aeration

Soil aeration refers to the process by which soil air is exchanged by air from the atmosphere and to the transport of gases through the pore-spaces filled with air. Gases move both into and out of soil and the rate of such gas exchanges depends largely on the volume of macro-pores and their continuity, soil water contents and drainage within the soil.

The volume of a soil not occupied by soil solids or water is the gaseous phase of soil called soil air. In a well-aerated soil, the soil air is similar in composition to the atmospheric air above the soil. Atmospheric air contains 79 % nitrogen, 20.97 % oxygen and 0.03 % carbon dioxide and 20-90 % water vapors. Soil air contains 79 % nitrogen, less oxygen, more carbon dioxide and water vapors (95-99 %) than atmospheric air but the sum of oxygen and carbon dioxide gases remains the same in soil and atmospheric air. Factors affecting composition of soil air are soil texture, structure, temperature, compaction, water contents, organic matter, soil depth, microbial and plant root activity. Soil properties influence the air-filled porosity and permeability of soil. More the plant roots and microbial activity more shall be the production of CO2 in soil.

Diffusion and mass flow are the two mechanisms responsible for the exchange of gases between soil and atmosphere. However, diffusion is considered the primary mechanism of gaseous exchange. The partial pressure gradient of carbon dioxide or oxygen in soil or atmosphere above is important for diffusion and total pressure gradient is important in mass flow. The partial pressure of oxygen in mixture of gases is the pressure oxygen gas exert if it was present alone in the soil air. Net movement of oxygen is generally towards the soil air because the oxygen content is lesser in soil than in the atmospheric air. On the contrary, CO2 moves in the opposite direction as its partial pressure is generally higher in the soil air.

Rainfall and irrigation water are also help in the renewal of soil air. Irrigation accounts for 5% and rainfall constitute about 6 to 9 % of the normal aeration. Rainwater displaces soil air from the soil pores and these are refilled with atmospheric air during percolation of water. Rainwater is also enriched by dissolved oxygen which is exchanged with the soil phase. One cm of rain over an area of one

hectare adds 100,000 liters of rainwater that contains 4339 g of oxygen at 20Co (Saha, 2004).

6.9.1. Measurement of soil aeration

The aeration status of a soil can be characterized by measuring: a) aeration or air-filled porosity; b) composition of soil air; c) the oxygen diffusion rate (ODR); and d) the oxidation-reduction (redox) potential. An early method to measure soil aeration is determining air-filled porosity which is expressed as a fraction or percentage of the bulk volume of soil that is occupied by air at any given time or specified water content. Aeration porosity is either calculated by subtracting volumetric water content from total porosity or measured directly with an air pycnometer. A typical poorly aerated soil has aeration porosity value of less than 10 %.

Measurement of the composition of soil air is the traditional approach and is a static indicator of soil aeration because soil gases are measured from gas samples collected from the soil profile. It gives more information whenever O2 concentration of soil air decreases significantly lower than the atmospheric air. The ODR determines the rate at which oxygen of soil air can be replenished by diffusion when it is used by respiring micro-organisms or plant roots. The growth of most plant roots ceases when ODR value is less than 20x10-4 g m-2 minute-1 and values greater than 40 × 10-4 g m-

2 minute-1 are considered sufficient for optimum growth of most plants. This method is most useful in fine-textured soils and for comparing ODR levels at different soil depths.

Redox potential (Eh) is the electrical potential of a soil created due to tendency of the chemical elements in it to donate or acquire electrons. It is usually measured in millivolts (mV) or volts (V). Soil Eh depends both on its pH and the presence of electron acceptor. Oxygen gas (O2) rapidly accepts electrons from other elements. In poorly aerated soils, Eh value is less than 350 mV and oxygen disappears at Eh value of about 300 mV. Well-aerated soils have Eh values of more than 400 mV. Oxidized form of important chemical elements such as nitrate, sulfate, carbon dioxide, ferric iron and tetravalent manganese (Mn4+) are dominant in well-aerated soils.

6.9.2. Effect of aeration on plant growth

Two life sustaining gases in soil air are CO2 and O2 since these are essential for photosynthesis and root respiration, respectively. Carbon dioxide is a primary greenhouse gas because it traps reflecting back long-wave radiation and prevents it from escaping into the outer space. Human related CO2 emissions are responsible for its increase in atmosphere occurred since industrial revolution. This high concentration of CO2 in atmosphere may be partially responsible in global warming.

Plants roots and microbes need O2 for respiration which is needed for life and growth. When ODR is too low and the redox values drops, plant roots cannot respire and their growth slows down or stops. Except rice, cultivated crop plants grow best in well-aerated or oxidized soils. Rice plants can move O2 internally through large diameter pores from their tops in atmosphere to their roots. If free O2 concentration is too low, some elements or ions become the electron acceptor. In poorly aerated soils, many

organic acids are formed which may be toxic to higher plants and to decomposing organism. The nitrates are denitrified and N2 and N2O gases formed are escaped to atmosphere, thus expensive N fertilizer input is wasted. Sulfate is reduced to sulfide and CO2 is converted to methane. Under anaerobic (lack of free O2) condition, the rate of organic matter decomposition is much slower than under aerobic condition. In the presence of free O2 (aerobic condition), glycolysis plus respiration releases about 19 times more energy (stored in ATP bonds, than anaerobic breakdown.

6.10. Soil Tillage, Tilth and Plant Growth

Tillage may be referred as the mechanical manipulation of soil aimed at improving soil conditions necessary for crop production. Major purposes of tillage include preparation of seed bed, eradication of weeds, destroying of soil crust, conserving water as well as soil, incorporating plant residues and farm manure into the soil, and creating optimum compactness for good soil-seed contact plus root penetration. Tilth is also a result of soil tillage and is discussed in Section 1.3.3. The soil must be friable and well aggregated, permitting free movement of water as well as air and easy cultivation in addition to planting crops.

6.11. Soil Health

Soil health and soil quality considerations are becoming popular worldwide. Soil quality is the capacity of soil to function within ecosystems according to the land use for sustaining productivity, maintaining environmental quality and improving plant and animal health. The National Resource and Conservation Service of USA have made it more meaningful by adding inherent and dynamic soil quality to its basic definition. The inherent soil quality is defined as the aspects of soil quality related to a soils natural composition and properties influenced by the factors and processes of soil formation in the absence of human influence. However, the dynamic soil quality relates to the soil properties which are result of soil use and management over time.

6.11.1. Indicators of soil health

There is dire need of establishment of the protocol for soil health assessment that may be cost effective and affordable, easy to understand and executed by the laboratory staff. There are number of soil health indicators that can be used for assessing soil quality in comparison to their effectiveness for reflecting true soil health status and its correlation with crop production. The soil health concept is an integrated approach comprising soil physical, chemical and biological parameters to address the soil constraints effectively and to enhance farmers’ profitability (Gugino et al. 2009).

These can be divided in to three categories:

20) Physical: The most important physical parameters include bulk density, macro-and meso-porosity, available water capacity, penetration resistance,

saturated hydraulic conductivity, wet aggregate stability and water infiltration rate of soil.

21) Chemical parameters comprised of pH, CEC, nitrate nitrogen, potassium, phosphorus, calcium, magnesium, iron, zinc and copper contents.

22) Biological parameters include soil organic matter content, potential mineralizeable nitrogen, particulate organic matter, active carbon, root bioassay and nematode population.

Most important physical parameters will be discussed here. The water stability of soil aggregates measures the extent to which soil aggregates resist the separation from one another through subsequent rain (or in water) and mechanical manipulation. Water content is important in structural stability and is an important factor in determining the degree to which particular mechanical forces will cause structural breakdown.

Available water capacity or plant available water refers to the difference in the amount of water retained in the soil at the field capacity and permanent wilting percentage or at the soil water potential values between -10 to -30 kPa and -1500 kPa. Available water capacity of the soil generally increases as the fineness of its texture or organic matter content increases. The amount of water retained in a soil one to three days after irrigation or rain or after downward movement of water by gravity become negligible, is called field capacity (θfc). Water potential at θfc is usually in the range of -10 to -30 kPa depending upon soil texture. Field capacity gives upper limit of water useful to plants but it is inexactly defined as its value changes with soil texture, structure, type of clay mineral, organic matter contents, depth of initial wetting and impeding layers. When plants growing in a soil do not regain their turgor and remain wilted when placed under humid conditions, the soil water content at this stage is called the permanent wilting point and for most plants it is the amount of water held by the soil when the water potential is -1500 kPa. Under this condition plants are not dead but will die if water is not provided.

The ease with which a standard cone penetrometer can be pushed into the soil at given water content is called penetration resistance. It is generally recorded in units of force, required to push the soil penetrometer into the soil. This is just a correlation measurement which is important in seedling emergence as well as establishment, root growth in addition to penetration and in tillage. Factors affecting penetration resistance are soil water content, its texture, organic matter content, type of exchangeable cations and clay minerals, bulk density and soil compaction. For a given bulk density, soil penetration resistance decreases with increase in water contents.

Hydraulic conductivity of a saturated soil is a measure of ease or ability of a soil to transmit water through its pores or it is an expression of readiness with which water flows through a soil in response to a given hydraulic gradient. It is the proportionality constant in Darcy’s law and is calculated by dividing water flux by water potential gradient. Factors affecting saturated hydraulic conductivity are pore-size distribution, soil texture, structure, bulk density, soil compaction, water temperature, nature of exchangeable cations and organic matter contents. It increases with increase in number of macropores, coarseness of texture and with aggregation, whereas

presence of high amount of exchangeable sodium, compaction of a soil and decrease in water temperature decrease saturated hydraulic conductivity of a soil.

The rate at which free water enters downward through the soil surface is called infiltration rate and is expressed in units of m s-1. It is generally measured with a double ring infiltrometer in which one metal cylinder is smaller in diameter than the outer one. The depth of water in the inner cylinder is recorded at given time intervals. The infiltration rate is highest when water first enters the dry soil, generally decreases rapidly at first and then become constant at large-time. The hydraulic conductivity is approximately equal to the constant rate of infiltration. Factors affecting infiltration rate are surface vegetation, initial soil water contents, texture and structure, size of pores, type of clay mineral, soil compaction, development of crust or seal and irrigation water quality.

6.11.2. Soil health, environmental quality and its impact on

crop growth

Soil health refers to soil quality and its suitability for crop growth and production. A healthy soil must possess some special qualities like good soil tilth and sufficient soil depth with good profile development, so that plant roots can perform their function properly. Sufficient supply of nutrients is necessary for maintaining nutrient balance within the system but excess of these may lead to leaching, ground water pollution, eutrophication, greenhouse gas losses and toxicity to soil microbes and plant life. In a healthy soil, the population of the plant pathogens and insect pests must be controlled; otherwise these will compete with the crops for nutrients making the crops poor in health. Healthy plants are better able to protect themselves against attacks of diseases, like human beings, due to their more effective and better resistance system. Well-aggregated soil can tolerate raindrop impact and hold optimal water content due to its high water holding capacity and suitable pore-size distribution. Water may cause the deterioration of aggregation in two ways. First, hydration causes a disruption of the aggregate through the processes of swelling and the exploding of entrapped air. Second, the impact of falling raindrops on exposed soil can break up the aggregates. The dispersed particles are then carried into the soil pores, causing decrease in porosity. Intense rains destroy the structure of the top few centimeters of soil to form a dense, impervious surface known as a crust or seal.

Various beneficial microbial populations are also essential feature of good soil health. They have significant role in organic matter decomposition, mineralization, nutrient cycling, structure stability, porosity, suppression of plants pests and thus maintaining good soils health.

Healthy soil must be free of toxic metals and chemicals that may damage crop growth and production. Healthy soils are managed by adsorbing heavy metals and making them less toxic for crops and save groundwater from pollution. Water pollution may damage human health and especially of the children who have weak immunity system and may suffer from diseases like blue babies (methemoglobinemia) due to high NO3 contents in drinking water and paralysis due to Cr metal. A healthy and well-structured soil can tolerate adverse events and climate like erosion by wind and water, extreme drought etc.

6.12. Soil Carbon Sequestration

Carbon sequestration refers to the process of capture and storage of the carbon by transfer of atmospheric CO2 into terrestrial (i.e., soil and biota) and geologic (i.e., deep strata and oceanic) pools and subsequent long-term storage, as a result it is not released back into the atmosphere. Soil carbon sequestration refers to transferring atmospheric carbon to the soil carbon pool in the soil profile either through humification of biomass residue and/or formation of secondary carbonates.

6.12.1. Soil carbon sequestration and soil health

The principal aims of soil carbon sequestration are to improve water holding capacity, enhancing biodiversity and sustain crop productivity to achieve food security for rapidly increasing population of the world. Restoration and protection of soil organic carbon above critical level improves soil quality and consequently results in increased crop production per unit input of water, land area and energy. This strategy is very important, particularly for developing countries like Pakistan having low organic carbon soils and intensive cropping pattern. Organic carbon level in soil varies with climatic zones, tillage practices, cropping pattern and rate of fertilizer application. Adoption of recommended management practices enhance and sustain biomass production and improve soil quality. Soil health can be sustained by sequestering more carbon by conservation tillage, efficient nutrient management, reduced grazing, erosion control by using cover crops and restoring degraded or desertified soils.

6.12.2. Soil carbon sequestration and environmental quality

Carbon is being lost as carbon dioxide and methane gases and is sources of pollution. Human activities are adding more CO2 in the atmosphere ever since industrial revolution. It is the primary greenhouse gas and due to its increasing concentration, there is increase in global temperature. Due to this, water level in oceans is increasing posing threat to the cities close to them. Soil carbon sequestration is a natural process that removes carbon dioxide from the atmosphere and stores it on long-term basis in the soil profile. It is one of the most efficient and cost-effective means of off-setting fossil fuel emissions to decrease the rate of CO2 enrichment in the atmosphere. The soil carbon sequestration process can defer or mitigate global warming and provide economic gains, environmental benefits and agro-biodiversity.

6.13. Conclusion

In this chapter, all the physical, chemical and biological aspects related to the soil quality have been addressed. The soil quality has been considered as integrated approach by covering all three aspects of soil quality to give more sound and practicable information’s for agriculture and environmental sustainability.

References

Blake, G.R. and K.H. Hartge (1986). Bulk density. In: A. Klute (Ed.) Methods of Soil Analysis. Part 1. Physical and Mineralogical Methods. Monograph 9, pp. 363–375. SSSA; Madison, WI, USA.

Gee, G.W. and D. Or (2002). Particle-size analysis. In: (J. Dane and G. Topp Eds.) Methods of Soil Analysis. Part 4. Physical Methods., pp. 255–293. SSSA; Madison, WI, USA.

Gugino, B.K., O.J. Idowu, R.R. Schindelbeck, H.M. van Es, D.W. Wolf, B.N. Moebius, J.E. Thies and G.S. Abawi. (2009). Cornell Soil Health Assessment Training Manual. NYSAES, New York, USA.

Saha, A.K. (2004). Text Book of Soil Physics. Kalyani Publishers, New Delhi, India.

Chapter 7

Plant Nutrients and Soil

Fertility Management

Abdul Wakeel, Muhammad Yaseen, Muhammad Aamer

Maqsood, Muhammad Sanaullah and Tariq Aziz†

Abstract

Plant nutrients are chemical elements essential for all higher plants to complete their vegetative and reproductive growth cycles. A plant nutrient cannot be replaced by any other element fully. An element required by some plants or only beneficial in enhancing the growth of some plants cannot be termed as a plant nutrient; however, may be called as a beneficial element. The development and use of diagnostic techniques for assessing nutrient status of plants and soils to manage soil fertility are of great importance. The most widely used technique for managing soil fertility and plant nutrition is soil testing; however visual deficiency symptoms and plant analysis are also useful for determining soil fertility and plant nutrient status for developing nutrient management strategies for attaining optimum crop growth and yield. Optimum and economic crop yields can be achieved only by improving nutrient use efficiency by crops and making site-specific balanced fertilizer use. Integrated use of chemical fertilizers, green manuring, composting, slowly decomposable organic amendments, such as biochar, and, crop residue recycling, not only contribute to profitable agricultural production but also contribute to soil fertility replenishment and reduced environmental consequences of high scale land cultivations.

Keywords: Plant Nutrient; Nutrient-use Efficiency; Soil Fertility Evaluation; N, P, and K Cycles; Integrated Nutrient-use

†Abdul Wakeel*, Muhammad Yaseen, Muhammad Aamer Maqsood, Muhammad Sanaullah and Tariq Aziz

Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: abdulwakeel77@gmail.com Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

142 A. Wakeel, M. Yaseen, M.A. Maqsood, M. Sanaullah and T. Aziz

7.1. Introduction

The elements essentially required for human life are catered by plant and animal sources, which in turn are supplied by soil. Even sea foods acquire sediment-bound nutrients after their solubility. Most of the essential elements for human nutrition are also essential plant nutrient and plants accumulate these nutrient elements in their various parts. Most parts of crop plants are either used as animal feed or as human food. Along with plant nutrients, about 50 additional elements can also be taken up by plants; some of those are essential for animal and human nutrition while others may even be toxic for domesticated animals and humans. Since the life started on this Earth, humans and animals are obtaining their nutrition from the natural sources, especially soil; and the demand is increasing with increase in human and animal populations. Thus, ever increasing population growth is leading to mining of natural resources so hastily that sustainability of agriculture, the prime source of food for population, is at risk. Most of the projections indicate that the world population will be doubled in the next 3-4 decades; therefore, sustainability of natural resources for enhanced agricultural production is vital for human survival. The decline in soil fertility due to intensive cultivation and adoption of high yielding crop cultivars is among the major challenges faced by modern agriculture. Therefore, conservation and wise management of soil resources to restore their productivity to provide adequate nutrients to crop plants for sustainable food production merit great emphasis. Thus, an understanding of plant nutrients and their dynamics in soils and plants is necessary to develop and adopt best management practices for soil fertility restoration for sustainable agricultural production.

7.1.1. Historical perspectives of plant nutrition

The history of plant nutrition is thousand years old; however, the first theory of about plant nutrition was given by a Greek philosopher, Aristotle (384-322 before Christ). According to his philosophical opinion, plants take their food from decomposed plants and other living organisms. This theory was named as “Humus theory” and it remained valid for a duration of about two thousand years due to the great influence of Greek philosophy. This theory was firstly doubted by Jan Van Helmont in 1600, who after five years of his quantitative experiments, concluded that plants take water from the soil and produce substances from it. In 1780, John Priestely stated that the substances produced by plants are necessary for animal life (Fig. 7.1). Later on this substance was named as oxygen and discovery of photosynthesis came into being.

French chemist Theodore de Saussure conducted first ever nutrient solution experiments and in 1800 he concluded that water and ions of chemical elements, taken up selectively by plants, are necessary for photosynthesis. Furthermore, he stated that plants can get carbon from atmosphere but not nitrogen. It provided a clear basis for the theory of mineral nutrition of plants, formulated by Carl Philip Sprengel in 1828. Later on, this theory was named as “Liebig’s law of minimum” after marketable arguments of Justus von Liebig (1803-1873; Fig. 7.2). According to the “Law of minimum”, plant growth is limited by the essential element which is deficient, even if all other nutrients are present in excess amount (Mengel 2001).

Plant Nutrients and Soil Fertility Management 143

Fig. 7.1 The simple experiment set by Priestly (1775) indicating release of gas from plant required by animals.

Fig. 7.2 Law of minimum, initially given by Sprengel and promoted by Justus von Liebig (1803-1873).

Today, plant nutrition is based on mineral theory, which actually explains that plants’ food is also like animals and that plant nutrients are chemical elements and provision of these elements in any system have potential to grow plants. Dennis R. Hoagland (1884-1949) is the father of modern plant nutrition. Although some experiments had already been conducted in solution culture, Hoagland developed the complete recipe of nutrient solution, including micronutrients which were not discovered earlier, for optimum plant growth. All fields of consequent sophisticated plant research depended on the recipe formulated by Hoagland, with slight modifications.

7.1.2. Chemical elements and soil fertility

The organisms which can grow, reproduce, have their own metabolism and take energy are considered as living organisms. All living organisms need nutrients to

perform these functions. However, different living organisms take up nutrients in different forms, and may also have different nutrient requirements. In general, animals fulfill their energy requirements by taking carbohydrates, fats, proteins, etc., whereas plants obtain energy from light by using inorganic elements. Plants can take up a large number of chemical elements; however, only 17 elements are considered essential for plants and these are called “plant nutrients”. In some text books, the term “essential plant nutrients” is also used, which is technically not appropriate because an element is termed as “plant nutrient” only when it is essential for normal plant growth and reproduction and functions of this essential element cannot be performed by any other chemical element. This definition is based on the following three-point criteria given by Arnon and Stout (1939):

• It is impossible for the plant to complete its life cycle without that element;

• A deficiency of that element can be rectified only by supplying that element; and

• The element is directly involved in nutrition of the plant.

Later on, criteria were simplified by Epstein and Bloom (2005), according to which:

• The element is a part of molecule that is an intrinsic component of the structure or metabolism of the plant; and

• The plant can be so severely deprived of the element that exhibits abnormalities in its growth, development, or production in comparison with plants not so deprived.

In case of inadequate supply of any one of 17 plant nutrients plants cannot perform normally; however, plant requirements of these essential elements are quite diverse. Some nutrients are required in greater amounts and others are required in minor amounts. Even excessive availability of a plant nutrient may be harmful for plant growth and yield. At a certain concentration range of each nutrient in plant tissue, plant growth and yield are maximum; this range is called critical range. The terms deficient, sufficient and toxic levels of plant nutrients are explained in Fig. 7.3.

Soil fertility is the branch of soil science which deals with the supply of essential elements (plant nutrients) to plants. In particular, it deals with the ability of soils to provide nutrients to plants for their growth and reproduction. All plant nutrients which are mineral in nature are provided from the soil. Presence of high concentration does not refer to soil fertility, because an element presence in higher concentration in soil may not be available to plants due to their forms, bindings to soil particles or other chemical compounds and other physicochemical properties of soil. The concentrations of plant-available essential elements refer to the soil fertility. Soil fertility is a dynamic domain of soils because it can be changed (managed) due to the influence of farming practices, climatic conditions and application of manures, green manures, fertilizers and biofertilizers. Other important factors which can influence soil fertility include soil texture, soil structure, soil pH, soil organic matter, soil moisture and soil organisms.

Fig. 7.3 Generalized relationship between concentration of nutrients and plant yield (Adapted after Havlin et al. 2014).

7.2. Plant Nutrients

Chemical elements which fulfill the criteria of essentiality, given by Arnon and Stout (1939), are called essential elements for plant growth or plant nutrients. However, the requirements of plants for various nutrients are not the same, because some are needed in higher amounts and the others are essentially required in a very less amounts.

7.2.1. Classification of plant nutrients

In general, plant nutrients are classified on the basis their average concentration in optimally grown plant tissues. The nutrients taken-up/required by plants in higher amounts (in g kg-1 or in % age of dry mass; i.e., > 0.5 g kg-1) are categorized as “macronutrients”, while the nutrients required in much lesser amounts (in mg kg-1 dry mass; i.e., < 0.5 g kg-1) are considered as “micronutrients”. Nine macronutrients include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S). Among macronutrients, three are non-mineral while all others are mineral in nature (Table 7.1). Eight other nutrients, required by plants in much lesser amounts and categorized as “micronutrients”, are iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), boron (B), chloride (Cl) and nickel (Ni).

In addition to 17 plant nutrients, some other elements have positive impact on growth of some plant species, but not of all plants. These are called as “beneficial elements”. Beneficial elements include sodium (Na), silicon (Si) and cobalt (Co). Beneficial

elements are not included in the list of plant nutrients as they do not fulfill the criteria established for plant nutrients.

Table 7.1 Classification of plant nutrients and their plant available forms and average concentrations in optimally-grown plant tissues.

Elements Abbreviation Average concentration in plant tissue

Macronutrients Non-minerals % DM Carbon C 44 Hydrogen H 42 Oxygen O 6 Minerals g per kg DM Nitrogen N 15 Phosphorus P 2 Potassium K 10 Sulfur S 1 Calcium Ca 5 Magnesium Mg 2 Micronutrients mg per kg DM Iron Fe 2 Manganese Mn 1 Copper Cu 0.1 Zinc Zn 0.3 Molybdenum Mo 0.001 Boron B 2 Chloride Cl 3 Nickel Ni 0.001

Source: Havlin et al. (2014), Schubert (2006)

7.2.2. Plant available forms of nutrients

In soil, nutrients are present in organic and inorganic forms. Although some studies have indicated that N can be taken up by plants as amino acids as well, an organic form (Moran-Zuloaga et al. 2015), usually plants take up the plant nutrients in inorganic forms. Plant roots have selective permeable systems for absorption of specific plant nutrients, nevertheless root absorption is not restricted to plant nutrients as non-essential and even toxic elements can also be taken up by plant roots. The uptake efficiency is different for different elements.

Commonly, plants take up plant nutrients in ionic forms which become available after the solublization of respective compounds. All plant nutrients are taken up by plant roots either in cationic or in anionic form, except for N which is taken up both as NH4

+ and NO3-. The plant available forms of nutrients are given in Table 7.1.

Carbon is absorbed by plants through leaf stomata, as CO2, and is used in photosynthesis, while H2O, taken up by plant roots, provides H and O. Oxygen is also taken up from the soil air by plant roots for performing respiratory functions.

7.2.3. Nutrient acquisition and uptake by plants

Nutrient acquisition is the process of approaching the nutrients present in the rhizosphere soil, whereas nutrient uptake means the entry of nutrients into plant root tissue across the plasma membrane. There is another relevant term “assimilation” which means the immersion of nutrients into the plant metabolism and/or structure.

Nutrients are acquired by plant roots from the rhizosphere by three modes of action, i.e., mass flow, diffusion and root interception.

7.2.3.1. Mass flow

The soluble fraction of nutrients present in soil solution, not held on the soil solids, flow to the root as water is taken up by plant roots. The amount of a nutrient thus taken up from the rhizosphere depends on the volume of soil water absorbed by plant roots and concentration of the nutrient in soil solution.

7.2.3.2. Diffusion

The nutrients present in soil solution move towards plant roots through diffusion because of concentration gradient, i.e., when concentration of a specific nutrient is decreased (or depleted) in soil solution in the vicinity of roots due to its strong absorption by plant roots. The movement of nutrient in soil in governed by Fick’s law of diffusion:

Where J is diffusion, D is diffusion coefficient, a is for nutrient activity and x is distance from roots.

7.2.3.3. Root interception

The plant roots grow and bump into nutrient ions present in the soil solution near the plant. Root interception is not an efficient mode of nutrient acquisition and only a small fraction of total nutrient taken up by the plant can be acquired through this mode. Extensive root architecture is of great importance for effective root inception extension to the soil’s nutrient depletion zones. Root morphological characteristics have vital role in determining the extent of nutrient acquisition. Root hairs are more active parts of roots for nutrient uptake. The plants having mycorrhizal association have expanded depletion zone for nutrients like phosphorus. Root exudates also contribute towards increasing nutrient acquisition by mobilizing nutrients in the soil.

When the nutrients are at the root surface, there are two ways of their uptake (or absorption) in to the root, i.e., either via symplast or through apoplast. Once inside the plant root, nutrients are transported through xylem to plant shoot to take part in important metabolic processes. When nutrients enter into plant-cell protoplast across the plasma-membrane of root epidermal cells and then move from cell to cell using plasmodesmata, it is called symplastic pathway of nutrient movement in root. Plasma-membrane is selectively permeable for different elements present on the root surface and halts the entry of toxic elements which hinder plant metabolism. In apolastic pathway, nutrients enter into apoplast of the root cells and move towards

xylem-vessel without entering into the plant cell protoplast. However, there is a barrier, known as the Casparian strip, surrounding the vascular cylinder of the root to prevent free apoplastic flow. Therefore, nutrients have to make transition from the apoplastic route to the symplastic pathway before entering xylem vessel or transpiration stream of the plants preventing or reducing the translocation of many harmful elements up to plant shoot.

Like all living cells, plant root cells also have a membrane surrounding the root cells, called cell membrane or plasma-membrane. Plasma-membrane is made up of double layer of phospholipids having phosphate, which are hydrophilic in nature, at outer sides of the membrane and hydrophobic double-strand lipids are on inner side of the membrane (Fig. 7.4).

Fig. 7.4 Model of a bio-membrane (adapted from Schubert 2006).

The cell membrane is a highly firm and organized, dynamic in structure through which small charged molecules, like ions and water, cannot pass without special support. However, trans-membrane proteins are an integral part of these membranes, in great numbers, throughout the membranes for transport of ions. These transport proteins can be divided in to three major classes, i.e., channels, carriers, and pumps. Channels are transmembrane proteins that can open and allow free and fast passage of different ions depending upon the type of the element and configuration of proteins. The transport of ions through ion channels may allow passage of up to a million ions per second. Water-specific channels are called aquaporins and that allow movement of water as dictated by water potential inside and outside the cell membrane. Potassium-specific channels show high specificity for K+ transport and may permit few other cations (like Na+). The movement of ions through ion channels is in response to electrochemical gradient, i.e., electric as well as chemical gradient, across the membrane. The transport of ions along the electrochemical gradient is called passive transport. High concentration of K+ inside the cell, compared to the external environment, may allow passive transport of K+ into the cell if electrical gradient is present.

Carriers are like channels as they also mediate passive transport through the membrane. But carriers have specific shapes and have high affinity to bind substrate molecules/ions and release them at other side of the membrane after undergoing a conformational change. Although selectivity of these proteins is higher but transport

rate is lower than channels. The rate of carriers for transport is up to 10,000 ions per second.

The third class of trans-membrane transport proteins, i.e., pumps, has binding sites for high-energy molecules such as ATP (adenosine triphosphate) and separate sites for substrate molecules as well. Conversion of ATP to ADP by transferring the high-energy third phosphate group provides energy to the pump which undergoes a conformational change and releases the substrate molecule across the membrane even against its electrochemical gradient. Transport against the electrochemical gradient, by using energy, is called active transport and the speed of active transport is only up to 500 ions per second.

7.2.4. Functions of plant nutrients

It is interesting to classify plant nutrients based on their physiological characteristics and functions in plants into four groups, i.e., assimilated nutrients, ester-forming nutrients, free and sorbed nutrients and nutrients in prosthetic groups.

7.2.4.1. Assimilated nutrients (C, H, O, N, S)

Carbon is taken by plants as CO2 from atmosphere and is synthesized into different molecules with the help of enzymes. In an important cycle, CO2 is converted into sugar with the help of a well-known enzyme, ribulosebisphosphate-carboxylase (Rubisco). Also, CO2 is incorporated into phosphoenolpyruvate (PEP) by the reaction catalyzed by PEP-carboxylase. Carbonic-anhydrases produce bicarbonates from CO2 and H2O. Carbon is an integral part of carbohydrates, fats and proteins, the building blocks in plants. Hydrogen is released as proton after the hydrolysis of water molecule during photosynthesis, and often takes part in reduction reactions. Oxygen is assimilated with other chemical elements such as H (hydratase reaction), C (carboxylase reaction), P (phosphorylation reaction) or S (sulfurylase reaction). Reduction of oxygen occurs in respiratory chain where it acts as electron receptor (Schubert 2006).

Nitrogen is a constituent of amino acids, amides nucleic acid, nucleotides, coenzymes, enzymes and other proteins. Sulfur is a constituent of three important amino acids, cysteine and methionine. Co-enzyme A, thiamine pyrophosphate, glutathione, biotin, 5´-adenylylsulfate, and 3´-phosphoadenosine also contain S as an integral part. Nitrogen and S are mostly taken up by the plants from soil and make covalent bonds via oxidation-reduction reactions to produce organic compounds. However, atmospheric N can also be used by leguminous plants in association with N-fixing microbes (Havlin et al. 2014).

7.2.4.2. Nutrients for energy storage or structural integrity (P, B)

This element-group is significant in energy storage reactions or maintenance of structural integrity of plant cell. In plant tissue, these elements are often present in the form of phosphate and borate esters in which the elemental group is bound to the hydroxyl group of an organic molecule (i.e., sugar phosphate). Therefore, these are also called ester-forming plant nutrients. Phosphorus is an integral part of sugar phosphates, phytic acid, coenzymes, nucleic acids, nucleotides and phospholipids

having key role in the reactions involving ATP. Boron develops complexes with polymannuronic acid, mannitol, mannan and other constituents of cell walls. Cell elongation and nucleic acid metabolism also involve contribution of B. Some role of B has been indicated in plasma membrane of plant cells; however, further studies are warranted to affirm this role.

Silicon, though a beneficial element, is also known to play a significant role in energy storage reactions or in structural integrity. Like P and B, Si is also often present in the form of silicate esters. Silicon is deposited as amorphous silica, especially in cell walls and improves the mechanical properties of cell wall making it rigid and elastic.

7.2.4.3. Free and sorbed nutrients (K, Cl, Ca, Mg)

The nutrients of this group are present within plant tissues as free ions in the cell sap or are sorbed to the substances due electrostatic forces. The intensity of sorption is governed by the Coulomb law considering charge of the ions, ionic radius and free concentration (Schubert 2006).

Where Q is quantity of charge, r is radius of ions and k is a constant.

Potassium is, principally, involved in three types of important functions, i.e., enzyme activity, osmotic functions and charge balancing contributing to a number of metabolic functions in plants cells (Schubert 2006; Wakeel et al. 2011; Wakeel 2013). Calcium is required as a cofactor by enzymes involved in hydrolysis of phospholipids and adenosine tri-phosphate (ATP). Furthermore, it is a constituent of cell walls and plays a significant role as second messenger in metabolic regulations.

The enzymes involved in phosphate transfer require Mg. Also, Mg is an integral part of the chlorophyll molecule. Photosynthetic reactions involve Cl in O2 evolution, whereas Mg activates some dehydrogenases, decarboxylases, kinases, oxidases and peroxidases in plant metabolism.

Although Na is not an essential nutrient for all plants, it is involved in regeneration of phosphoenolpyruvate in C4 and crassulacean acid metabolism (CAM) plants and also can substitute some functions of K (Taiz and Zeiger 2010).

7.2.4.4. Nutrients in prosthetic groups (Fe, Mn, Cu, Zn, Mo, Ni)

Nutrients of this group are all micronutrients. These are divalent or tetravalent elements and can chelate the molecules to build a ring structure, and have important roles in reactions involving electron transfer. Manganese has chelating properties to bind with chlorophyll to perform its functions. The elements of this group can also bind the molecules by sorption; however, their functions under this status are still unknown.

Cytochromes and nonheme Fe proteins are involved in photosynthesis, N-fixation and respiration, which require Fe as an integral part (Noggle and Fritz 1986). In Fe-S proteins, Fe has strong involvement in light-dependent reactions of photosynthesis (Barker and Pilbeam 2007). Zinc is a constituent of alcohol dehydrogenase, glutamic

dehydrogenase, carbonic anhydrase, etc., whereas Cu is a component of ascorbic acid oxidase, tyrosinase, cytochrome oxidase, uricase, monoamine oxidase, phenolase, plastocyanin and laccase. Molybdenum functions in plants as a metallic component of enzymes such as nitrogenase, nitrate reductase, and xanthine dehydrogenase. It is required for biological N fixation by microorganisms as a part of nitrogenase enzyme; Mo also catalyzes the reduction of nitrate to nitrite in the cytoplasm. Nickel is the lastest element established as nutrient for higher plants; it is a constituent of urease and its deficiency causes urea toxicity in plants. It is also a constituent of hydrogenases and N fixing bacteria.

7.3. Cycling of Plant Nutrients

All essential plant nutrients are permanent part of this ecosystem and always remain in this closed system; however, the forms of these elements constantly change in response to environmental and climatic conditions. Drylands occupy approximately 40% of total land area around the globe, wherein nutrient cycling is even more critical due to low soil organic matter content, low precipitation and poor vegetation. Generally, low precipitation and high temperature accelerate decomposition of soil organic matter; still a number of factors such as soil mineralogy, soil physico-chemical properties, crop-nutrient interactions and nutrient interactions significantly affect nutrient cycling. This section will focus on cycling of agriculturally important macro-nutrients like N, P and K.

7.3.1. Nitrogen cycle

Seventy eight percent of the Earth’s atmosphere is composed of N in the form of di-nitrogen gas (N2). However, 98% of the Earth’s N is present in rocks and soils. As compared to the atmosphere, rocks contain 50 times more N and the atmospheric N is nearly 5,000 times greater than that of the soil N (Stevenson 1982).

7.3.1.1. Nitrogen fixation

The production of nitrates from atmospheric N2 is called nitrogen fixation. This fixation may be through non-biological or biological means.

i) Non-biological nitrogen fixation

Thunderstorms and lightning are the natural photochemical and electrochemical reactions. Such lighting and thunder storms convert atmospheric gaseous nitrogen (N2) to oxides of nitrogen. These gases get dissolved in rainwater forming nitrous acid and nitric acid, which further combine with other salts to produce 'nitrates’. This natural phenomenon produces 7.6 x 106 metric tonnes of nitrogen per year of nitrogen.

Chemical reactions are represented as:

In the industrial arena, Haber’s process is employed to produce ammonia (NH3) by combining nitrogen and hydrogen under high pressure of 200 atmospheres and extreme high temperature of 400oC (Smith et al. 2004).

ii) Biological nitrogen fixation

The process of transformation of gaseous nitrogen into nitrates by living organisms is called biological nitrogen fixation (BNF). Biological nitrogen fixation occurs by two different processes, i.e., symbiotic nitrogen fixation and non-symbiotic nitrogen fixation.

Symbiotic nitrogen fixationis brought about by certain bacteria, such as Rhizobium species, present in root nodules of legumes, Nostoc and Anabaena (cyanobacetria) in the coralloid roots of cycas and Actinomycetes present in the root nodules of Alnus, Casuarina, etc. An average of approx. 54 × 106 metric tonnes / year of nitrates are fixed by this mode of BNF.

Non-symbiotic or free living organisms are also is nitrogen fixers that function under poor aeration conditions. These include: i) Obligatory aerobes such as Azotobacter; ii) Facultative aerobes such as Escherichia, Bascillus, etc.; iii) Anaerobic bacteria like Clostridium; and iv) Photosynthetic bacteria like Rhodospirillum (purple bacteria), etc. (Fig. 7.5).

7.3.1.2. Nitrification and denitrification

Bacteria, such as Bascillus ramosus, Bascillus vulgaris and Bascillus mycoides, are involved in the decomposition of proteins of dead plants and animals, and nitrogenous compounds such as urea, uric acid, etc. to ammonia. These bacteria are referred to as ammonifying bacteria and this process is called ammonification.

Animals consume plant proteins which, after digestion, are broken down to nitrogenous wastes, like urea, uric acid and ammonia. This nitrogenous waste produced in the animal’s body is excreted which is then further decomposed by microorganisms such as actinomycetes and fungi.

Proteins → Amino acids → Urea → Ammonia

Oxidation of ammonia to nitrites and finally to nitrates in the presence of nitrifying bacteria (chaemosynthetic autotrophs) is called nitrification (Smil 2000).

First, ammonia is converted into nitrites by Nitrosomonas and Nitrococcus bacteria

The nitrites are then converted into nitrates by Nitrobacter and Nitrocystis these forms of nitrogen in soil are taken up by plant roots.

The biological process in which nitrates and nitrites are reduced to molecular nitrogen (N2) in the presence of denitrifying bacteria, including Bascillus subtilis, Micrococcus denitrificans, Pseudomonas stutzeri, Pseudomonas aeruginosa, is called denitrification. It results in reduced soil fertility and is stimulated by water logging conditions which cause poor drainage, lack of aeration and accumulation of (undecomposed) organic matter in the soil.

Fig. 7.5 The nitrogen cycle.

7.3.2. Phosphorus cycle

Phosphorus (P) exists in different forms in soil; these forms can be grouped into four types. These include plant available inorganic P and three forms which are not plant available, viz., organic P, adsorbed P and primary mineral P. The processes that bring about P transformation are weathering and precipitation, mineralization and immobilization, and adsorption and desorption (Fig. 7.6). Plant available P is increased by weathering, desorption and mineralization, while immobilization, precipitation and adsorption decrease plant available P.

Fig. 7.6 The phosphorus cycle.

Immobilization occurs when plant available P forms are taken up by microbes, turning inorganic P into organic P forms, which is not available to plants. However, over time, the microbial P may become available to plants when microbes die. An important strategy for improve P fertility in soils is to maintain an adequate level of organic matter in soil. Mineralization is opposite to immobilization and results in the slow release of P in the soil solution during the crop growing season, making P available for plant uptake and thus reducing the need for P fertilizer applications. Phosphorus mineralization is favored by a soil temperature range of 65 to 105oF (Filippelli 2002).

Adsorption is chemical binding of plant available P to soil particles, which makes it unavailable to plants. The release of adsorbed P from bound state into soil solution is termed as desorption. Adsorption is a quick process while desorption usually occurs slowly. Adsorption is different from precipitation: adsorption is a reversible process involving chemical binding of P to soil particles whereas precipitation involves a permanent change in chemical properties of P as it (P) is removed from the soil solution. Availability of soil P to plants is maximum in between pH 6 and 7. At higher pH in alkaline and calcareous soils, P can precipitate with Ca making it less available for plant uptake. At lower pH (in acid soils), P tends to get fixed by Fe and Al oxides (i.e., sesquioxides).

In soils, P is generally present in the following three forms: solution P, labile P and fixed P. Usually, solution P comprises only a fraction of the total soil P. Most of the solution P prevails in orthophosphate form, but small amounts of organic P may exist as well. The solution P fraction is of great significance for crop growth as it is the P pool which is immediately available for plant uptake. Labile P is the solid phase P, which, on depletion of soil solution P, is readily released into soil solution. The concentration of phosphate in soil solution decreases because of P uptake by plants. This lowers the level of P in soil solution; consequently, some of the labile pool P

becomes soluble and replenishes soil solution P. Fixed soil P comprises of highly insoluble inorganic phosphate and organic compounds which are resistant to mineralization by microorganisms. Phosphate in this pool may remain in unavailable form to plants for years and, hence, is insignificant regarding soil fertility and plant growth.

7.3.3. Potassium cycle

Generally, soil potassium (K) exists in four forms (also referred to as K fractions) and their availability to crop roots is not similar. These forms include soil solution K (K+ ions), exchangeable K (easily exchangeable and slowly exchangeable K), non-exchangeable K (reserves of K) and structural K in soil minerals (Krauss 2003).

These forms of K are actively transformed (as depicted in K cycle) and quickly equilibrate when a certain quantity of exchangeable and soil solution K is taken up by plants (Fig. 7.7). However, transformation of K from the structural fraction to any other form is extremely slow and is considered on the long-term scale only. Potassium transformation is significantly affected by both physical and chemical processes. Clay mineralogy strongly influences the soils capacity for fixing or releasing potassium. A soil’s ability to fix or release K is strongly affected by its clay mineralogy. Clay minerals, especially illites, vermiculites, montmorillonites and smectites, play an important role in soil K dynamics. A major part of total K in soils is bound to clay minerals and, thus, is not available to plants. The soils containing illite and vermiculite minerals have ability to fix the applied K fertilizer, making it less available to plants (Wakeel et al. 2013). Therefore, while making K fertilizer use recommendations, it is crucial to consider clay mineralogy and K dynamics in the soil. Although K leaching is not considered in agricultural lands, however, its leaching in sandy soils cannot be ignored.

As K is a macronutrient, it is required by plants in large amounts; therefore, its removal by crop plants may lead to its deficiency in soils if K fertilizers are not applied.

Fig. 7.7. The potassium cycle.

7.3.4. Factors affecting nutrient availability

Soil characteristics, including pH, structure, composition and content of organic matter, and climatic factors, mainly temperature and moisture, affect nutrient availability to plants. The mobility of a nutrient in soil and within plant has also great bearing its availability to plants. Information regarding mobility of nutrients in soil and plants is given in Table 7.2.

Temperature range of 15oC to 40oC is favorable for most agricultural crop. Each crop requires a specific temperature range for different growth processes. Temperature also affects absorption and uptake of nutrients by plants. Temperature is important in regulating rate of soil chemical reactions which make nutrients available for plant uptake. Under cool soil temperatures, chemical reactions and root activity decrease, resulting in reduced nutrient availability to crop plants.

Table 7.2 Mobility of plant nutrients in soil and plant.

Mobility of nutrients In soil In plants

Mobile Nitrogen, Sulfur, Boron Nitrogen, Phosphorus Potassium, Magnesium

Somewhat immobile Potassium, Calcium, Magnesium

Sulfur, Iron, Manganese, Copper, Zinc

Very immobile Phosphorus, Iron, Manganese, Copper, Zinc

Boron, Calcium

Source: Jones and Jacobsen (2001), Bray (1954), Aziz et al. (2011), Goldy (2013)

7.3.4.2. Moisture

Majority of the soil nutrients are water soluble. Reduction in soil moisture results in reduced movement of nutrients to plant roots and thus nutrient uptake by plant roots is hampered. Excess water (soil moisture) can result in nutrient loss by leaching beyond the root zone. Soil moisture levels also affect microbial activity in soils. Excessive water in the soil for extended periods of time may lead to oxygen (O2) depletion and buildup of carbon dioxide (CO2) levels. Thus, too high or too low moisture results in decreased nutrient transformations and processes which transform nutrients into plant-available forms.

7.3.4.3. Soil structure and composition

The structure and texture of soil influences the soil’s capacity for nutrient retention by altering compaction, cation exchange capacity (CEC) and porosity. In comparison to fine textured soils, coarse, sandy soils have less capacity to hold nutrients. Fine textured soils tend to bind nutrients in forms less available to plants. In general, loam, clay loam and clay soils have least nutrient deficiencies. Highly compacted soils can limit the amounts of water and air in these types of soils, which are important for nutrient breakdown into plant available forms of nutrients. Root penetration can be

hindered or completely restricted by soil compaction; thus, reducing the amount of nutrients and water uptake by plants.

7.3.4.4. Soil pH

Soil pH significantly affects the solubility of nutrient compounds and biological activity responsible for release of nutrients into soil solution. Plant nutrients availability is maximum between soil pH 6.2 to 7.0. In acidic soils below pH 5.5, and in alkaline soils above pH 8.0, many nutrients change their form and their availability to plants is affected adversely. Aluminum toxicity is known to occur in acidic tropical soils because of high Al solubility at pH below 5.8. Basic cations such as calcium, magnesium and sodium are increased in alkaline soils having pH above 8.0, while P deficiency in plant may be induces because of precipitation of P with free lime / Ca.

7.3.4.5. Organic matter

Nutrient availability for plants may be limited when organic matter forms a complex with nutrients. In addition to increasing CEC and nutrient storage and release, organic matter causes aggregation of soil particles. Aggregation of soil particles facilitates water infiltration, root penetration, gas movement and restriction of soil erosion by developing large soil pores. Organic matter improves soil tilth, and the water holding capacity of soils. Approximately 0.03% of the total soil weight consists of living biomass, which plays important roles in plant growth by decomposing organic materials, recycling of plant nutrients and CO2, and transforming molecular forms of nutrients.

7.4. Soil Fertility Evaluation

Soil is a medium of growth for higher plants which provides essential nutrients, water, and anchorage to plants. Capacity of a soil to provide essential elements to plants is termed as soil fertility (Havlin et al. 2014). High amount of a total nutrient concentration in a soil does not always mean that that particulate soil is productive (or even fertile in that nutrient) as a number of other growth factors significantly influence nutrient availability within soil and the plant’s capacity for nutrient uptake. Continuous availability of essential nutrients throughout the plant growth period is necessary for normal growth and yield; hence soil fertility is one of the most important factors determining productivity (Mengel et al. 2001).

Fortunately, farmers can control fertility by managing the fertilizer application. However, site-specific estimates of the nutrient fertility status of the soils are very important to rationalize the fertilizer use (Black 1993) for economic agriculture. One and the most important aspect of fertilizer management is the evaluation of soil fertility as both low and over dose of fertilizer application may cause serious problems for crop production and environment, respectively. Reliable information can only be accomplished through a well-defined and managed program of soil fertility evaluation (Black 1993). Soil fertility evaluation is the process of estimating the amount of native and residual nutrient elements which could be available for use by growing crops in a particular soil and the amount and type of fertilizers to be supplemented and the appropriate application method for profitable crop production

(Fageria 2009). Soil fertility is generally evaluated by visual observations, soil analysis, plant analysis and greenhouse and/or field trials.

7.4.1. Visual symptoms

Plants, being living entities, respond to biotic and abiotic stresses and exhibit various symptoms if there is any change in their environment (Bould et al. 1983). These symptoms vary with change in type of stress. In a similar fashion, plants respond to any change in nutrient concentration within soil and ultimately within plant tissues. These responses include reduced or stunted growth, changes in leaf color, chlorosis, necrosis, dead tissues, de-shaping of growing buds, delayed maturity, etc. These symptoms are generally known as deficiency symptoms and can be a very effective tool in diagnosing the plant’s nutrient status (Fageria et al. 2010). However, visual symptoms only are not sufficient to make a definitive diagnosis of any deficiency or toxicity, and in field situations only an experienced professional (plant nutritionist) may avail the plant deficiency symptoms for diagnosing nutrient deficiencies correctly.

The interaction between nutrient mobility in the plant and plant growth rate can be a major factor influencing the type and location of deficiency symptoms that develop (Mengel et al. 2001). For very mobile nutrients such as nitrogen and potassium, deficiency symptoms develop predominantly in the older and mature leaves. This is a result of these nutrients being preferentially mobilized during times of nutrient stress, from the older leaves to the newer leaves near the growing regions of the plant. Additionally, mobile nutrients newly acquired by the roots are also preferentially translocated to new leaves and the growing regions. Thus, old and mature leaves are depleted of mobile nutrients during times of stress while the new leaves are maintained at a more favorable nutrient status (Marschner 1995). Many of the classic deficiency symptoms such as tip burn, chlorosis and necrosis are characteristically associated with more than one mineral deficiency and also with other stresses that by themselves are not diagnostic for any specific nutrient stress (Fageria et al. 2010). Stresses such as salinity, pathogens, and air pollution induce their own characteristic set of visual symptoms. Often, these symptoms closely resemble those of nutrient deficiency. Pathogens often produce an interveinal chlorosis, and air pollution and salinity stress can also cause tip burn (Marschner 1995). Although at first these symptoms might seem similar in their general appearance to nutrient deficiency symptoms, they do differ in detail and/or in their overall developmental pattern. Pathological symptoms can often be separated from nutritional symptoms by their distribution in a population of affected plants. If the plants are under a nutrient stress, all plants of a given type and age in the same environment tend to develop similar symptoms at the same time (Epstein and Bloom 2005). However if the stress is the result of pathology, the development of symptoms will have a tendency to vary between plants until a relatively advanced stage of the pathology is reached.

In the vast majority of cases, nutrient deficiencies can substantially reduce production without showing any clear symptoms. This problem is referred to as “hidden hunger” whereby a deficiency is having a negative effect without being recognized, though if an early diagnosis is made, effective action can usually be taken

(Havlin et al. 2013, 2014). Most common nutrient deficiencies in our soils are of nitrogen, phosphors, potassium, zinc, boron, and iron. However, deficiencies of other nutrients may occur because of specific soil and environmental conditions. Specific nutrient deficiency symptoms of various crops are described briefly in Table 7.3.

Table 7.3 Specific nutrient deficiency symptoms of various crops

Nutrient Deficiency Symptoms Pictures

Nitrogen

Plants often have stunted growth with yellow to pale lower leaves, which is a condition known as chlorosis. Since nitrogen is a mobile nutrient within the plant, its deficiency first appears in older leaves. When nitrogen is severely deficient, chlorotic leaves may die and fall off the plant (Epstein and Bloom 2005).

Phosphorus Plants often have overall stunting,

particularly during the early stages of growth. When deficient, older leaves develop a dark green to blue green color. In corn and certain grass species, older leaves may develop a purple coloration. (Photo Source Tariq Aziz, 2011; P deficiency in Ptilotus polystachyus;

Unplublished).

Potassium Plants often experience stunted growth. Like nitrogen and phosphorus, potassium is a mobile nutrient. Older leaves may develop chlorosis along the margin, or edge, of leaves (V-shaped) (Photo source: Epstein and Bloom 2005).

Sulfur Uniform chlorosis of leaves, may

resemble nitrogen deficiency symptoms, except that the symptoms first appear on new growth of most crops, since sulfur is mostly immobile within the plant. Growth may be stunted, with spindly and thin stems. (Photo source: Bergmann 1992)

Calcium New leaves, buds, and root tips, fail to develop, and eventually turn brown and die. Leaf tips are often chlorotic or colorless and young leaves may bent down. Young leaves of certain crops may develop a cupped or crinkled appearance. Blossom end rot in fruits. (Photo source: Bergmann 1992)

Magnesium Commonly, plants develop interveinal

chlorosis on older leaves. If case of severe deficiency, the entire leaf may become chlorotic and eventually die. Younger leaves affected with continued stress. Chlorotic areas may become necrotic, brittle, and curl upward. (Photo source: Bergmann 1992)

Boron New leaves may be thickened, curled,

and brittle. Boron deficiency also induces premature flower and fruit shedding. Stems may also become cracked. Other symptoms include rotting and discoloration of fruits and roots. (Photo source: Bergmann 1992)

Copper Plants may have chlorosis, stunted

growth, and curling of young leaves. Leaf tips and leaf edges may begin to die back. Leaves may develop a dark bluish-green cast. In case of severe deficiency, wheat ears look normal but are devoid of grains. (Photo source: Epstein and Bloom 2005)

Iron Deficiency symptoms include interveinal chlorosis, which first appears on young growth. In severe cases, entire leaf may turn chlorotic and die. (Photo source: Epstein and Bloom 2005)

Manganese Like iron, interveinal chlorosis may

develop on young leaves, except the chlorosis appears as yellow dots. Chlorosis is less marked near veins Some mottling occurs in interveinal areas In monocot plants, black spots also appear on the base of young leaves. Photo source: (Bergmann 1992)

Molybdenum Older leaves may become chlorotic. Its

deficiency symptoms may be similar to the symptoms of N deficiency, as molybdenum is involved in major nitrogen processes in plants. Spots of dead leaf tissue may appear on the margins of leaves. Classic molybdenum deficiency in poinsettia is shown as a thin, leaf margin chlorosis.” as overall yellowing, leaf margin necrosis and some interveinal chlorosis. (Photo source: Bergmann 1992)

Zinc Interveinal chlorosis may appear on younger leaves as appear in the case of iron deficiency. Unlikely to iron deficiency, distinctive chlorotic bands appear between midrib and edges of leaves, especially in maize. In some crops, interveinal chlorosis develops on older leaves, leading to eventual death of the leaves (Irshad et al. 2004).

7.4.2. Soil analysis

Soil analysis is a very valuable and practically feasible tool in effective nutrient management (Jones and Beaton 2001). It is a very rapid chemical analysis to assess/monitor available nutrient status of the soil. Soil testing also includes interpretation, evaluation and recommendation. Above all, it helps us in prediction and determination of the proper amount of nutrients for a particular soil based upon its fertility requirements (Jones and Beaton 2003). Accurate soil analysis can lead to precise application of nutrients and thus can reduce wastage of valuable resources. When fertilizers are applied based on soil test results, we can predict the probability of getting the profitable response to the fertilizers.

7.4.3. Plant tissue analysis

The principal advantage of visual diagnostic symptoms is that they provide an immediate evaluation of nutrient status of plants. However, their main drawback is that visual symptoms do not develop until after there has been a major adverse effect on plant growth, development and yield.

Tissue sampling allows us to determine whether plants are receiving proper nutrition. Plant analysis is a laboratory determination of nutrient concentration in plants or certain diagnostic plant parts. Plant analysis is used to monitor nutrient status of crops, troubleshooting of the problem, making nutrient management recommendations for perennial fruit crops. Plant analysis is the only way to know whether or not crop plants are adequately nourished during the growing season. Plant analysis can detect unseen nutrient deficiencies, confirm visual symptoms of deficiencies and detect toxic levels of nutrients in plant tissues (Epstein and Bloom 2005). In case of field crops, plant analysis is usually used as a diagnostic tool for correcting nutrient problems in the future crop analysis of young plants can allow a corrective fertilizer application to the same crop.

Plant tissue sampling is the most critical step for success of any plant analysis program. Like soil sampling, a representative tissue sample must be collected, which characterizes the entire plant population. Thus, plant analysis is a tool which must be used with great caution. The sampling procedure (i.e., the diagnostic plant part and appropriate plant growth stage for sampling) is unique to each crop or a group of similar crops. The sample must be of a specific plant tissue taken at a specific growth stage (Jones and Beaton 2001). Representative sampling can be achieved by sampling a multitude of plants. Getting truly representative s`ample from a crop in the field is a real challenge. Sample collected from plant may not be representative of all the plants in a field or a yard. To get a true sample, highly damaged plants due to insects or disease attack must not be sampled. Sample must be taken, if possible, from different plants in the field and healthy and damaged plants must be sampled separately and send to the laboratory for comparison. For plant sampling, some important points to be kept in mind are: identify which plant part is recommended for sampling; sample healthy plant parts, and not the damaged, diseased or dead plants; remove any soil or dust from the sampled plant parts; place plant samples in clean paper bags; protect the sample bags from dirt and contamination; and wash,

dry, grind and homogenize the plant tissue samples very carefully, avoiding any chances of contamination.

Plant tissue analysis is nutrient-specific but relatively a slow process. The diagnostic plant part must be sampled at the recommended growth stage, processed (i.e., washed, dried, and ground) carefully, and analyzed accurately. A nutrient concentration in the selected plant part, when compared against the relevant critical level values (which are available in the literature for most crop plants) can help in evaluating the plant nutrient status at the time of sampling with a relatively high degree of confidence and can be extrapolated to project nutrient status till harvest (Jones and Beaton 2011).

Improper sampling methods for sampling particular crop will lead to misleading results. The data obtained from the analysis of samples collected from the field infested with insects, diseases and weeds, drought or moisture stressed may have no or little importance. For any type of sampling, certain principles must be followed to get correct information from the plant analysis program.

Plant nutrient concentration will vary depending on the type of plant sampled, specific plant part sampled and the crop growth stage when sampled. For example, young, immature leaves will have a different nutrient concentration than older, mature leaves. Effective diagnosis of nutrient deficiencies by plant tissue analysis can be achieved only if relevant plant analysis criteria are used to interpret the laboratory results. A salient example of erroneous critical levels of B in cotton leaves was observed by Rashid et al (2002) who determined that actual critical level of B in cotton leaves was multiple times higher, i.e., 53 mg kg-1, compared with much lower values of 15–20 mg kg-1 published in the literature (Jones 1991; Reuter et al. 1997).

7.5. Plant Nutrients and Soil Fertility Management

Catering of an ever-increasing food supply for a fast increasing population in a sustainable way is a daunting challenge for agriculturists. Continuous decline of soil fertility because of intensive cropping and inadequate and imbalanced application of fertilizers has made this task quite difficult. Many countries have main focus on soil resources management maintaining the adequate fertility level to increase crop yields to fulfill the needs of projected population (Ahmad 2011). This indeed is more needed in Pakistan where soil fertility is declining rapidly with relatively rapid increase in population.

Agriculturists must set priorities to restore, maintain and build up soil fertility particularly in areas where soils are developed from parent materials inadequate in essential plant nutrients or where population pressure has posed rapid increase in demand for food and raw materials. In such situations, enhanced crop productivity per unit land area is a prerequisite to meet the growing demand for food, which is probably achievable by crop production intensification. A fertile soil allows growing of a wide range of crops by fulfilling their nutrient needs, to address food security issues for an ever-increasing population in the face of climate change challenges. Therefore, maintaining adequate soil fertility is imperative to attain sustainable crop production. Nutrient management strategies provide guideline to farmers for

managing soil fertility by applying appropriate amount, source, placement and time of the deficient plant nutrients (as chemical fertilizers and/or manures, in appropriate proportions). The overall objective of this management system is to supply plant nutrients in adequate and balanced amounts for optimum plant growth and crop yields with least environmental pollution and contamination of groundwater and maintaining and/or improving soil conditions for sustainable crop production.

Plant nutrients and soil management go hand in hand to maintain fertility and productivity of soil to: 1) optimize plant production (with maximum yield and quality, and profit); 2) conserve resources; and 3) enhance soil quality and productivity. Plant nutrients essential for crop growth and production come primarily from soil, chemical fertilizers and manures. Application of nutrients, especially nitrogen, phosphorus, and potassium, in proper quantities and at appropriate times help to obtain maximum economic crop yields. Improper application of nutrients can cause significant decline in crop yield along with polluting the soil environment. Nutrient management must aim at wise use of fertilizers and/or manures to get optimum economic benefit with minimum impact on the environment.

For efficient use of available nutrients, soils must have good structure, adequate drainage and good moisture holding capacity. In the broader perspective, all soil properties, i.e., physical, chemical and biological, have definite roles in maintenance of soil fertility. Profitable and sustainable crop production depends on proper nutrient management. Therefore, a sustainable nutrient management program must meet the following goals:

• Fulfil the crop nutrient requirements

• Ensure profitability and yield quality

• Generate least pollutants to degrade soil environment

• Minimize the cost of supplying nutrients

• Utilize local nutrient sources, like manure and organic farm materials, to get the best advantage

• Must be applicable and feasible for most of the farmers

These goals are compatible to maintain soil fertility by one way or the other. For example, efficient use of nutrients available on the farm according to crop needs can reduce the risk of damage to the soil environment and water quality.

Plant biomass largely builds up because of interaction of water and CO2 (present in air), light and energy of the sun and nutrients from soil and water. To achieve optimum plant growth, adequate nutrients must be present in available forms in soil solution which is accessible to the root system as per need of crops particularly at vegetative and reproductive growth stages. A fertile soil has inherent ability to supply nutrients essential for growing crop plants on the one hand and activate a diverse microbial soil community on the other hand. To build up soil fertility, some traditionally practiced strategies are crop rotations including leguminous crops, use organic manures and cover crops. Animal manure use has been a common practice to replenish soil nutrients around the globe since time immemorial. Manures are

generally applied to field in either fresh or processed form. Composted manure has many advantages including supply of nutrients, addition of organic matter, and promoting biological processes in the soil. However, it is rather more important to know about the contribution of manure to the soil, because manures vary in composition particularly nutrients and contaminants depending upon the type of animal, method of bedding and storage. It is better to compost the manure as heat produced during the process of composting may break down contaminants, kill weed seeds and some pathogens. Generally, fresh manure contains high amount of available N and its long-term overuse can lead to salt build up. Therefore, soil testing is considered a prerequisite to monitor soil fertility status and, thus, is recommended, to enable farmers apply appropriate quantity of fresh/raw or processed manure to avoid imbalances of nutrients in the soil.

7.5.1. Chemical fertilizers

Fertilizers are indispensable component of crop production system because these play vital role in increasing crop productivity per unit field area, particularly under shrinking land and water resources. Because of their alkaline and calcareous nature, our soils are prone to nutrient deficiencies, particularly of N, P, and certain micronutrients (like Zn, B and Fe), and their fertility is further declining with the passage of time due to continuous nutrient mining because almost all the crop biomass is removed out of the fields and fertilizer use is imbalanced. It is evident from the data of soil testing and field trials conducted by public and private research organizations that soils are deficient in N due to low organic matter content. In case of P, nearly 90% of soils have inadequate available soil P or deficient levels of P. For K, the picture is not so clear. Some research reports state that deficiency of K prevails in up to 40% of soils (land area), but crop responses to K are erratic and, consequently, use of K fertilizer is negligible. Field-scale deficiencies of economic significance of the micronutrients exist in the case of Zn, B and Fe. The first ever identified and established field scale deficiency of a micronutrient in Pakistan was of Zn in rice (Kausar et al. 1976). Now the extent of Zn and B deficiency in the country is observed in about 50% cultivated areas (Rashid, 2005, 2006; Ahmad 2011; Yaseen et al. 2013; Yaseen 2014). This situation is a consequence of inadequate and imbalanced use of fertilizers in the country. Moreover, organic matter content in majority of cultivated land averages around 0.5%, which is considered inadequate for obtaining high crop yields.

Intensive cropping with yielding crop varieties combined with inadequate and imbalanced nutrient management is causing continual mining of soil nutrients. This unscientific use of fertilizers is limiting productivity in many Asia-Pacific developing countries, including Pakistan (FAO 2011). Research data estimates indicate that more than 2 million tonnes of nutrients are removed from the soil per year in South Asia, seriously threatening sustainable agricultural production in the region. The challenges to sustainable soil fertility management in the region include: (i) declining trends in soil fertility and mining of soil nutrients; (ii) decline in soil organic matter levels, and; (iii) overuse and inefficient utilization of mineral fertilizers in certain locations and the resulting deterioration of environmental quality.

Mostly used mineral fertilizers are of N, P and K. No doubt, these play important role in boosting crop production, however, in spite of their increased use over the years, per hectare yield of crops are stagnant or even have declined in Pakistan (NFDC 2007; GoP 2008). A recent field survey (Ahmad 2011) indicates that farmers use 43 % less nitrogen, 74 % less P2O5 and 99 % less K2O than their recommended doses on five major crops, i.e., wheat, cotton, rice, sugarcane and maize. These crops consume more than 80 % of the total fertilizers used in Pakistan (Din and Jafery 2007; Ahmad 2011). Therefore, yield gap of 35-50 % between potential and farmers’ yield can only be filled if proper balance between N and P use is achieved (Ahmad and Tila 1998). Farmer’s failure in applying fertilizer in recommended dosage and in balanced proportions has been attributed to some socio-economic constraints such as inadequate supply or inadequate availability of fertilizers at proper time, greater market distances and lack of credit facilities and high prices.

Fertilizer needs of a crop will depend upon the characteristics of the preceding crop in the rotation as fertilizers applied to one crop can benefit the succeeding crop(s) due to their residual effects. For efficient fertilizer management, it is obligatory to evaluate precisely the role of preceding crops and also the residual effect of applied nutrients in sustaining the productivity of soils. Therefore, the role of systematic an approach of nutrient management on cropping system basis in improving fertilizer use efficiency and economizing their use cannot be over emphasized. This can be attained by accounting for the residual effect of applied fertilizers to the preceding crops. Therefore, a technology package has to be developed depending upon soil test results, the cropping system (crop rotation), soil type, ecological situation and socio-economic conditions of the farmer.

Statistics on fertilizer use in the country indicate the over use of N over the years. It has resulted in serious imbalanced use among N, P, and K. For example, the year 2010-11 c usage ratio of N: P2O5: K2O was 1:0.25:0.01 against the recommended use ratio of 2:1:1 (NFDC 2011), which has further widened in favor of N. The use of micronutrient fertilizers is very limited and inadequate. Consequently, not only crop yields in the country are low, but in many cases, quality of the crop produce is deteriorated. Salient examples of micronutrient deficiency induced crop produce impairment in Pakistan are impairment of rice grain quality with B deficiency (Rashid et al. 2007; Rashid and Ryan 2008) and low-Zn wheat grains with Zn deficiency (Zou et at. 2012). Organic sources of nutrients has been discussed in chapter 8.

7.5.2. Integrated nutrient management

The role of fertilizers in boosting crop production is inevitable and this fact is well acknowledged in Pakistan. However, in spite of increased fertilizer input over the years, per hectare yield of crops has remained low compared to many other countries. As mentioned above, farmers use 43 % less N, 74 % less P2O5 and 99 % less K2O. About 35-50 % gap between potential and farmers’ yield could be filled, if proper balance between N and P fertilizer use is maintained. Farmers’ failure to apply fertilizer in recommended amounts and in balanced proportions has been attributed to some socio-economic constraints such as inadequate supply, inadequate

availability at proper time, greater market distances, lack of credit facilities and high prices (NFDC 2008).

In Pakistan, fertilizer use efficiency is very low in almost all cropping zones and is direly needed to be improved. Nitrogen use efficiency seldom exceeds 50%, whereas remaining is lost through different means like leaching, volatilization, denitrification etc. (Abbasi et al. 2003). Similarly, P use efficiency is 10-25 % and K is up to 50 % while micronutrient use efficiency is 5-10 %. Lift over nutrient remains in soil in unavailable forms to plant. This situation compels to pay attention to develop “Integrated Nutrition Management (INM) - using organic manures with mineral fertilizers”. This will not only maintain but also enhance soil productivity by the balance use of mineral fertilizers in combination with different organic sources of plant nutrients. The INM sustains crop productivity in a manner suitable to the cropping systems relevant to their ecological, social and economic situations. It considers nutrients from various sources, e.g. organic wastes, residual nutrients, nutrients transformations, nutrients interactions, and nutrients availability in space and time. INM is ecologically, socially and economically viable system to increase both soil fertility and crop yields because it takes into account: 1) seasonal and annual cropping systems rather than an individual crop; 2) management of plant nutrients in the whole farming system; and 3) the concept of consideration whole fields rather than individual field.

Prior to green revolution in Pakistan, organic manures were the only source of plant nutrients for crop production. Animal wastes contributed the major proportion. Green manuring was another source. Around the 1950s, rhizobium inoculation of barseem was introduced by the Agriculture Research Department. With the introduction of mineral fertilizers on a commercial scale in the early 1960s, emphasis on the use of farmyard and green manure started decreasing. The miraculous crop responses to N fertilizers and later to P fertilizers diverted farmers’ attention to the use of inorganic fertilizers. With the passage of time, inorganic fertilizer use increased but the farmers continued using the FYM available at their farms. In a survey conducted by NFDC (2000), it was found that 49% of farmers use FYM. The work on INM in Pakistan can be summarized as follows:

• Organic and biosources cannot completely substitute for chemical fertilizer in the scenario of intensification of agriculture and need for national food security. However, these are very important to maintain soil fertility and crop productivity.

• Chemical fertilizers particularly nitrogenous fertilizers can be replaced by composted manure, green manure or FYM upto 50% depending upon soil type and amount of fertilizer applied.

• Organic sources like crop residues, animal droppings, filter cake from sugarcane industry etc., can potentially supplement nutrients supplied by chemical fertilizers provided the former are properly managed. However, fresh (undecomposed) FYM much less beneficial than well composted FYM.

• It is well proven that green manuring well before the transplantation of rice is more beneficial than the green manuring immediately before transplanting.

Biological fertilization research is showing promising results in Pakistan. National Institute for Biotechnology & Genetic Engineering (NIBGE), Pakistan Agricultural Research Council (PARC) and the provincial agricultural institutes sell packets of microbial fertilizers/ biofertilizers to farmers for application to the soil before sowing of a legume crop and rice. Nevertheless, biological fertilization on a large scale appears to be the only a long-term prospect.

At this stage recommendations on INM based on soil analysis are not formulated and disseminated to farmers. It is important that researchers and extension workers translate research results and transfer INM technology information to farmers. The message could be conveyed even through electronic and print media that farmers must use organic and biological resources to supplement inorganic fertilizers and improve use efficiency of fertilizers.

Most chemical fertilizers supply one or two essential plant nutrients for the plant growth. Organic amendments improve the soil physical conditions and chemical functions of soils, like their sorption capacity, mobilization of nutrients by mineralization, short term immobilization into soil organisms, long term fixation into stable humic substances and supply of organic substances. Keeping in view the useful specific benefits of chemical fertilizers and organic matter for crop and soil, the idea of integrated use of plant nutrients from various sources is being promoted in various farming systems. This system maintains soil fertility and plant nutrient supply from various sources through an integrated approach. Integrated use of organic, and/or biofertilizer with small quantity of chemical fertilizers results into higher crop yield than chemical fertilizers alone (Ahmad et al. 2006). Combined use of organic and chemical fertilizers increases each other’s efficiency. Interaction of chemical, organic and biological sources and their effective management not only in sustain crop productivity and soil health but they also optimize the use of chemical fertilizers for different crops in different cropping systems.

7.5.3. Environmental perspectives of soil fertility management

Plant nutrition management primarily enhances the quality of land resources for sustainable crop production to meet demands for food and raw materials. Degradation of environment due to imbalance use of fertilizers should be reduced by applying plant nutrients as per the crop requirements as well as by observing the soil and water conservation methods. Agricultural crops inevitably remove plant nutrients from the soil and this removal must match with the replenishment of these nutrients to the soil. Therefore, to make a sustainable farming system, the removed nutrients must be returned to the soil by whatever sources and means available to farmers. In Pakistan, major losses of N are through volatilization and leaching. Therefore, timely application of N fertilizer by efficient methods is recommended to minimize the losses. Nitrate pollution in drinking water has not been reported where N is applied. However, in areas where organic manures are dumped over time may have chances of nitrate pollution in underground drinking-water. There is no

regulatory measure or mechanism to check these N losses. However, with increased awareness concern about environmental issues is growing.

No doubt, nutrient losses are wastage of money for the farmer because loss of soil fertility due to continual nutrient mining by crop removal without adequate replenishment in combination with imbalanced plant nutrition practices poses a serious threat to soil productivity in narrow sense and agricultural production in the broader. Nutrient depletion has already caused greater yield decreases. Therefore, recycling and transfer of nutrients from crop residues and animal manures can particularly make up a pool of nutrients after harvested products. Therefore, use of external sources such as mineral fertilizers in combination with recycled nutrients is essential to meet crop requirements and to increase crop production. Environmental hazards and economic constraints limit increasing the use of plant nutrients which is necessary for the intensification of agriculture. In industrialized countries, environmental issues and international trade agreements restrict production of surplus food and restrictions on further intensification. However, in developing countries, the high cost of external sources of nutrients and their inadequate availability limit intensification. Therefore, the importance of plant nutrients in agricultural production necessitates establishing relationships between yield, use of plant nutrients, economic feasibility and environmental quality. Here some questions arise that farmers need to know how much and which plant nutrients should be supplied to obtain the optimum economic increase in yield without or minimum damaging the environment.

7.5.4. Soil fertility management for biofortification

Micronutrient deficiencies in soils not only limit crop growth and yields, but also decrease the nutritional quality of agricultural produce causing diseases in human. The people of developing countries and/or poor countries whose populations are mostly fed on staple crops are more prone to such diseases. Most of the micronutrient required by plants are also essential for humans and plants are the only source of these minerals for animals and humans, otherwise food supplements are required which are out of approach of poor populations of in developing world. Multiple micronutrient deficiencies in humans have been reported in african countries which are due to their deficiencies in soils. Application of micronutrients to soil or plant leaves to get it accumulated in edible parts for human and animal nutrition is called biofortification. Application of Zn has given very successful results in Turkey to eliminate the malnutrition due to Zn deficiency. Mining of the micronutrients need to be replenished micronutrients to fulfill the crop as well as human requirements. Application of organic sources is also providing diversity of nutrients to soils including micronutrients.

7.6. Conclusion

Provision of the soil test based diagnosed deficient plant nutrients in adequate and balanced dosage to plants is vital for their optimum growth and productivity. However, calculation of optimal dose of fertilizer nutrients for maximum growth is

quite complex. There are several factors which affect availability of nutrients for plants even if they are present in soil in higher concentrations. Movement of nutrients within the plants also involves a number of physiological and biochemical processes. Therefore, best management of fertility to provide optimum nutrition is integrated to many processes. Use of chemical fertilizers has contributed a lot to increase crop yield and food production, however keeping in view environmental factors and agricultural sustainability it is the time use integrate nutrient management using green manuring, composting, biochar, etc. along with chemical fertilizers.

References

Abbasi, M.K., Z. Shah and W.A. Adams (2003). Effect of the nitrification inhibitor nitrapyrin on the fate of nitrogen applied to a soil under laboratory conditions. J. Plant Nutr. Soil Sci. 166: 1-6.

Ahmad, N. (2011). Pakistan, In: Case studies on policies and strategies for sustainable soil fertility and fertilizer management in South Asia. Food and Agriculture Organization of the United Nations, Regional Office for Asia and The Pacific, Bangkok, 2011. Rap Publication 2011/09, pp.73-120.

Ahmad, N. and M. Rashid (2003). Fertilizers and their use in Pakistan. National Fertilizer Development Corporation, Planning and Development Division, Islamabad, Pakistan

Ahmad, N.A. and M. Tila (1998). Fertilizer, plant nutrition management and self-reliance in agriculture. The Pakistan Development Review 37: 217–233. Annual reports 2008FFC, ECPL and DH

Ahmad R., A. Naseer, Z.A. Zahir, M. Arshad, T. Sultan and M. Arshad-Ullah (2006). Integrated use of recycled organic wastes and chemical fertilizers for improving maize yield. Int. J. Agric. Biol. 6: 840–843.

Alberts, B., A. Johnson, J. Lewis, M. Raff, K. Roberts and P. Walter (2007). Molecular Biology of the Cell, 4th edition. Garland Science, New York, USA.

Arnon, D.I. and P.R. Stout (1939). The essentiality of certain elements in minute quantity for plant with special reference to copper. Plant Physiol. 14: 371–375.

Arshad, M., A. Khalid, M.H. Mahmood and Z.A. Zahir (2004). Potential of nitrogen and L-tryptophan enriched compost for improving growth and yield of hybrid maize. Pak. J. Agri. Sci. 41: 16–24.

Aziz T., I. Ahmed, M. Farooq, M.A. Maqsood and M. Sabir (2011). Variation in phosphorus efficiency among Brassica cultivars I: internal utilization and phosphorus remobilization. J. Plant Nutr. 34: 2006−2017.

Barker, A.V. and D.J. Pilbeam (2007). Handbook of Plant Nutrition. CRC Press, Taylor & Francis Group, New York, USA.

Black, C.A. (1993). Soil Fertility Evaluation and Control. Lewis Publishers, Boca Raton, FL, USA.

Bould, C, E.J. Hewitt and P. Needham (1983). Diagnosis of Mineral Disorders in Plants, Volume 1, Principles. Chemical Publishing, New York, USA.

Bray, R.H. (1954). A nutrient mobility concept of soil plant relationships. Soil Sci. 78: 9−22.

Jones, C. and J. Jacobsen (2001). MSU Extension Soil Scientist Nutrient Management Module No. 2. Montana State University Extension Service. 4449–4452.

Dhaliwal, S.S., J.S. Manchanda, S.S. Walia and M.K. Dhaliwal (2013). Differential response of manures in transformation of DTPA and total zinc and iron in rice transplanted on light textured soils of Punjab. Int. J. Sci. Environ. Technol. 2: 300–312.

DIN S.M. and S.H. Jafery (2007). Pakistan Fertilizer Sector Review. Fertilizer Pakistan IGI Securities. www.igisecurities.com.pk.

Eck, H.V. and B.A. Stewart (1995). Manure. In: J.E. Rechcigl (Ed.) Environmental aspects of soil amendments. Lewis Publishers, Boca Raton, Florida, pp. 169–198.

Epstein, E. (1996). The Science of composting. Technomic Publishing Company, Lancaster, PA, USA.

Epstein, E. and A.J. Bloom (2005). Mineral Nutrition of Plants: Principles and Perspectives, 2nd edition. Sinauer, Sunderland, MA, USA.

Fageria, N.K. (2009). The Use of Nutrients in Crop Plants. CRC Press, Boca Raton, FL, USA.

Fageria N.K., V.C. Baligar and C.A. Jones (2010). Growth and Mineral Nutrition of Field Crops. CRC Press, Boca Raton, FL, USA.

FAO (2011). Case studies on policies and strategies for sustainable soil fertility and fertilizer management in South Asia. Food and Agriculture Organization of the United Nations Regional Office for Asia and The Pacific Bangkok, 2011. Rap Publication 2011/09.

Filippelli, G.M. (2002). The Global Phosphorus Cycle. Rev. Mineral. Geochem. 48: 391–425.

Giller, K.E., G. Cadisch, C. Ehaliotis, E. Adams, W.D. Sakala and P.L. Mafongoya (1997). Building soil nitrogen capital in Africa. In: R.J. Buresh, P.A. Sanchez, F. Calhoun (Eds.), Replenishing Soil Fertility in Africa, Soil Science Society of America Special Publication 51. Madison, WI, USA. pp. 151–192.

Goldy, R. (2013). Knowing nutrient mobility is helpful in diagnosing plant nutrient deficiencies. Michigan State University Extension. http://msue.anr.msu.edu/news Accessed on on 14 November, 2016

GoP (2008). Agricultural statistics of Pakistan. Ministry of Food, Agriculture and Livestock, Government of Pakistan, Islamabad.

Havlin, J.L., S.L. Tisdale, J.D. Beaton and W.L. Nelson (2014). Soil Fertility and Fertilizers: An Introduction to Nutrient Management, 8th edition. Pearson Education, Upper Saddle River, NJ, USA.

Ibrahim, M., Anwar-ul-Hassan, M. Iqbal and E.E. Valeem (2008). Response of wheat growth and yield to various levels of compost and organic manure. Pak. J. Bot. 40: 2135–2141.

Jones, Jr., J.B., B. Wolf, and H.A. Mills (1991). Plant Analysis Handbook: A Practical Sampling, Preparation, Analysis and Interpretation Guide. Micro-Macro Publishing, Athens, GA.

Jones Jr., J. Benton (2001). A Laboratory Guide for Conducting Soil Tests and Plant Analyses. CRC Press, Boca Raton, FL, USA.

Jones Jr., J. Benton (2003). Agronomic Handbook: Management of Crops, Soils, and Their Fertility. CRC Press, Boca Raton, FL, USA.

Jones Jr., J. Benton (2012). Plant Nutrition and Soil Fertility Manual, 2nd edition. CRC Press, Boca Raton, FL, USA.

Jones Jr., J. Benton (2011). Hydroponic Handbook: How hydroponics growing systems work? Gro Systems, Anderson, SC, USA.

Kaihura, F.B.S., I.K. Kullaya, M. Kilasara, J.B. Aune, B.R. Singh and R. Lal (1999). Soil quality effects of accelerated erosion and management systems in three ecoregions of Tanzania. Soil Tillage Res. 53: 59–70.

Kausar, M.A., F.M. Chaudhry, A. Rashid, A. Latif and S.M. Alam (1976). Micronutrient availability to cereals from calcareous soils. I. Comparative Zn and Cu deficiency and their mutual interaction in rice and wheat. Plant Soil 45: 397–410

Krauss, A. (2003). Balanced Fertilization in Contemporary Plant Production. The Role of Potassium. Regional IPI Workshop. Lithuania, India.

Lasaridi, K.E. and E. Stetiford (1999). Composting of source separated MSW: An approach to respirometric techniques and biodegradation kinetics. International Symposium on “Composting of organic matter”, August 31 – September 6, Kassandra-Chalkidiki, Greece.

Lehmann, J. (2007). A handful of carbon. Nature 447: 143–144. Lupwayi, N.Z. and I. Haque (1999). Leucaena hedgerow intercropping and cattle

manure application in the Ethiopian highlands, 3: Nutrient balances. Biol. Fert. Soils 28: 204–211.

Lupwayia, N.Z., P.E. Olsena, E.S. Sandeb, H.H. Keyserb, M.M. Collinsa, P.W. Singletonb and W.A. Ricea (2000). Inoculant quality and its evaluation. Field Crops Res. 65: 259–270.

Marshner, H. (1995). Mineral Nutrition of Higher Plants, 2nd ed. Academic Press, London, UK.

Mengel, K., E.A. Kirkby, H. Kosegarten and T. Apple (2001). Principles of Plant Nutrition. Kluwer Academic Publishers, Dhodrecht, Germany.

Miles, N., J.H. Meyer and R. Van Antwerpen (2008). Soil organic matter data: What do they mean. Proc. South Afric. Sugar Technol. Ass. 81: 324–332

Moran-Zuloaga, D., Dippold, M., Glaser, B. Kuzyakov, Y. (2015). Organic nitrogen uptake by plants: reevaluation by position-specific labeling of amino acids. Biogeochemistry 125: 359-374.

Nahm, K.H. (2005). Factors influencing nitrogen mineralization during poultry litter composition and calculations for available nitrogen. World’s Poul. Sci. J. 61: 238–253.

Naidu, D.K., B.M. Radder, P.L. Patil, N.S. Hebsur and S.C. Alagundagi (2009). Effect of integrated nutrient management on nutrient uptake and residual fertility of chilli (cv. Byadgi dabbi) in a Vertisol. Karnataka J. Agric. Sci. 22: 306–309.

NFDC (2011). Fertilizer related statistics. National Fertilizer Development Corporation, Planning Commission, Islamabad, Pakistan.

Noggle, G.R. and G.J. Fritz (1986). Introductory Plant Physiology, Prentice-Hall of India, New Delhi, India.

Probert, M.E., J.R. Okalebo and R.K. Jones (1995). The use of manure on smallholders’ farms in semi-arid eastern Kenya. Exper. Agric. 31: 371–381.

Radke, L.F., D.A. Hegg, J.H. Lyons, C.A. Brock, P.V. Hobbs, R. Weiss and R. Rasmussen (1988). Airborne measurements on smokes from biomass burning, in Aerosols and Climate, edited by P. V. Hobbs and M.P. McCormick, pp. 411–422, A. Deepak Publishing, Hampton, Va., 1988.

Rashid, A. (2005). Establishment and management of micronutrient deficiencies in Pakistan: a review. Soil Environ. 24: 1–22.

Rashid, A. (2006). Boron Deficiency in Soils and Crops of Pakistan: Diagnosis and Management. Pakistan Agricultural Research Council, Islamabad, Pakistan, pp 34.

Rashid, A. and E. Rafique (2002). Boron deficiency in cotton grown in calcareous soils of Pakistan. II. Correction and criteria for foliar diagnosis: p. 357–362, In: H.E. Goldbach et al. (Eds.), Boron in Plant and Animal Nutrition. Kluwer Academic/Plenum Publishers, New York.

Rashid, A., and J. Ryan (2008). Micronutrient constraints to crop production in the Near East: Potential significance and management strategies. p 149–180, In: Alloway BJ (Ed), Micronutrient Deficiencies in Global Crop Production. Springer, Heidelberg, Germany.

Rashid, A., M. Yasin, M.A. Ali, Z. Ahmad, and R. Ullah (2007). An alarming boron deficiency in calcareous rice soils of Pakistan: Boron use improves yield and cooking quality. p 103–116, In: F Xu (Ed) Advances in Plant and Animal Boron Nutrition. Proc 3rd Int Symp on All Aspects of Plant and Animal Boron Nutrition, Wuhan, China, 9–13 Sep 2005. Springer, Dordrecht, The Netherlands.

Reuter, D.J., Edwards, D.G., and Wilhelm, N.S. (1997). Temperate and Tropical Crops. In Plant Analysis: An Interpretation Manual, 2nd ed.; Reuter, D.J. and Robinson, J.B. (Eds.). CSIRO Publishing: Collingwood, Victoria, Australia, p. 81–284.

Sanford, R.L. Jr. (1989). Fine root biomass under a tropical forest light gap opening in Costa Rica. J. Trop. Ecol. 5: 251–256.

Schjonning, P., S. Elmholt, J. Munkholm and K. Debosz (2002). Soil quality aspects of humid sandy loams as influenced by organic and conventional long-term management. Agri. Ecosys. Environ. 88: 195–214.

Schubert, S. (2006). Pflanzenernaehrung. Verlag Eugen, Ulmer, Stuttgart, Germany Smil, V. (2000). Cycles of Life. Scientific American Library, New York, USA. Smith, B., R.L. Richards and W.E. Newton (2004). Catalysts for nitrogen fixation:

nitrogenases, relevant chemical models and commercial processes. Kluwer Academic Publishers, Dordrecht, Germany.

Soumare, M., F. Tack and M. Verloo (2003). Characterization of Malian and Belgian solid waste composts with respect to fertility and suitability for land application. Waste Manage. 23: 517–522.

Stamatiadis, S., M. Werner and M. Buchanan (1999). Field assessment of soil quality as affected by compost and fertilizer application in a broccoli field (San Benito County, California). Appl. Soil Ecol. 12: 217–25.

Stevenson, F.J. (1982). Origin and Distribution of N in Soil. In: F.J. Stevenson (Ed.), Nitrogen in Agricultural Soils, American Society of Agronomy, Madison, WI, USA.

Tahir, M., M. Arshad, M. Naveed, Z.A. Zahir, B. Shaharoona and R. Ahmad (2006). Enrichment of recycled organic waste with N fertilizer and PGR containing

ACC-deaminase for improving growth and yield of tomato. Soil Environ. 25: 105–112.

Taiz, L. and E. Zeiger (2010). Plant Physiology, 5th Edition. Sinauer Associates, MA, USA.

USDA (2000). Composting. Part 637, National Engineering Handbook, NRCS, U.S. Department of Agriculture, Washington, DC, USA.

USDA-NRCS (Natural Resources Conservation Service) (2010). Chapter 2, Composting. Part 637 Environmental Engineering National Engineering Handbook. 210–VI–NEH, Amend. 40 November 2010. Publisher, city and country?

Wakeel, A. (2013). Potassium-sodium interactions in soil and plant under saline-sodic conditions. J. Plant Nutr. Soil Sci. 176: 344–354.

Wakeel, A., M. Farooq, M. Qadir and S. Schubert (2011). Substitution of potassium by sodium in plants. Crit. Rev. Plant Sci. 30, 401–413.

Wakeel, A., M. Gul and M. Sanaullah (2013). Potassium dynamics in three alluvial soils differing in clay contents. Emir. J. Food Agric. 25, 39-44.

Warman, P.R. and J.M. Cooper (2000). Fertilization of a mixed forage crop with fresh and composted chicken manure and NPK fertilizer: Effects on soil and tissue Ca, Mg, S, B, Cu, Fe, Mn and Zn. Can. J. Soil Sci. 80: 345–352

Yaseen, M. (2014). Efficient use of nitrogen and phosphorus fertilizers for crop yield development. Paper presented and Shield received from IPI-ISES Capacity Building Seminar on “Balance Use of Fertilizers for Quality Produce and Sustainable Agriculture” organized by Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad in collaboration with International Potash Institute on August 20, 2014 at Serena Hotel, Faisalabad.

Yaseen, M., W. Ahmad and M. Shahbaz (2013). Role of foliar feeding of micronutrients in yield maximization of cotton in Punjab. Turk. J. Agric. Forest. 37: 420–426.

Zahir, Z.A., M. Naveed, M.I. Zafar, H.S. Rehman, M. Arshad and M. Khalid (2007). Evaluation of composted organic waste enriched with nitrogen and l-tryptophan for improving growth and yield of wheat (Triticum aestivum L.). Pak. J. Bot. 39: 1739–1749.

Zou, C.Q., Y.Q. Zhang, A. Rashid, H. Ram, E. Savasli, Z. Arisoy, I. Ortiz-Monasterio, S. Simunji, Z.H. Wang, V. Sohu, M. Hassan, Y. Kaya, O. Onder, O. Lungu, M.Y. Mujahid, A.K. Joshi, Y. Zelensky, F.S. Zang, and I. Cakmak (2012.) Biofortification of wheat with zinc through zinc fertilization in seven countries. Plant Soil 361: 43–55.

Chapter 8

Soil Organic Matter:

Significance, Sources and

Functions

Muhammad Aamer Maqsood, Tariq Aziz, Muhammad Sabir,

Abdul Wakeel, Muhammad Sanaullah and Muhammad Zia-Ur-

Rehman†

Abstract

Soil organic matter (SOM) is a dynamic component in soil and acts as a source of food for microbes and inorganic nutrients for plants. It improves soil structure resulting in better aeration, water movement and increases resistance to soil erosion. Being highly porous and having high surface area, it improves nutrient and water holding capacity. Organic matter content in soil is a reflector of soil health. Sources of organic matter include animal manures, municipal sewage sludge, logging and wood manufacturing refuse, industrial organic residues, and food processing residues. Several soil factors are responsible for soil organic matter buildup including soil microbial activity, temperature, soil pH, soil moisture etc. Microorganisms, including bacteria, actinomycetes and fungi decompose organic matter in the soil. Carbon/nitrogen ratios (C:N ratio) of residues controls the rate of organic matter decomposition. In order to overcome current stagnation in agriculture and to increase profitability on a sustainable basis, understanding the dynamics of organic matter is

†Muhammad Aamer Maqsood*, Tariq Aziz, Muhammad Sabir, Abdul Wakeel, Muhammad Sanaullah and Muhammad Zia-Ur-Rehman Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: mohamgill@uaf.edu.pk Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

176 M.A. Maqsood, T. Aziz, M. Sabir, A. Wakeel, M. Sanaullah and M. Zia-Ur-Rehman

the need of the day. This chapter is a discussion on the available sources of organic matter, factors affecting its buildup and its effects on soil and environment.

Keywords: Organic matter, Sources, Significance, C:N ratio, Decomposition

8.1. Introduction

Soil is the most important natural resource which serves as a natural medium for plant growth and a natural sink for variety of environmental pollutants. Soil is a complex medium which is composed of solid, liquid and gaseous components. In mineral soils, half volume is occupied by solids and rest half is occupied by pore space, shared jointly by water and gases. Solid portion of mineral soils is occupied by 48 % mineral particles and 2% by organic matter. Most agricultural soils contain organic matter in the top 25 cm of soil (Duxbury et al. 1989). The organic component of soil, mostly comprising dead organisms, plant matter and other organic materials at different stages of decomposition, plays a significant role in nutrient management in the soil as it acts as a reservoir or storehouse of plant nutrient. Although in mineral soils, OM is very low but still it can considerably modify a soils’ physical properties and strongly affect its chemical and biological properties. Soil organic matter is a very dynamic and vital portion of the soil (Magdoff and Weil, 2004). It is a reservoir of nitrogen; it supplies large portions of the soil phosphorus and sulfur; protects against soil erosion; improves aggregate formation; and it loosens up the soil to provide better aeration and water movement. Organic matter must be readily decomposable and continuously replenished with fresh residues.

Organic material is decomposed by microorganisms; predominantly by bacteria, actinomycetes, and fungi (Holland and Coleman 1987). These decomposers produce enzymes that are the proteinous substances directly responsible for the decomposition, by reducing the activation energy, necessary to break the bond of the organic materials. Many different enzymes are required for decomposition of the complex variety of organic substances. Nitrogen, either from organic residue or nitrogen already in the soil, greatly affects the rate of decomposition. Residues with carbon/nitrogen ratios (C:N ratio) wider than about 30:1 will have a low decomposition rate due to deficiency of available N. Nutrient balance is one among others factors that promote optimum organic matter decomposition (Dalal and Mayer 1986; Rice 2002). Organic matter residues are collectively called humus, after the active decomposition has taken place.

Organic matter (humus) is a dynamic portion of soil and must be replenished continuously to maintain soil productivity. This can be supplemented by applying organic amendments, such as animal manures, municipal sewage sludge, logging and wood manufacturing refuse, industrial organic residues, and food processing residues. Crop residues comprise of about 70%, animal manures about 23%, logging and wood manufacturing wastes about 5%, and each of the other groups less than 1 percent of total residues. In many countries including Pakistan, some of the residues available are used for other purposes such as fodder for animals, building structures and fuel.

Soil Organic Matter: Significance, Sources and Functions 177

Overall organic matter plays a pivotal role in sustainable and profitable agriculture. Understanding the dynamics of organic matter may help in alleviating the current stagnation in various agricultural systems. This chapter aims to discuss the current sources of organic matter, factors affecting its buildup and its effects on soil and environment.

8.2. Sources of Soil Organic Matter

8.2.1. Animal manure

Animal manures supply most of the macronutrients as well as micronutrients necessary for plant growth. Its fertilizing effect on crops can be compared to that of mineral fertilizers. Application of manure in a certain year influences not only the first crop grown but its residual effect also benefits the succeeding crops in later crop seasons/years because of extended time span required for decomposition of the manure. Application of farmyard manure is synergistic to mineral fertilizers for various nutrients as it improves soil growth conditions as well as replenishes plant nutrients. Decomposition of farmyard manure by soil microorganisms results in the release of CO2, water and plant nutrients, such as N, P, K and micronutrients. In general, after about one year, almost all the applied farmyard manure becomes part of soil organic matter and a small portion is transformed to humus, which is resistant to further microbial breakdown. Enhancement in soil organic matter adds to cation exchange capacity (CEC) of the soil, thus increasing its retention capacity for plant nutrients through adsorption. It improves soil structure which is less susceptible to compaction or erosion. Soil organic matter also raises water holding capacity of the soil. Improvement in soil organic matter decreases soil bulk density by improving total porosity. Besides improving physical, chemical and biological properties of soil, organic matter application also improves availability of micronutrients which are gaining increase importance because their deficiency is known to impair yield as well as quality of crop produce (Dhaliwal et al. 2013). Animal manure is used for maintaining and restoring fertility and productivity of soil since ancient time and is now considered an important and cheap method of recycling nutrients (Schjonning et al. 2002; Radke et al. 1988).

Although, composition of manures largely depends on the type of diet and animals (Eck and Stewart 1995; Probert et al. 1995) but collection, storage and application methods of manures are also important (Giller et al. 1997). Storage method has high influence on the composition of manure, even keeping all other factors constant. For instance, Sanford (1989) observed increase in N, P and K concentration by 108, 20 and 62%, respectively in buried manure compared to loosely heaped manure in open air. Application of manure to the soil can enhance the level of nutrients in soil. For example, an application of 3 tonnes of dry matter ha-1 would add macronutrients like N (35-82), P (7-21), K (32-163), Ca (30-74), and Mg (10-37) kg ha-1, while micronutrients such as Fe, Mn, Cu, and Zn in the range of 11-67, 0.8-5.7, 0.02-0.26 and 0.15-0.65 kg ha-1 respectively. It is not possible that all the nutrients from the manure will be released in the applied season (Lupwayi et al. 2000). Only 28, 19 and 90% of N, P and K, release was observed respectively in the first season (Lupwayi

and Haque 1999). Kaihura et al. (1999) and Warman and Cooper (2000) have reported an increase in soil ammonical and nitrate nitrogen i.e. total nitrogen in accordance with increasing rates of annual manure addition.

Animal manure in Pakistan consists mainly of animal dung and urine, FYM, poultry manure, crop residues, and slaughterhouse waste. None of these are being produced or marketed in an organized manner in the country. The farmers collect and use what is available on-farm. This is primarily FYM consisting mainly of animal droppings, straw and litter used in animal bedding and fodder residues. FYM has a variable nutrient value depending on the type of livestock and storage conditions. It has been estimated by the National Fertilizer Development Centre (NFDC) that over 1.5 million tonnes of nutrient are available from FYM in the country. Of this, nitrogen accounts for 726000, P2O5 for 191000 and K2O about 617000 tonnes. About 50% of animal dung in Pakistan remains uncollected. Out of the collected animal dung, 50% is used as fuel in the form of dried cake. The remaining is usually heaped on top of ground surface with fodder residues and other house sweepings for manuring. The nitrogen of the manure undergoes transformations and mostly lost by volatilization and leaching. Therefore, the remaining manure material that is finally spread on the fields may have very low nitrogen content. Poultry farmers sell bird droppings directly to farmers. None of the poultry farmers maintains records about either sales or use. Most crop residues are used as animal fodder or as fuel, leaving very little for recycling. Although composting plants have been installed at Lahore and Islamabad, difficulties have been experienced in marketing the finished product because of high production and transportation costs.

8.2.2. Green manure

Green manuring is another strategy to add organic matter to soil. Green manure is referred to as “A cover crop, mostly legume, which is ploughed into the soil when it is still in lush green stage”. Green manuring is an important practice on some progressive agricultural farms because this practice adds organic matter and nutrients, particularly biologically fixed N, to the soil. Incorporation of green plants into soil as manure can add a reasonable amount of N and moisture and also serve as a carbon source for soil organisms including microbes and earthworms. After decomposition by the microorganisms in the soil, the constituents of green manure like organic acids and many nutrients become available to crop plants (Rasmussen and Collins 1991). Another benefit of green manure is that its addition suppresses weeds and most of the soil-borne plant diseases.

Crops grown for green manuring are referred to as soil fertility building crops. Green manures are a gift from nature and a suitable alternative to improve organic matter level in the soil that in turn improves soil physical and biological properties. These properties assist soil and crop plants with pest, disease and weed management. Addition of several green manure crops grown over a period of perhaps five to ten years can have prominent and significant effects on soil physical properties.

Green manures are either grown and used in situ, or used as green leaf manure. In the in-situ method, green manure crops are grown in a field prior to crop cultivation and then cut and buried when approximately 50% of all plants are flowering. The use

of janter and sunn hemp is popular and well-practiced by most of the farmers. Because of their ability to grow fast and efficient nitrogen fixing capacity, these plants are grown to optimum vegetative stage and ploughed in to improve the living condition of the main crop.

8.2.3. Compost

Composting is a microbiologically mediated process, whereby manure and other organic wastes are partially converted into readily degradable organic matter and finally into stabilized contents. During this process, carbon of the organic matter is released partly as CO2, incorporated into microbial cells or humified contents. The organic N, primarily present as proteins, is mineralized into inorganic N (NH4-N and NO3-N) during the composting process, which is then re-synthesized into humic substances and microbial biomass. The rate of organic C breakdown during compositing by bacteria, fungi, and actinomycetes depends on the stage of degradation, characteristics of materials and temperature (Epstein 1996; USDA 2010). Composting could convert organic wastes into humus like substances which are easy to handle, store and to apply, also safe for the environment. It could stabilize nutrients in manures while reducing the total biomass (Lasaridi and Stetiford 1999; Nahm 2005).

The organic materials like city waste, poultry manure, and wastes from cotton, sugar and rice industries could be made accessible in large amounts and if not managed properly and accumulated for long time, could adversely affect land, water and air environments. Additionally, because of their anecdotal composition, water percentage and bulk nature, the use and haulage of these organic wastes is a real concern in today’s fuel conscious society. Pakistan lacks appropriate waste compilation and discarding system, so farmers use a variety of wastes without evaluating the effectiveness and economic feasibility (Arshad et al. 2004; Zahir et al. 2007).

Uninterrupted release and accumulation of different organic wastes constitute a real confront to environmentalists. Various remedies could be used but composting is the most economical, not only protecting the environment but also yielding an inexpensive and a quality soil amendment. Composting is the best solution for reducing the gigantic piles of organic wastes and yielding a value added product.

Direct use of fresh (un-composted) organic waste has many drawbacks due to wider C:N ratio compared to the composted material. Mature compost is normally superior to uncomposted organic materials, because well decomposed compost has a narrow C:N ratio, more concentrated nutrients, and is free from weed seeds, pathogens, and likely contaminants (Tahir et al. 2006).

8.2.4. Biochar

Biochar is a charcoal created by pyrolysis of biomass at high temperature in the absence of oxygen. It is considered a good soil amendment that has the potential to improve soil for sustainable crop production. Biochar has been used commonly in traditional as well as modern agricultural practices because of its very specific and

unique properties among organic soil amendments. Beneficial effects of biochar on both functions of soil microbial and availability of soil water are highly and well documented (Pramod et al. 2010; Chan et al. 2007; Yeboah et al. 2009).

Biochar improves sustainability of soils by converting agro-wastes into a powerful soil booster that holds carbon and thus makes soils more fertile. It can preserve cropland diversity to enhance food security by discouraging deforestation. Some of the salient benefits include reduction in leaching of nitrate into the ground water; reduction in emissions of nitrous oxide to the atmosphere; improvement in soil fertility by increasing CEC and water holding capacity and proliferation of variety of beneficial soil microbes

Biochar is more persistent in soils compared with all other forms of organic matter commonly applied to soil. Biochar is considered long lasting and alternative management tool to farmyard manure because of its benefits associated with nutrient retention and soil fertility. This long lasting effect of biochar in soil makes it a prime candidate for the mitigation of climate change. Biochar has the potential to create a carbon sink, as about 50% of the original carbon is retained in the biochar during the process of conversion (Lehmann 2007).

Biochar offers a unique opportunity to improve soil fertility, either used alone, or in combination with compost and manure. Moreover, biochar application rates are reduced significantly when used with biochar as a soil amendment. A single application of biochar can provide benefits over many years because it persists in the soil for a long period. Without a doubt, all types of organic materials added to soil improve different soil properties and functions such as to retain several essential nutrients to plant growth, but one of the special features associated with biochar is its effectiveness in retaining most of nutrients in available form to plants compared to leaf litter, compost or manures (Lehmann 2007).

Biochar reduces soil acidity more than soil alkalinity. Biochar application to an acidic soils brings more benefits on crop growth than in alkaline soils. Nevertheless, it is possible to produce biochar suitable for alkaline soils with little or no liming capacity. Upon addition of biochar to an acidic soil, there is increase in pH in ranges that minimize detrimental effect of nutrient deficiencies on crop yields. Biochar made from manures and bones is exceptionally good as it has potential to reduce fertilizer requirements by holding soil nutrients in easily available form. This results in reduction of fertilization costs and retention of fertilizer (organic or chemical) in the soil for longer time. Biochar develops plentiful negative charge on its surfaces to hold nutrients and buffer acidity in the soil as does organic matter in general.

The stability of biochar provides basis to determine the environmental benefits of biochar due to two reasons: 1) How long carbon in biochar will remain sequestered in soil and contribute for mitigating climate change? 2) How long biochar can benefit soil and water quality? Characteristics of biochar vary depending upon the source material and method used to prepare biochar. Most biochars have both a small amount of easily decomposable fraction as well as much larger amount of stable fraction.

8.3. Decomposition of Organic Matter

Decomposition is a biologically mediated process that includes the physical breakdown and biochemical conversion of complex organic molecules of dead material into simpler organic and inorganic molecules (Juma 1998). When organic substances are manufactured by plants, energy from the sun through the process of photosynthesis is stored in these substances. When these substances are decomposed, the stored energy is again released. Carbon dioxide (CO2), water, plant nutrients and energy are liberated in the decomposition process, different. The process of decomposition has an energy barrier, called activation energy, which must be overcome. When wood is burned, the activation energy is elevated. In nature, few reactions such as lightening are available to provide this heat energy. For most biological processes to occur under natural conditions, enzymes are a prerequisite to reduce this activation energy (Alvarez and Alvarez 2000).

8.3.1. Composition of organic matter

Organic matter (OM) comprises of carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, and many other elements (Paustian 2002). Major fraction of OM consists of carbon atoms, linked together into carbon chains of numerous lengths and linkages and thus constitute the basic “frame” of organic compounds. The remaining elements fill out the frame to make different groups of organic matter substances called proteins, lignins, carbohydrates, oils, fats, waxes, and many other materials (Wander 2004). Humic substances are the dark brown, colloidal, amorphous, polymeric components of soil organic matter.

Soil humus is left after extensive chemical and biological breakdown of fresh plant and animal residues and makes up 60-70 percent of the total organic carbon in soils. Humus is often divided by solubility separations into fulvic acid and humic acid. Both fulvic acid and humic acid are soluble in dilute sodium hydroxide solutions, but humic acid will precipitate when the solution is made acidic. Humin is the portion of humus which is insoluble in dilute sodium hydroxide.

The nature of soil humus is extremely complex. In addition to humin, humic acid, and fulvic acid, some of other specific substances comprising soil humus are sugar amines, nucleic acids, phospholipids, vitamins, sulfolipids, and polysaccharides. All of these substances are of complex nature and uncertain origin. They may be residual from plant tissues, synthesized by microbes, or residues of microbial degradation.

8.3.2. Products of decomposition

In soils with sufficient aeration, the final products of decomposition are CO2, NH4+,

NO3-, H2PO4

-, SO42-, H2O, resistant residues, and numerous other essential plant

nutrient elements in smaller quantities (Alvarez and Alvarez 2000). If soil is not well aerated, less desirable products are formed. For example, in anaerobic conditions significant amounts of methane (CH4), also called “swamp gas”, is produced, as well as some organic acids (R-COOH), ammonium (NH4+), various amine residues (R-

NH2), the toxic gases like hydrogen sulfide (H2S), dimethyle sulfide, ethylene (H2C=CH2), and the resistant humus residues are formed.

8.3.3. Factors affecting organic matter decomposition

The most important conditions that alter the accumulation of soil organic matter are:

Low temperature retards plant growth and organic matter decomposition. If temperature is high enough to produce considerable vegetation during the growing season, but are cold for long periods at other times of the year, organic matter accumulation in and on the soil, will be high. Continuous low temperature lowers soil humus because even though, humus persists, plant growth is lowered. Continuous moderate to warm temperature aid high plant production but also promote faster decomposition.

8.3.3.2. Soil moisture

Both plant growth and organic matter decomposition require moisture. Extremes of both arid (dry) and water logged (anaerobic) conditions reduce plant growth and microbial decompositions. Poorly drained soils with growing vegetation usually have relatively high humus contents; such conditions have been the cause of formation of some organic soils.

8.3.3.3. Nutrients

Lack of nutrients, particularly nitrogen, usually reduces plant growth more than it slows decomposition because microorganisms use the nutrients in the dead organic material before plant roots can absorb it (Duxbury et al. 1989).

8.3.3.4. Soil pH

For most common microorganisms, pH 6-8 is best suited for growth and are significantly reduced below pH 4.5 and above pH 8.5. Strongly acid soils are even more inhibiting to microbial growth than are strongly alkaline ones. Plants tolerate pH extremes more readily than microbes.

8.3.3.5. Soil texture

Soils higher in clays tend to retain larger amounts of humus. Most organic substances adhere to mineral surfaces by many kinds of bonds, particularly to clays. The many active bonding sites of minerals and humus include the =O, ̶ OH, ̶ Al ̶ OH, ̶ Fe ̶ OH, and cation exchange sites of minerals, and the –NH3 , -SH , -OH , and –COOH portions of organic materials.

8.3.3.6. Other factors

Other decomposition inhibitors include toxic levels of elements (aluminum, manganese, boron, selenium, chloride), excessive soluble salts, shade, and organic phytotoxins (toxic to plants). The type of plant is also important, as legumes are more readily decomposed than grasses. Soil organic matter buildup is a resultant of all these effects plus the degree of mixing into the soil. Mixing the plant residues into

soil (tillage) speeds up decomposition and lessens accumulation. Cultivation reduces vegetative growth (by destroying weeds) and speeds up decomposition by mixing.

8.3.4. Rate of decomposition and carbon: nitrogen ratio

Most nutrients needed by plants are also required by microorganisms that decompose organic matter. However, nitrogen concentration (which is in smaller proportion than carbon) most often controls the rate of organic matter decomposition because it is needed to build proteins in new bacterial and fungal populations. The nitrogen content in the microorganisms and in organic materials is given in proportion to the carbon content and is called the carbon: nitrogen ratio (C: N ratio) (Ge et al. 2013). A wide organic carbon: total nitrogen ratio indicates a material relatively in low nitrogen content. Table 8.1 lists some common organic materials and the percentages of the total weight that are organic carbon and total nitrogen. Bacteria requiring one kilogram of nitrogen for each four or five kilogram of carbon (C: N ratio of 4:1 or 5:1), are heavy users of nitrogen. If straw with its lower proportion of nitrogen (C: N ratio of 80:1) is incorporated into a soil low in nitrogen, bacteria will increase slowly because the straw is a low nutrient “food” for the decomposing microorganisms. The process of decay can be enhanced by adding more nitrogen (usually from fertilizer) to meet microbial and plant needs. Bacteria (or fungi) will use any available nitrogen in the soil. Plants growing in a nitrogen deficient soil are deficient in nitrogen because the soil microorganisms, which are more abundant and in more intimate contact, are able to use most available nitrogen before it can become accessible to plant root surfaces. The same is true for phosphorus and sulfur in organic residue and soil and to a lesser extent is true for other nutrients as well.

Table 8.1 C:N Ratio of Common Organic materials Applied to Agriculture Soils

Organic Material Organic Carbon (%) Total Nitrogen (%) C: N Ratio

Crop Residues

Alfaalfa (very young) Clovers (mature) Bluegrass Cornstalks Straw, small grain Sawdust Soil Microbes

Bacteria Actinomycetes Fungi Soil Humus

40 40 40 40 40 50

50 50 50 50

3.0 2.0 1.3 1.0 0.5 0.1

10.0 8.5 5.0 4.5

13: 1 20: 1 30: 1 40: 1 80: 1

500: 1

5: 1 6: 1

10: 1 11: 1

As progressive decay of organic matter continues, much of the carbon escapes into the atmosphere as carbon dioxide (CO2). Eventually the easily decomposable residues decompose rapidly while resistant fraction of OM decompose slowly and persist in the soil for longer time. The food and energy source is now in short supply and some of the bacteria and fungi die. Their bodies, having high nitrogen content, are decomposed by other living organisms, evolving carbon as carbon dioxide and

releasing some nitrogen to the soil solution. This “released” nitrogen is available to growing plants.

In soil, the oxidation or breakdown of chemical complexes in organic matter into plant available forms is known as minerlaiztion. This is the opposite of immobilization. If C: N ratio is 20:1 or narrower in plant residues, then there would be net mineralization of OM because sufficient nitrogen will be available meet the requirements of decomposing microorganisms. If plant residues have C: N ratios of 20:1 to 30:1, sufficient nitrogen will be available for decomposing microorganisms but not enough to result in much release of nitrogen for plant use. The first few weeks after incorporation, residues with C: N ratios wider than 30:1decompose slowly because they lack sufficient nitrogen for the microorganisms to use for increasing their numbers, resulting in the use of nitrogen already present in the soil. If environmental conditions are favorable, the rate of decomposition of plant residues is more rapid during the first two weeks after incorporation in to the soil. Sometimes residues with a wide C: N ratio may not have nitrogen immobilization, when incorporated into soils. If rates of decomposition are slow (large particle size, cool weather, quite dry), the need for the nitrogen by population of the decomposing micro-organisms is low (Rasmussen et al. 1991).

8.4. The Role of Soil Organic Matter

The importance of organic matter in soil is related to its control over fertility. Organic matter has a major influence on both physical and chemical fertility. Chemical fertility is concerned with the supply of nutrients to the growing crop, whereas physical fertility is concerned with the provision of soil conditions that allow plants to make optimum use of those nutrients. It involves the structure of the soil, or the physical arrangement of soil particles, whereby air and water supply to plants is optimal for the seasonal conditions and root penetration is not impeded. The third component of soil fertility is biological fertility which relates to the amount of organic matter in a soil and the activity of the microflora and microfauna that it supports. Cation exchange capacity of soil is improved due to the presence of organic matter. This soil chemical property is important in holding nutrients against leaching, and is particularly important for light textured soils. (Bell et al. 1998; Hudson 1994). Typically, soil population may consist of fungi, bacteria, actinomycetes, algae and protozoa, as well as the larger animals such as worms, molluscs, nematodes and small insects. Their importance in this context is their effect on soil properties, both direct and indirect, and this is related with the role of organic matter on the fertility of the soil.

8.4.1. Greenhouse gas control

Soil organic matter is also important as a possible sink/source for greenhouse gases, which trap radiated solar energy and thus keep the earth’s surface at a temperature level necessary to support life. These atmospheric gases, mostly carbon dioxide (CO2) but also methane (CH4) and nitrous oxide (N2O) being increased by anthropogenic activity such as burning fossil fuels and land clearing, and many

scientists believe that significant global warming is attendant with sea level and regional changes. The consequences of such changes could be social, economic and environmental disastrous (Ryals and Silver 2013).

Carbon dioxide is the main gas contributing to this enhanced greenhouse effect, and its small but highly significant concentration in the atmosphere has increased by about 30% in the last two centuries. This has occurred largely because of the burning of coal, oil and natural gas and the clearing and burning of natural vegetation (Krull et al. 2004).

The condition of the soil, while being vital for agricultural production, is also important in terms of providing a long-term ‘sink’ for atmospheric carbon. In the form of carbon dioxide, this carbon is tied up (sequestered) through the process of photosynthesis by plants to give organic compounds that ultimately come to reside in the soil for varying lengths of time.

The soil ecosystem has the potential to sequester carbon for very long time periods. Depending on soil conditions, these compounds are eventually broken down and the carbon is returned to the atmosphere through decomposition, and respiration of the micro-organisms involved. Methane and nitrous oxide may also be released as a result of the decomposition (Corsi et al. 2012).

This ‘sequestering potential’ of soils becomes important in the overall strategy to reduce green-house gas emissions and ties up carbon dioxide in the form of soil organic matter. Increasing organic carbon in soils also has benefits in terms of productivity and sustainability. Soils that have less organic matter (through over cropping, for example), and may be in a degraded condition, have substantial sink capacity for sequestering more carbon dioxide. In a stable ecosystem the emission of the gas from soils (due to decomposition and respiration) is more or less balanced by H input to soils (due to photosynthesis). However, recent quantification of these processes shows that emissions are exceeding inputs as a result of clearing and cultivation of natural vegetation. As the soil carbon pool is approximately twice the size of the atmospheric pool, there is considerable scope for this to continue if land management practices are not improved.

8.4.2. Effect of organic matter on soil health

The amount of organic matter in soils is affected by soil type, previous cropping history, climate, tillage practices, the amount and kind of plant or animal material returned or removed, and crop residue burning. The most significant of these in the long term is the amount of organic material returned to, or removed from the soil. In this context, plant roots have an important role since, when they decompose, not only the humic substances are added to the soil, but also the nutrients are released throughout the root zone, thus benefiting both the structure and the uptake of nutrients by the subsequent crop.

In unfertilized soils (virgin soils), organic matter is the source of 90-95% of the nitrogen and is a major source of available phosphorus and sulfur, where humus is present in appreciable amounts (>2%). Organic matter supplies polysaccharides directly, or indirectly through microbial action, that greatly improves soil

aggregation. Organic matter typically enhances water retention at field capacity, available water content in sandy soils, and rate of water infiltration in heavy textured soil. This latter effect is probably due mainly to soil aggregation, which produces larger soil pores. Organic matter acts as a chelating agent. Soluble chelates improve the retention and mobilization micronutrients and increase their availability to plants. Soil OM is source of carbon for many microbes that perform other beneficial functions in soil (e.g. free dinitrogen fixers, dinitrifires). As mulch on top of soil, organic matter reduces erosion, shades the soil (which prevents rapid moisture loss), and keeps the soil cooler in very hot weather and warmer in winter. Humus buffers the soil against a rapid change in acidity, alkalinity, and salinity; and damage by pesticides and toxic heavy metals. Surface organic matter acts as an insulator, retarding heat movement between the atmosphere and the soil. In hot summers, this benefits some plant roots, but in cool areas it slows soil warming in the spring.

Many benefits of organic matter in the soil are offset in certain conditions by detrimental influences. Organic matter is an energy and carbon source for many pathogens, ensuring their longer survival in the soils. Excessive amounts of organic matter are physically difficult to incorporate into the soils and hinder easy planting. Residues (or virgin vegetation on land being cleared) are often burned to reduce bulk. Although it is common practice (e.g. sugarcane and rice harvests), burning off crop residues is not desirable. Burning is harmful because it removes organic material that protects the soil against erosion, some ash that contains plants nutrients can be lost easily by wind or water erosion and most nutrients in the ash are soluble and some are easily leached through the soil. Numerous plants contain or produce phytotoxins (plant poisons: for example juglone from black walnuts), which make them undesirable as organic matter; so any and all plant materials should not be used indiscriminately in the soil. Unfortunately, the problem cannot always be avoided because the decomposition of many crop residues produces such toxins.

8.5. Conclusion

Sustainable productivity in any agricultural system, demands the maintenance of soil organic matter levels and the optimization of nutrient cycling. Humus, is the active fraction of organic matter, and it plays a very important function in aggregate stability and water infiltration. Maintaining soil organic matter content requires a balance between addition and decomposition. Intensive crop production globally is causing a reduction in soil organic matter content and consequently, a decline of soil fertility. Some part of the organic matter lost can be returned with improved land management practices. By adopting zero or minimum tillage practices, soil organic matter levels can be increased and agricultural soils can act as carbon sinks. Residue accumulation, including cover crops, farm yard manure, green manuring and crop residues improve the soil organic carbon and nutrient levels. Agricultural profitability can be substantially increased with careful management, preservation, and accumulation of soil organic matter.

References

Alvarez, R. and C.R. Alvarez (2000). Soil organic matter pools and their associations with carbon mineralization kinetics. Soil Sci. Soc. Am. J. 64: 184–189.

Arshad, M., A. Khalid, M.H. Mahmood and Z.A. Zahir (2004). Potential of nitrogen and L-tryptophan enriched compost for improving growth and yield of hybrid maize. Pak. J. Agri. Sci. 41: 16–24.

Bell, M.J., P.W. Moody, R.D. Connolly and B.J. Bridge (1998). The role of active fractions of soil organic matter in physical and chemical fertility of Ferrosols. Aust. J. Soil Res. 36: 809-819.

Chan, K. Y., L. Van Zwieten, I. Meszaros, A. Downieand and S. Joseph (2007). Agronomic values of green-waste biochar as a soil amendment. Aust. J. Soil Res. 45: 629–634.

Corsi, S., T. Friedrich, A. Kassam, M. Pisante and J. de Moraes Sà (2012). Soil Organic Carbon Accumulation and Greenhouse Gas Emission Reductions from Conservation Agriculture: A literature Review. Integrated Crop Management Vol.16. Food and Agriculture Organization of the United Nations. Rome.

Dalal, R.C. and R.J. Mayer (1986). Long-term trends in fertility of soils under continuous cultivation and cereal cropping in Southern Queensland. II. Total organic carbon and its rate of loss from the soil profile. Aust. J. Soil Res. 24: 281–292.

Dhaliwal, S.S., J.S. Manchanda, S.S. Walia and M.K. Dhaliwal (2013). Differential response of manures in transformation of DTPA and total zinc and iron in rice transplanted on light textured soils of Punjab. Int. J. Sci. Environ. Technol. 2: 300–312.

Duxbury, J.M., M.S. Smith and J.W. Doran (1989). Soil organic matter as a source and sink of plant nutrients. In: D.C. Coleman, J.M. Oades and G. Uehara, (Eds.), Dynamics of soil organic matter in tropical ecosystem, University of Hawaii Press, USA. pp. 33-67.

Eck, H.V. and B.A. Stewart (1995). Manure. In: J.E. Rechcigl (Ed.) Environmental aspects of soil amendments. Lewis Publishers, Boca Raton, Florida, pp. 169–198.

Epstein, E. (1996). The Science of Composting. Technomic Publishing Company, Lancaster, PA, USA.

Ge, S., H. Xu, M. Ji and Y. Jiang (2013). Characteristics of Soil Organic Carbon, Total Nitrogen, and C/N Ratio in Chinese Apple Orchards. Open J. Soil Sci. 3: 213-217.

Giller, K.E., G. Cadisch, C. Ehaliotis, E. Adams, W.D. Sakala and P.L. Mafongoya (1997). Building soil nitrogen capital in Africa. In: R.J. Buresh, P.A. Sanchez, F. Calhoun (Eds.), Replenishing Soil Fertility in Africa, Soil Science Society of America Special Publication 51. Madison, WI, USA. pp. 151–192.

Holland, E.A. and D.C. Coleman (1987). Litter placement effects on microbial and organic matter dynamics in an agro-ecosystem. Ecology 68: 425–433.

Hudson, B.D. (1994). Soil organic matter and available water capacity. J. Soil Wat. Con. 49: 189–194.

Juma, N.G. 1998. The pedosphere and its dynamics: a systems approach to soil science. Volume 1, Edmonton, Canada, Quality Color Press Inc. pp 315.

Kaihura, F.B.S., I.K. Kullaya, M. Kilasara, J.B. Aune, B.R. Singh and R. Lal (1999). Soil quality effects of accelerated erosion and management systems in three ecoregions of Tanzania. Soil Tillage Res. 53: 59–70.

Krull, E., J. Skjemstad and J. Baldock (2004). Functions of Soil Organic Matter and the Effect on Soil Properties: A Literature Review. Report for GRDC and CRC for Greenhouse Accounting. CSIRO Land and Water Client Report. Adelaide, Australia.

Lasaridi, K.E. and E. Stetiford (1999). Composting of source separated MSW: An approach to respirometric techniques and biodegradation kinetics. International Symposium on “Composting of organic matter”, August 31 – September 6, Kassandra-Chalkidiki, Greece.

Lehmann, J. (2007). A handful of carbon. Nature 447: 143–144. Lupwayi, N.Z. and I. Haque (1999). Leucaena hedgerow intercropping and cattle

manure application in the Ethiopian highlands, 3: Nutrient balances. Biol. Fert. Soils 28: 204–211.

Lupwayia, N.Z., P.E. Olsena, E.S. Sandeb, H.H. Keyserb, M.M. Collinsa, P.W. Singletonb and W.A. Ricea (2000). Inoculant quality and its evaluation. Field Crops Res. 65: 259–270.

Magdoff, F. and R.R. Weil (2004). Soil Organic Matter in Sustainable Agriculture. CRC Press, 398 pp.

Nahm, K.H. 2005. Factors influencing nitrogen mineralization during poultry litter composition and calculations for available nitrogen. World’s Poul. Sci. J. 61: 238–253.

Paustian, K. (2002). Organic matter and global cycle. In: Encyclopedia of soil science, New York, USA, Marcel Dekker Inc. pp. 895–898

Pramod J., A.K. Biswas, B.L. Lakaria and A.S. Rao (2010). Biochar in agriculture – prospects and related implications. Current Sci. 99: 1218–1225.

Probert, M.E., J.R. Okalebo and R.K. Jones (1995). The use of manure on smallholders’ farms in semi-arid eastern Kenya. Exper. Agric. 31: 371–381.

Radke, L.F., D.A. Hegg, J.H. Lyons, C.A. Brock, P.V. Hobbs, R. Weiss and R. Rasmussen (1988). Airborne measurements on smokes from biomass burning. In: P. V. Hobbs and M.P. McCormick. (Eds.), Aerosols and Climate. Deepak Publishing, Hampton, Va. pp. 411–422.

Rasmussen, P. E. and H. P. Collins (1991). Long-Term Impacts of Tillage, Fertilizer, and Crop Residue on Soil Organic Matter In Temperate Semiarid Regions. In: Nyle C. Brady (Ed.) Advances in Agronomy, Volume 45. Academic Press, USA. pp. 93–134.

Rice, C.W. (2002). Organic matter and nutrient dynamics. In: Encyclopedia of Soil Science. New York, USA, Marcel Dekker Inc. pp. 925-928.

Ryals, R. and W.L. Silver (2013). Effects of organic matter amendments on net primary productivity and greenhouse gas emissions in annual grasslands. Ecol. Appl. 23: 46–59.

Sanford, R.L. Jr. (1989). Fine root biomass under a tropical forest light gap opening in Costa Rica. J. Trop. Ecol. 5: 251–256.

Schjonning, P., S. Elmholt, J. Munkholm and K. Debosz (2002). Soil quality aspects of humid sandy loams as influenced by organic and conventional long-term management. Agri. Ecosys. Environ. 88: 195–214.

Tahir, M., M. Arshad, M. Naveed, Z.A. Zahir, B. Shaharoona and R. Ahmad (2006). Enrichment of recycled organic waste with N fertilizer and PGPR containing ACC-deaminase for improving growth and yield of tomato. Soil and Environ. 25(2): 105-112.

USDA. (2010). Composting. Part 637, National Engineering Handbook, NRCS, U.S. Department of Agriculture, Washington, DC, USA.

Wander, M. (2004). Soil organic matter fractions and their relevance to soil function. In: Magdoff, F. and R.R. Weil, (Eds). Soil Organic Matter in Sustainable Agriculture, CRC Press, USA.

Yeboah, E., P. Ofori, G.W. Quansah, E. Dugan and S.P. Sohi (2009). Improving soil productivity through biochar amendments to soils. Afr. J. Environ. Sci. Technol. 3: 34–41.

Zahir, Z.A., M. Naveed, M.I. Zafar, H.S. Rehman, M. Arshad and M. Khalid (2007). Evaluation of composted organic waste enriched with nitrogen and L-tryptophan for improving growth and yield of wheat (Triticum aestivum L.). Pak. J. Bot. 39: 1739–1749.

Chapter 9

Salt-affected Soils: Sources,

Genesis and Management

Muhammad Zia-ur-Rehman, Ghulam Murtaza, Muhammad

Farooq Qayyum, Saifullah, Muhammad Saqib and Javed Akhtar†

Abstract

The extent of salt-affected soils is proliferating because of different natural and anthropogenic factors like high temperature, low rainfall, poor quality of irrigation water etc. Different nature salts are being accumulated at the surface of soils and make environment difficult for plants to grow on such soils due to the reduced hydraulic conductivity and the low permeability. This leads to alter physical and chemical properties of soils making them non-productive for general cropping. Different management and remedial technologies are available to combat with the problem but the most striving concern is to opt the most economical and environment friendly technology. Different halophytic species can be used for the productive use of saline soils. Sodic and saline-sodic soils can be reclaimed using different amendments, which can provide soluble calcium to replace exchangeable sodium adsorbed on clay surfaces. There are two main types of amendments: those that add calcium directly to the soil and those that dissolve calcium from calcium carbonate already present in the soil. Studies demonstrated that under adverse conditions tree plantations may provide positive returns to investment and significant economic and social benefits to land users. These findings suggest that there is an opportunity for capital investment in afforesting abandoned salt-affected lands with multipurpose

†Muhammad Zia-ur-Rehman*, Ghulam Murtaza, Saifullah, Muhammad Saqib and Javed Akhtar

Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: ziasindhu1399@gmail.com Muhammad Farooq Qayyum

Department of Soil Science, Faculty of Agricultural Sciences and Technology, Bahauddin Zakariya University Multan, Pakistan. Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

192 M. Zia-ur-Rehman, G. Murtaza, M.F. Qayyum, Saifullah, M. Saqib and J. Akhtar

tree species. This chapter covers the introduction of salt-affected soils, associated aspects, management, and their reclamation.

Keywords: Salinity, Brackish Water, Root Zone Salinity, Reclamation, Management.

9.1. Introduction

Salt-affected is a general term used for soils which contain soluble salts or exchangeable sodium and/or both, in such amounts that can retard growth and development of plants. Such soils cause reduction in crop yield and are required to be managed and remediated for sustainable agriculture. Mostly salt-affected soils exist in arid and semi-arid regions but also found in some humid to sub-humid climatic areas, where conditions are favorable for their development. In Pakistan 6.67 × 106 ha area is under salt contamination (Khan, 1998) mainly due to unavailability of good quality water for irrigation. Ground water may supplement irrigation needs because of increased cropping intensity and competition from non–agricultural sectors for fresh water. At present, in Pakistan, more than 1.07 × 106 tube wells are pumping out 9.05 × 106 ha–m ground water (Anonymous, 2011) and 70-80 % of this water is unfit (Latif and Beg, 2004; Ghafoor et al. 2004) for agricultural crops having high electrical conductivity (EC), sodium adsorption ratio (SAR) and/or residual sodium carbonate (RSC) that have negative impacts for crop growth and development. In arid and semi-arid climatic zones use of low quality irrigation water has become a common practice to fulfill the needs of ever increasing population demands for food crops. (Qadir et al. 2007). Pakistan is situated in an arid to semi-arid region. As the fresh water supplies are getting short, farmers are pumping low quality (high EC, SAR and RSC) ground water for irrigation which is further aggravating the soil and ground water salinity and related hazards. These soils are adversely affecting the economic yields of crops and consequently leading to uneconomical crop production and rural poverty. In the suburbs of Indus Basin in Pakistan various research studies have been conducted and results reveal that almost 20-43% yield loss occur in salt affected fields as compared to normal ones. Qadir et al. (2014) reported that 36-69% yield loss with the average of 48% for rice crop occur due to salinity hazards. In this chapter, different aspects of salt-affected soils along with their management and remedial measures have been discussed.

9.2. Sources of Salts

Salts may originate from various sources acting either alone or in combination. However, the primary and major source of salts in soils and oceans is rocks and minerals present in the Earth crust which are weathering with the passage of time. Although the salts currently occurring in the ocean arise mainly from the weathering process of the rocks and minerals in Earth crust, now the ocean is functioning as an important “source” for the redistribution of salts.

Salt-affected Soils: Sources, Genesis and Management 193

9.2.1. Parent material and weathering process

As a result of in-situ weathering process, salts are released into soils and are accumulated or removed depending on the prevailing environmental conditions. Under humid conditions, salts leach through soils and are transported to the nearby streams and rivers resulting in formation of inland salt-affected areas. However, under arid to semi-arid climatic conditions, the weathering products accumulate in-situ and result in the development of salinity and/or sodicity. This process of formation of salt-affected soils as result of accumulation of salts released during weathering is called primary salination/sodication. In Pakistan salt-affected soils have been formed by: (i) deposition of physically transported salts along with parent material (PM) such as NaCl and CaSO4 in the salt range belt of Pakistan; and (ii) mineral weathering in-situ, i.e., transformation of soil mineral and dissolution of sparingly soluble salts deposited along with PM as well as those formed later e.g. gypsum, lime etc.

9.2.2. Irrigation water

All the natural waters contain dissolved salts. The expected effect (adverse or favorable) is highly dependent upon type and amount of salts and volume of irrigation water used. Canals of Pakistan contain best quality irrigation water as it contains salts varying from 120 to 200 mg L-1. As an estimate, 10-cm deep irrigation with canal water in one hectare may add 120-180 kg salts. Other common source of irrigation and salts is ground water, which are mostly brackish in arid regions like Pakistan but the levels of EC, SAR and RSC in ground waters are quite variable. On an average, the ground waters in Pakistan contain up to 1250 mg salts L-1 (Ghafoor et al. 2004). A 10-cm deep irrigation using groundwater may add 1.2 Mg salts ha-1. Such additions of salts in the soils highly depend upon depth of ground water table, volume of water used for irrigation, and type of salts as well as upon the evaporative demand of the atmosphere.

9.2.3. Flood waters and waste effluent

Flood water mostly redistributes the already present salts but may become important in some parts of the world such as during monsoon in Pakistan. Similar is the case with untreated sewage water as a source of salts, particularly in the Third World countries where it is used to irrigate crops, mainly vegetables, around cities or is disposed-off into the existing irrigation channels. Such irrigation waters are of particular concern with respect to heavy metals entry in the food chain of human beings and because of many pathogens as well as toxic organic materials.

9.2.4. Sea water

Sea water (EC > 4 dSm-1, SAR > 50-55) intrusion as well as sea water sprays could contribute large quantities of salts but the action is a bit localized along coastal areas. Almost similar is the mode of inland saline seeps to contribute salts. However, importance of playas (Lakes having input but no output of effluent) need special

consideration in some areas of the world. The soils in coastal areas are enriched with salts coming from sea through various ways, such as:

a) Striking of sea water high-tides with nearby surface soil;

b) Entry of sea water through rivers, estuaries, etc.;

c) Ground-water inflow; and

d) Salt-enriched sprays transported up to many kilometers inland from the sea coast and deposited as dry “fall-out” or “wash-out” by showers. Inland deposition of NaCl at a rate of 20-100 kg ha-1 year-1 is quite common and values of 100-200 kg ha-1 year-1 for nearby coastal areas have been reported. Although these amounts may appear small, but their regular deposition over long periods of time may lead to salinization of the soils.

9.2.5. Lacustrine and marine deposits

According to geological information, once whole of the Indian sub-continent was under sea. Gradually, sediments from Himalayas produced up-lands which were later developed for agriculture. Hence, some of the salts could be considered as fossil salts. Irrigation with low quality water reveals that salts already present in the soil profile are transported to the soil surfaces with irrigation which are left behind after evaporation. Thus, after a longer period of time salts that were previously evenly distributed in the whole profile may selectively accumulate on the soil surface and give rise to saline soils. Accumulation of salt-laden runoff water and its subsequent evaporation in the un-drained basin is also a cause of salinity in many low-laying areas.

9.2.6. Fossil salts

Salts accumulation in the arid regions often involves “fossil-salts” which are a consequence of earlier deposits or entrapped solutions in former marine or lacustrine deposits. Salt release may occur through natural as well as anthropogenic activities. An example of the former situation is the rise of salt bearing ground water through an originally impervious cap (which became permeable as a result of weathering process) overlaying saline strata. Examples of latter scenario are assembly of canals along with water works within the saline strata and use of ground water for irrigational purposes. In Rajasthan, India, a canal built on an underlying gypsum layer has resulted in development of salinity in the area within only a few years of its construction. This has been due to perched water table and contribution of salts from the underground gypsum layer.

9.2.7. Chemical fertilizers and waste materials

Utility of inorganic fertilizers is increasing and that of organic manures is decreasing in agricultural fields but their contribution to overall salt build-up in soils is insignificant. However, certain situations, such as dumping of cow’s dung slurry, sewage sludge or industrial by products such as press mud or pyrites, can contribute to excessive accumulation of certain ions those could limit soil productivity.

9.3. Genesis of Salt-affected Soils

The mode of soils-origin and/or the processes and factors involving in soil formation from un-consolidated parent material is defined as soil genesis, i.e., it is a process of developing soils from parent material. Genesis is a continuous but slow process that includes decreasing the particle size of the parent material, reordering of mineral particles, addition of certain materials such as organic matter and salts, changing the kinds of minerals, creating horizons, and producing clays.

9.3.1. Genesis of primary salt-affected soils

The following factors mostly contributed towards the genesis of salt-affected soils in Pakistan.

9.3.1.1. Salty parent material

Presence of primary minerals as the special constituent of parent material is the most important factor for genesis of salt-affected soils. Arid to semi-arid climatic zones of the world including Pakistan have more common soil salinity concerns due to low precipitation which is inadequate for leaching of salts below root zones. Under these circumstances soluble salts coupled with exchangeable Na+ have accumulated over thousands of years during the process of soil formation. This is the case of primary/old/ancient salt-affected soils. These soils existed before the advent of the canal irrigation system in the Indus Plains of Pakistan.

9.3.1.2. Aridity and uneven distribution of rainfall

Most of the soils of Pakistan exist in arid to semi-arid climatic regions. Most of the rainfall occurs during monsoon (July-August) while during major part of the year the salts present in the soil tend to move upward with water through capillary action. The rainfall that is received (mostly < 500 mm annually) is not sufficient to leach down the salts away from the root zone. Moreover, the net upward movement of water in the soil along with evaporation at the surface provokes the accumulation of mineral salts in the surface soil.

9.3.1.3. Physiographic unevenness

Micro unevenness of the soil surface is generally not observable. This situation can be visualized from different depths of the standing water after a heavy rainfall. The rainwater flows from the convex parts over the sloping parts and is accumulated on concave parts. In parts where there is low effective leaching (convex and sloping parts), accumulation of salts takes place. Hence, patches of salts develop in an uneven soil. In Pakistan, the natural drainage is poor due to lower slope of 30 cm per 1609 m which promotes the salinization and sodication processes.

9.3.2. Genesis of secondary salt-affected soils

Introduction of canal water irrigation system in Pakistan is the major cause of evolution of secondary or man-made salt-affected soils. However, the extent of secondary salt-affected areas is very small than the primary salt-affected areas.

Several factors act alone or in combination to form secondary salt-affected soils. Insufficient or unequal application of irrigation water, imperfect soil drainage, waterlogging, brackish ground water, improper soil and water management, seepage from canals and water courses or combination of these factors are the principal causes for the formation of secondary salt-affected lands.

9.3.2.1. Sodication

Sodication can be defined as process of accumulation of exchangeable Na+ content in the soil that results in the formation of poor soil structure along with unavailability of essential nutrients (Qadir et al. 2004). The salts of Na+ Ca2+ and Mg2+ as well as Cl- and SO4

2- are present in excess under salt-affected soils while those present in smaller amounts are cations like K+ and NH4

+ and anions like CO32-, HCO3

- and NO3-.

When salt concentration in soil is very high, a part of Ca2+ and Mg2+ precipitates as CaCO3, MgCO3, CaSO4 and MgSiO3. The precipitation of these salts results in the increased proportion of Na+ in soil solution as well as on the exchange complex. In this way, saline soils can be regarded as responsible for genesis of sodic soils. For this reason, most of the moderately to strongly saline soils in Pakistan are generally saline-sodic/sodic as well. Sodication generally leads to deflocculation (dispersion), poor drainage and poor aeration in soil (Shainberg and Letey, 1984). In addition, severe nutrient imbalance results in these soils which may be in the form of deficiency as well as toxicity of certain vital elements. Such physical and chemical impairments lead towards low yield and production due to negative impacts on root growth activity coupled with soil micro-organisms. The color of sodic soils is most often dark that is due to deposition of discrete and suspended organic matter prevailing in soil solution at the soil surface. In such soils, after evaporation, darkening of the soil color is increased which may extend up to blackish in.

9.4. Classification of Salt-affected Soils

Salt-affected soils are usually characterized into three main groups 1) saline, 2) sodic and 3) saline-sodic.

9.4.1. Saline soil

Saline soil is referred as a soil that contains plenty of soluble salts that have adverse effects on plant growth but does not contain excessive exchangeable Na+. Most of the soluble salts in saline soils are composed of cations Na+, Ca2+, and Mg2+ and of anions Cl-, SO4

2-, and HCO3-While in minute concentrations other cations such as K+

and NH4+ along with anions including NO3

-, CO32- and BO4

2- also occur in these soils. Saline soils have ECe ≥ 4 dS m-1, SAR < 13 (mmol L-1)1/2, ESP < 15and pHs < 8.5.

9.4.2. Sodic Soil

Sodic soil can be defined as a soil that restrains adequate concentrations of exchangeable Na+ that have serious impacts on plant growth and development but not having excessive concentration of soluble salts. Soil structure, aeration, and hydraulic conductivity are deteriorated by the excessive amount of exchangeable

Na+. Sodic soils have ECe < 4 dS m-1, SAR > 13 (mmol L-1)1/2, ESP > 15 and pHs > 8.5.

9.4.3. Saline-sodic soil

Saline-sodic soil refers to a soil having both soluble salts as well as exchangeable Na+ in sufficient amounts that cause harmful impacts on all type of crop plants Saline-sodic soils are characterized as the soils that have: ECe > 4 dS m-1, pHs > 8.5, SAR > 13 (mmol L-1)1/2 and ESP > 15.

In some literature, the term "alkali" is used in place of "sodic", i.e., for soils having excess exchangeable Na+. Hence, the terms "saline-alkali" in place of "saline-sodic" and "alkali" in place of "sodic" are used. However, the use of the term "alkali" is being discouraged because of its ambiguity with the term "alkaline" which refers to the soils having pH > 7.0. According to an estimate (Khan 1998), the salt-affected soils of Pakistan cover on area of about 6.67 × 106 ha.

On global basis, the salt-affected soils exist mostly under arid and semi-arid climates in more than 100 countries covering about 9.55 × 106. These soils cover about 25% and 60 % of the world’s irrigated and cultivated land, respectively. Overall, about 62% of the salt-affected soils of the world are saline-sodic/sodic while 38% are saline (Tanji 1990).

Table 9.1 Extent of soil salinity/sodicity problem in Pakistan

Province Area (Million ha)

Punjab 1.234 Sindh 3.04 Balochistan 0.12 KPK 0.11 Pakistan 4.50

Source: WAPDA (2003)

Table 9.2 Salt-Affected Area (m ha) of Punjab, Pakistan

Year Area

Survyed

Salt Affected

Uncultivted Cultivated Total %age

1945-46 4.84 0.42 0.49 0.91 18.80 1955-56 5.96 0.05 0.69 1.20 20.64 1965-66 6.88 0.44 0.68 1.12 16.28 1975-76 7.34 0.37 0.61 0.98 13.35 1985-86 7.57 0.30 0.58 0.88 11.62 2000-01 7.92 1.16 1.51 2.67 33.71

Source: Ahmad and Chaudhry (1997)

9.5. Chemistry of Soil Solution in Salt-affected Soils

9.5.1. Soil solution

The soil system is composed of three phases of matter; 1) solid, 2) liquid and 3) gas. The solid part is comprised of a mixture of mineral and organic material and provides the skeletal frame work of the soil. In this frame work, a system of pores exists which is shared jointly by the liquid and gaseous phases. The gaseous phase, or soil air, is a mixture of gases. The liquid portion of soil matrix also known as soil solution, is comprising of water, small quantities of dissolved gases and dissolved solutes. Soil solution is the medium in which most soil chemical reactions occur. It bathes the plant roots and forms the source from which the roots of plants and other organisms obtain their required, nutrients and water.

Table 9.3 Saline area (in 000’ ha) in different districts of southern Punjab

Sr. No District Area surveyed Salt affected area

1 Bahawalnagar 623.7 130.4 20.9 2 Bahawalpur 468.5 23.3 5.0 3 RahimYar Khan 720.8 119.8 16.6 4 Dera Ghazi Khan 150.7 24.6 16.3 5 Muzafar Garh 474.8 92.9 19.6 6 Layah 246.4 0.9 0.4 7 Rajan Pur 237.2 25.9 10.9 8 Vehari 431.4 28.4 6.6 9 Khanewal 377.5 61.2 16.2 10 Multan 361.0 59.8 16.6 11 Lodhran 173.2 25.3 14.6 12 Sahiwal 258.7 28.7 11.1 13 Okara 439.4 44.1 10.0 14 Pak Pattan 235.3 14.4 6.1 15 Faisalabad 544.3 90.3 16.6 16 Toba Tek Singh 308.5 38.1 12.4 17 Jhang 482.9 109.0 22.5 18 Kasur 280.9 46.0 16.4 19 Shiekhupura 523.6 70.6 13.5 20 Gujranwala 416.5 52.1 12.5 21 Hafiza Abad 60.7 20.4 33.6 22 Mandi Bahudin 182.2 4.0 2.1 23 Sargodha 497.1 59.5 12.0 24 Khushab 181.1 0.8 0.4 25 Bakhar 314.8 1.5 0.5

Source: Punjab Development Statistics (2006)

9.6. Soil Salinity Evaluation

9.6.1. Root zone

The area of the soil matrix from which plant roots uptake water and other essential nutrients is known as root zone or rhizosphere. Plants absorb water from the soil by applying immense absorptive force more than that with which it is held with soil. When plants fail to apply enough absorptive force for the uptake of sufficient water from the soil, they face water stress. This situation prevails when soil becomes too dry or the osmotic potential of the soil solution decreases significantly. Mainly salts decrease the free energy of the water molecules which ultimately decrease the water potential of soil solution consequently plant suffers with water deficiency. If we take two soils having similar physicochemical properties except that one is normal and the other is salt affected soil, plants have to exert more force for the absorption of water from salt affected soils compared to that with the normal soil. Salts have more affinity for water due to its polarity and plants require higher absorptive force to take in water from the salt affected soil as compared to the normal land having same amount of water.

9.6.2. Evaluation of average root zone salinity

The average of five points in the root depth can be helpful in the evaluation of average root zone salinity in the soils. These points can be assumed as:

1) The soil surface (ECsw0)

2) Bottom of the upper quarter of the root zone (ECsw1)

3) Bottom of the second quarter depth (ECsw2)

4) Bottom of the third quarter depth (ECsw3)

5) Bottom of the fourth quarter or the soil water draining from the root zone (ECsw4)

The following assumptions are used to estimate the average root zone salinity to which crop responds.

1) Salinity of the applied irrigation water = 1 dS m-1

2) Crop water demand (ET) = 1000 mm per season

3) The crop water use pattern is 40-30-20-10. This means that the crop will get 40 % of its ET demand from the upper quarter of the root zone, 30 % from the next quarter, 20 % from the next, and 10 % from the lowest quarter.

4) Crop water use will increase the concentration of the soil-water which drains into the next quarter, i.e., ECsw0 < ECsw1 < ECsw2 < ECsw3 < ECsw4

5) Desired leaching fraction (LF) = 0.15. The leaching fraction of 0.15 means that 15 % of the applied irrigation water entering the surface percolates below the root zone and 85 % is used by the crop to meet its ET demand and water lost by surface evaporation.

9.6.3. Salinity control in the root zone

In the root zone the salinity control depends on adequate leaching of excess salts that is directly proportional to the heavy irrigation and rain fall which reduces the soil infiltration capacity. High rainfall receiving areas also known as humid regions have sufficient water to flush out the salts from the rhizosphere or root zone. Controversial to this phenomenon in arid to semi arid climatic zones where rain fall is very low while temperature is very high soil salinity problem prevails. Water balance of the crop root zone provides the calculations for the amount of irrigation water required for the proper growth and development of the plants. Water flows through the root zone of crops in the following forms:

1) Irrigation water (Di)

2) Rainfall (Dr)

3) Upward movement of the ground water (Dg)

Water flows out of the root zone due to:

1) Evaporation (De)

2) Transpiration (Dt)

3) Drainage (Dd)

Variation between water flowing into the root zone and out of the root zone is equal to The change in water storage can be calculated by subtracting the water flowing out of the root zone from the water flowing into the root zone. Therefore, water balance equation for change in storage (Ds) may be written as:

Ds = (Di + Dr + Dg) - (De + Dt + Dd) . . . . . . . . . . . . . . . . (1)

While change in salt storage (root zone salinity), i.e. Ss can be explained by the following equation:

Ss = (DiCi + DrCr + DgCg + Sm + Sf) - (DdDd + Sp + Sc) . . . (2)

C = Salt concentration

Sm = Salt dissolved from minerals in soils

Sf = Salt concentration contributed as the fertilizers or a constitute of amendment

Sp = Salt in the form of precipitations

Sc = Salt removed due to crop harvesting

If Di + Dr + Dg in equation (1) are less than De + Dt, the water deficit in soil is compensated by the absorption of water from the soil storage along with lowering the drainage process. With the passage of time the deficiency is completely fulfilled and thus become zero. When Ds become less, soil becomes dry that leads to reduction in De and crops face water stress that causes the Dt reduction. In the beginning due to these processes water loss occur in the root zone that remains equal to the water supplied at the zero drainage. Nevertheless, in the absence of drainage water higher

salt concentration in the root zone results in the saline stored water. As salinity increases, the osmotic stress of the plant increases, which further reduces transpiration and thus plant dies when salts increase continuously.

In the presence of shallow water tables, deficiencies in Di + Dr may be offset by Dg. If movement of ground water is upward drainage becomes zero. This situation cannot continue forever. Under the dynamic field conditions, upward water movement coupled with drainage remain continue alternately throughout the year especially in the cultivated areas. If upward flow continues while leaching remains insufficient, soil salinity will retard the plant growth and development and ultimately plants die. That is why if salinity problem prevails there is the need of net downward water movement for the sustainability of the crop production. The conditions that control the inward water flow as well as outward from the root zone are not true for the steady-state conditions permanently. Due to these processes salt concentration in the soil solution varies over time. The primary objective of water management is the maintenance of this variation that controls the excess drainage as well as reduction of plant growth and development.

9.6.4. Salt precipitation

The equation (2) shows that the salt balance of a root zone is influenced by the precipitation of soluble salts. As a result, concentration of salts that leach down may be less than the applied quantity. At low leaching fractions (LF=0.1), almost ≥ 20% salts become precipitated from the irrigational water and thus not present into the drainage water. Therefore, salt precipitation component is an important factor for the calculation of salt balance especially under less leaching fraction.

9.7. Reclamation of Salt-affected Soils

Several techniques are adapted to reclaim salt-affected soils. The fitness of each technique depends upon a number of factors, e.g., 1) Physical, chemical and mineralogical characteristics of the soil; 2) Internal soil drainage; 3) Presence of pans in the subsoil; 4) Climatic conditions; 5) Content and types of salts present; 6) Quality and quantity of water available for leaching; 7) Quality and depth of ground water; 8) Desired rate of replacement of excessive exchangeable Na+, if present; 9) Presence of lime or gypsum in the soil; 10) Availability and cost of the amendments; 11) Availability of the equipment for soil tillage, if needed; 12) Crops grown in the region; 13) Topographic features of the land; and 14) Time available for reclamation.

Good internal soil drainage, land leveling, and deep ground water (preferably below 3 m) are considered essential prerequisites for successful reclamation. From reclamation point of view, the salt-affected soils may be divided into two categories; 1) saline and 2) sodic/saline-sodic.

9.7.1. Reclamation of saline, sodic/saline-sodic soils

Saline soils restrain only higher concentration of soluble salts and their reclamation is done by leaching with excess of good quality irrigation water that carries salts into

the deeper soil layers. Amount of water to be applied is important and it depends on several factors such as initial soil salinity and moisture levels, techniques of water application, and soil type etc. Good quality irrigation water is normally required for soil reclamation.

For reclamation of sodic/saline-sodic soils, a soluble source of Ca2+ such as gypsum is added in the soil followed by flooding with good quality irrigation water. The Na+ ions on exchange complex are replaced by Ca2+, and removed from root zone along with dissolved salts in leaching water. Thus reclamation of both soils (saline and sodic/saline-sodic) requires flow of water through the profile.

Overall, the methods of reclamation of saline-sodic/sodic soils may be grouped into: 1) Physical methods; 2) Chemical methods; 3) Biological methods; 4) Hydro-technical method; 5) Electro-reclamation method; and 6) Synergistic approach. Apart from decrease in salinity/sodicity hazard, the method used at a particular site must be able to perk up the physical soil conditions by minimizing exchangeable Na+ that deteriorates the physical properties of sodic soils. Soil aggregates in sodic soils slake and disperse and hence reduce porosity (Qadir and Schubert 2004). An effective amendment/method improves porosity, hydraulic conductivity and infiltration rate and decreases bulk density (Murtaza et al. 2009). physical properties of sodic soils maybe refined by the reclamation processes due to the incorporation of high amount of Ca2+ as compared to Na+ in soil solution as well as on exchange sites. This flocculates the dispersed soil thereby improving water conducting soil properties.

9.7.1.1. Physical methods

Several methods, viz. deep ploughing, subsoiling, hauling, sanding, and horizon mixing are used to improve salt-affected soils by physical/mechanical treatments:

i. Deep ploughing

Deep ploughing involves ploughing to a depth from about 40 cm to 150 cm. This is a beneficial method on stratified soils having impermeable layers. After a series of experiments, it was found that a single deep ploughing having 40 to 75 cm depth economically improved the calcareous sodic soils both physically and chemically. Under conditions where the subsoil is more sodic than the surface soil, then deep ploughing should be avoided. However, this method is very helpful to speed up soil reclamation if the subsoil is gypsiferous, i.e. the subsoil contains a good quantity of gypsum.

ii. Subsoiling

Sub-soiler is comprised of erect steel/iron strips also known as knives/tines that are almost 60 to 90 cm apart and are pulled by the use of high power tractor through the soil. In this way soil channels are opened and permeability is increased. Significance of sub-soiling lies in the fact that the favorable impacts of sub-soiling remain continue till many years due to break down of lime layer. Even if breakdown of lime layer does not occur it is beneficial for one season.

iii. Sanding

In this practice, sand is mixed with a fine-texture soil that does not contain high clay content to make it more porous for accelerating the permeability process. By sanding the soil texture of the surface soil is changed permanently. Moreover, it improves root penetration, water and air permeability that facilitate the leach down of salts from root zone. For better results, sand should be mixed with at least 10 cm of surface soil.

iv. Hauling

In this technique, surface of the salt affected soil is removed and a layer of good quality soil is applied there. Hauling is absolutely useful but it might not be applicable everywhere because this method is considered expensive.

v. Horizon mixing

This method is used when the soil profile has good surface horizon but lower horizon has undesirable characteristics. Such characteristics are found in saline-sodic/sodic soils which have a favorable surface soil underlain by a slowly permeable, sodium-affected B horizon which is underlain by a more permeable gypsum-horizon. Benefit of the profile mixing is that it preserves the surface soil but upturn the subsoil along with substratum. This process is done by removal of upper surface, deep mixing of underlining subsoil coupled with substratum and at last again substituting the upper soil surface.

Profile of a virgin soil Profile of an amended soil

____________ ____________

A _______ A

____________ ____________

9.7.1.2. Chemical methods

Chemical methods employ use of chemical amendments to improve soil properties and crop growth. Chemical amendment at any place is chosen depending upon various factors such as its availability, cost, handling and application difficulties, and the time required to react within the soil profile and to reinstate the adsorbed Na+. Various amendments reveal different levels of effectiveness for the reclamation of sodic as well as saline-sodic soils of varying characteristics. Chemical amendments generally used for renovation of saline-sodic/sodic soils can be categorized into two basic groups:

i. Inorganic amendments

These can be further subdivided into three types.

a) Soluble calcium salts, such as CaCl2, gypsum (mined gypsum) and phosphor-gypsum that results from the assemblage of high analysis phosphatic fertilizers.

b) Slowly soluble calcium salts, like ground limestone (CaCO3).

c) Acidifying materials. These amendments mobilize Ca2+ in calcareous soils by enhancing the conversion of CaCO3 to more soluble CaSO4, Ca(HCO3)2, Ca(NO3)2 or CaCl2. These amendments include H2SO4, HCl, HNO3, sulphur, pyrite (FeS2), lime sulphur (CaS5), FeSO4, and Al2(SO4)3.

Inorganic fertilizers may furnish soluble Ca2+ directly like calcium nitrate Ca(NO3)2 and single superphosphate (SSP) and/or indirectly by the addition of ammonium sulphate [(NH4)2SO4] and urea that enhance the physiological acidity (pH < 7) in the vicinity of their application. However large scale application of such fertilizers to reclaim the soil sodicity problem is not an economical approach.

Among various inorganic amendments gypsum has declared as the most efficient, cheap, environment friendly and easily available amendment that is the rich source of Ca+2 (Ghafoor et al. 2004). It is a proximal approach to reclaim the calcareous as well as non-calcareous sodic and/or saline-sodic soils. The gypsum required for reclamation, in Mega-gram per hectare (Mg ha-1, 1Mg = 1000 kg = 1 ton), of sodic and saline-sodic soils is called gypsum requirement (GR) of the soils. A laboratory method (Schoonover's method) is generally used to determine the GR of the sodic and saline-sodic soils. Other inorganic amendments used for soil reclamation can be applied under suitable conditions. Equivalent quantities of chemically pure amendments relative to one Mg of gypsum are given in the following Table.

Table 9.4 Amount of amendments is equivalent to one mega gram of gypsum

Amendments Formula Amount equivalent

to 1 Mg of gypsum

Gypsum Calcium chloride Sulphur Ferrous sulphate Ferric sulphate Aluminium sulphate Sulphuric acid (36N) Hydrochloric acid (12N)

CaSO4.2H2O CaCl2.2H2O S8 FeSO4.7H2O Fe2(SO4)3 .9H2O Al2(SO4)3 .18H2O H2SO4 HCl

1.00 0.85 0.19 1.61 1.09 1.29 0.57 1.71

Source: Qadir et al. (2001)

The solubility and relative effectiveness of gypsum depends upon its mesh size. The suitable particle size of gypsum used is between the 8-30 mesh, such that the particles should pass through a 2 mm sieve while 50% among them must also pass through 0.5 mm (30 mesh) sieve (Talib and Akram 2001).

ii. Organic amendments

Organic matter is needed to maintain and even to improve the physical, chemical and fertility characteristics of normal as well as salt-affected soils. The organic amendments include green manures, farm manures, poultry manures, slaughter house waste, etc. The use of some organic polymers (polyvinyl alcohol, PVA) has also been suggested for the reclamation of sodic soils. By-products of some industries, such as pressmud and molasses meal from sugar industry may be effective for reclamation of saline-sodic/sodic soils but their extensive use is limited because of limited availability and slow reaction rates.

Table 9.5 Properties of loam soil as affected by ECe:SARss receiving gypsum @ 50 % soil GR.

Treatment Gyp. mesh size pHs ECe (dS m-1) SAR

ECe:SARss :: 8:8 Passed through 5 mesh 7.76 1.25 1.12 ECe:SARss :: 8:8 Passed through 16 mesh 7.56 1.21 1.18 ECe:SARss :: 8:8 Passed through 30 mesh 7.75 1.37 1.50 ECe:SARss :: 8:48 Passed through 5 mesh 7.84 2.04 1.97 ECe:SARss :: 8:48 Passed through 16 mesh 8.05 2.13 2.26

Source: Farid (2000)

9.7.1.3. Biological methods

The term "biological reclamation" is used to describe the reclamation of a salt-affected soil by growing crops on the affected area. Sometimes, addition of organic matter to the salt-affected soils as farm yard/green manure is also included under the same heading. Use of manures/other organic materials to reclaim the sodic/saline-sodic soils must be done separately rather together to avoid confusion between the organic and the biological amendments.

Plant parts either above or below ground have great influence on soil. Plant parts that are present below the ground through root-soil interaction have great impact on soil conditions. For example, roots tend to change the soil pH, lower oxygen concentration, release organic compounds and complex energy sources such as exudates, secretions, and mucilages, produce chelating and/or reducing substances, increase CO2 partial pressure, endow it the channels that support soil solution flow, improve various microbial processes and reveal impact on the soil physical as well as chemical properties. The above-ground plant parts change the microclimate by providing soil cover, reducing the temperature of the soil, improve the soil mulching, slow down the evaporation process and therefore resist the upward flow of salts by reducing capillary rise. Even after the harvesting of the crops, below ground residual plant parts incorporate the soil organic matter through root parts coupled with rhizomes and other constituents. The possible mechanisms of biological reclamation may be associated with long chain of various reactions. These involve: 1) release of CO2 in the rhizosphere as a result of root and microbial respiration; 2) formation of carbonic acid (H2CO3) via CO2 dissolution in water; 3) reaction of H2CO3 with the native CaCO3 to form relatively more soluble Ca(HCO3)2; 4) release of Ca2+ ions from Ca(HCO3)2; and 5) replacement of exchangeable Na+ by the Ca2+.

Plants growing in saline/sodic soils have limited biomass production. In saline soils, crop yields are reduced by disturbing the water along with nutrient balance for plants while in sodic soils, plant growth is affected due to deteriorated physical conditions of soils. Moreover, in sodic soils, the excess Na+ in the root medium disturbs the nutrition of plants. The selection of plant species to reclaim the salt affected soils should be very careful. Plant species vary in their tolerance to soil salinity/sodicity and irrigation requirements resulting in variable efficiency of growth. Generally, salt dilution supports the water loving plants due to heavy irrigation whereas the salt tolerant plants get benefits through both natural as well as adaptive modifications when cultivated in saline water environment.

Stage of vegetative growth and kind of vegetation play a vital role in modifying the environment of the host soil. At early stages of growth, crop roots occupy some of the soil macropores that would otherwise be available for infiltration. The amount of root mass, its rate of decay as well as ability to form root channels can markedly be different among crops. Regarding kind of vegetation, plant species that are stress tolerant especially under salt affected conditions are important for reclamation. Plant species that are stress tolerant and grow efficiently in wide range of stresses conditions could render them in an expanded range of adaptability and utility compared to others. Some research workers favored the inclusion of kallar grass, sesbania or sudan grass as the first crop to start and speed up the reclamation process of salt affected soils. The salt tolerant plant species generally perform more efficiently in calcareous salt affected soils than the non-calcareous soils. In calcareous soils, their roots act as Ca2+ mobilizers via dissolution of the native CaCO3. In some experimental studies, amount of soluble Ca2+ in calcareous sodic soils cultivated with salt tolerant plants were observed sufficient for the marked reduction in the salinity and sodicity levels.

Although growing of certain salt tolerant plant species for improvement of salt-affected soils is an age old practice, yet little work has been reported to evaluate the role of these species in terms of soil amelioration over a certain period of time and at different growth stages. Many workers have simply correlated a good stand and harvest of certain salt tolerant forage plants from the salt-affected areas with the decrease in salinity/sodicity hazard without analyzing the soil characteristics. Very few studies give the requisite information on actual changes in ECe and SAR/ESP of saline-sodic/sodic soils during reclamation through biological means. Generally, reclamation of saline-sodic/sodic soils through biological means is considered a slower than the application of inorganic amendments. However, biological reclamation can be started at a relatively low initial cost.

9.7.1.4. Hydro-technical technique

Using this technique saline water that has high concentration of electrolyte is applied that affect the soil permeability and thus continuous addition of water for dilution purposes leads towards the "valence dilution" effect. Eaton and Sokoloff (1935) described the "valence dilution" effect for the very first time when they were conducting an experiment regarding reclamation of sodic soils. In soil water system where monovalent and divalent cations in solution as well as in absorbed form is equal, application of further water leads the equilibrium towards the preferable

adsorption of divalent cations such as Ca2+ as compared to the monovalent cations, such as Na+. Reverse to this phenomenon takes place when evapotranspiration makes the soil solution too much concentrated.

The ratio of divalent cations to the total cations (with concentrations expressed in mmolc L-1) of the irrigation water must be ≥ 0.3 that leads towards the less use of water for proficient reclamation process. Rarely a few natural water sources sustain this ratio while for all other situations use of extra Ca2+ source is required that can be incorporated by various processes including; 1) application of gypsum into the soil after subsequent irrigation and/or 2) placement of gypsum stones into the water channels for the sufficient addition of Ca2+ into saline water. The basic problem for the conduction of this technique is the unavailability of primary facilities including collection, transport and reclamation of saline water

9.7.1.5. Electro-reclamation approach

Electro-reclamation approach can be defined as the amelioration process of salt affected soils using the principle electrodialysis technique. Numerous research studies including laboratory as well as field experiments reveal that use of electric current for the reclamation process speed up the reclamation mechanism manifolds although it is not the complete substitute for the traditional reclamation processes. This method of soil reclamation has shown some encouraging results which indicate increased solubility of CaCO3 to supply more Ca2+ to replace the exchangeable Na+. Moreover, this method created an environment which was effective for leaching of soluble salts and exchangeable Na+.

It is too early to recommend this method for practical use in agriculture of Pakistan and elsewhere in the world.

9.7.1.6. Synergistic approach (combination of reclamation methods)

Under certain conditions, reclamation can be speeded up by combining the various reclamation methods, e.g. a saline-sodic soil having an impermeable layer of 15 cm width at a soil depth. In this case, use of physical and chemical approaches collectively may be much better than the use of either chemical or physical method alone. In most of the cases, this approach is practiced for the reclamation of salt affected soils at farmers' level.

Combined use of gypsum along with various organic amendments decreased the salinity/sodicity problem to great extent. Gypsum application with various organic amendments is reported like gypsum in combination with FYM (Murtaza et al. 1999); gypsum in combination with sesbania green manure (Baig and Zia, 2006); gypsum in combination with rice husk (Chang and Sipio 2001) shown remarkable effects in reducing salinity/sodicity problem.

As already discussed, use of gypsum for the reclamation of salt affected soils is a wide spread approach. However, in a developing country like Pakistan, although gypsum is available in abundance yet its prospective use is restricted because of the bitter reality that an amount of more than Rs.28000 per hectare (considering an average gypsum requirement of sodic soils = 14 Mg ha-1) is needed to purchase the amendment only. This high price is not acceptable by the small farmers occupying

the greater part of the affected soils. Thus high cost of reclamation process makes it out of reach approach for common person and there is very low progress regarding sodic reclamation in county. It is highly recommended that Government should provide gypsum at subsidized rates on credit to poor farmers.

9.8. Management of Salt-affected Soils

Management of salt-affected soils can be done by following certain measures. These measures can be divided into two categories, i.e. measures for the management of reclaimed salt-affected soils, i.e. normal soils, and measures for the management of salt-affected soils.

9.8.1. Management of reclaimed soils

General measures for prevention of salinization in reclaimed salt-affected soils aim to protect the soils from the development/reoccurrence of salt build up. These measures include:

1) Maintenance of a downward balance of movement of salts and water

2) Reduction in the replenishment of the ground waters and ingress of salts into irrigated areas

3) Reduction in ground water evaporation

9.8.1.1. Measures for maintaining a downward balance of salt and water

movement in the soils

Wherever natural drainage is available or artificial drainage has been provided, prevention of salination can be done if the balance of moisture movement (water) is maintained downward in the soil profile, i.e. more water is applied than the amount of water moving upward in the soil profile under evapo-transpiration forces. This can be achieved by the use of irrigation depth greater than the consumptive use of crops or by including such crops in the rotations which require excess irrigation depth (high delta water crops).

9.8.1.2. Measures for reducing the replenishment of ground waters and

ingress of salts into irrigated areas

i. Planned, rationed water utilization

Planned water utilization can be practiced in accordance with the nature of the soil, the depth of ground waters, type of agricultural crops grown and the type of economy in each irrigation system. This effort makes it possible to reduce the ingress of water and easily soluble salts into the irrigated territory by as much as 20-30 % of the head water intake. However, this requires the equipment for water measurement and control.

ii. Water usage according to weather conditions

A study of the autumn, winter, spring and summer weather forecasts should be done so that in the wet period of time no watering is done.

iii. Control of surplus irrigation

Surplus irrigation water must never be spread in any part of the irrigated area and flood water has to be controlled.

iv. Control of seepage

Seepage must be kept to a minimum. The losses in areas where the canals and water courses are not lined may be as high as 45 %. It is necessary to line the canals and water courses to control the conveyance losses as much as possible. Good results may be obtained in the initial stages by coating with clay materials.

v. Remodeling of ancient irrigation systems

Many of the ancient irrigation systems have not been rebuilt. Some canals lack the requisite hydrotechnical equipment, are meandering and too long. Measures are needed to reconstruct these systems according to the requirements of modern agriculture.

vi. Provision of water for domestic purposes

The use of irrigation canals for the delivery of water for domestic purpose during the period without irrigation must be avoided to control water seepage. For this purpose, special canals, storage ponds or wells have to be constructed.

vii. Field leveling

The fields must be carefully leveled under conditions where surface irrigation methods are used. This practice improves water-use efficiency.

viii. Correct planning for rice growing

Rice requires huge amount of irrigation water. If a greater part of an area is under rice cultivation, a sharp rise in the ground water may occur. Rice growing areas must be specially selected. They must lie at some distance from the main areas of irrigated land, and have good artificial drainage. Some areas, like the Indus Plains of Pakistan, are suitable only for rice growing because of the large volume of irrigation water available only during the summer.

9.8.1.3. Measures for reducing ground water evaporation

Ground water can move from the lower depths to the surface soil where water evaporates and leaves behind salts. The following measures can help reduce the ground water evaporation.

i. Plant cover over the field

To reduce ground water evaporation, it is necessary to keep a plant cover over the field. This is especially important in irrigated farming. Plant cover provides shade to the field, act as mulch and thus reduce surface evaporation.

ii. Improvement of soil structure

A granular water-resistant soil structure weakens the capillary rise and thus reduces the evaporation. Soil structure can be improved by the addition of organic matter (green manure), stubble incorporation in soil instead of burning, deep ploughing, cultivation in relation to irrigation schedule, and avoid overflowing of water after which the soil forms a crust upon drying.

iii. Tree plantation along roads and canals

Strip afforestation slows down the speed of winds and increases the air humidity thereby reducing the evaporation. On the other hand, the water consumption of trees is very high, thus the water table is maintained/lowered.

iv. Use of ground water for irrigation

Some ground waters having salt concentrations under permissible limits can be used for irrigation. This practice lowers the water table and decreases direct evaporation.

9.9. Management Strategies for Salt-affected Soils

Management of salt-affected soils can be divided into different aspects including leaching requirement (LR), selection of salt tolerant crops, irrigation practices, balanced fertilization, and planting techniques.

9.9.1. Leaching requirement

Part of the irrigational water that has to pass through root zone for the control of soil salinity problem at a specific level is referred as leaching requirement. It can also be described as the ratio between equivalent depth of drainage water (Ddw) to the equivalent depth of irrigation water (Diw). Similarly, LR can be calculated from the knowledge of the amount of salts present into the irrigation water (ECiw) and the permissible level of salt concentration in the drainage water (ECdw). Importance of LR can be depicted by the following simple equation as:

LR = Ddw ∋ Diw = ECiw ∋ ECdw . . . . . . . . . . . . . . (1)

Leaching requirement may be demonstrated in fraction form as well as percentage. The calculations for LR are made by assuming that there is always a steady-state water flow along with uniform application of irrigation water, no removal of salts in the harvested crop, no rainfall and no precipitation of soluble salts in the soil. By considering such assumptions drainage conditions of soil, depth of root zone, moisture and salt storage in soil, and cation exchange reactions remain neglected. On the other hand, it is assumed that the soil drainage will permit the specified leaching. Regarding field crops if ECdw = 8 dS m-1 it can be tolerated and thus formula for the calculation of LR would be as:

LR = Ddw ∋ Diw = ECiw ∋ ECdw = ECiw ∋ 8

For irrigation waters with EC values of 1, 2, 3, and 4 dS m-1, respectively, the LR will be 13, 25, 38, and 50%. These are the maximum values because rainfall, removal of salts by crops, and precipitation of salts in soils are seldom zero. The predicted value of LR may reduce if these factors are properly taken into consideration.

Equation 1 must be used with great care as the provision of steady-state and/or longtime average in this case is assumed. In equation 1 average EC of the irrigation water must be used over averaged longtime for the conductivities of the rain water (ECrw) and irrigation water (ECiw) as described in the given equation:

EC (rw+iw) = (Drw ECrw + Diw ECiw)∋ (Drw + Diw) . . . . . . . . . . . . (2)

Where Drw and Diw are indicating the depths of rain water along with the irrigational water that enters into the soil respectively. In order to restrain the soil salinity to cross a specified value, knowledge related to the consumptive use of water is an important factor if the LR concept has to be under consideration while determining either the depth of irrigation water that must be applied or the minimum depth of water that must be drained. The depth of irrigation water (Diw) is related to consumptive use (Dcw) and the depth of drainage water (Ddw) by the equation:

Diw = Dcw + Ddw . . . . . . . . . . . . . . (3)

Using equation 1 to remove Ddw from equation 3 gives:

Diw = Dcw / (1 - LR) . . . . . . . . . . . . . . (4)

Expressing the LR in equation 4 in terms of conductivity ratio in equation 1 gives:

Diw = [ECdw / (ECdw - ECiw)] Dcw . . . . . . . . . . . . . . (5)

Thus, the depth of irrigation water (Diw) can explained using the EC of irrigation water, consumptive use and salt tolerance of a crop. The crop salt tolerance is taken into account by the selection of the permissible values of EC of the drainage water or EC of the soil saturation extract.

9.9.2. Crop selection for salt-affected soils

In salt-affected soils, the wise selection of crops that can provide suitable yields (50% lower) under saline conditions may clearly differentiate between success and failure of any management option, particularly during early phase of colonization of such soils. Plant’s ability to endure the hazards of soil salinity within the root zone and provision of proficient growth is declared as the salt tolerance of the plants. salt tolerance potential of various plants can be evaluate using the following criteria as:

1) The ability of the crop to survive on salt-affected soils.

2) The acceptable yield of the crop on salt-affected soils, mostly 50 % reduced yield

3) The relative yield of the crop on a salt-affected soil as compared with its yield on a normal soil under the similar growing conditions.

The salt tolerance of a plant is not an exact value. It depends on many factors, viz. environmental and edaphic factors (soil fertility, physical condition of soil, salt

distribution in soil profile, irrigation practices, climate) and biological factors (stage of growth, varieties and rootstocks). The salt tolerance of some plants is given in Table 9.6.

Table 9.6 Tolerance of some crops to saline conditions. Salinity expressed as electrical conductivity

Sensitive

(0-4 dS m-1)

Moderately tolerant

(4-6 dS m-1)

Tolerant

(6-8 dS m-1)

Highly tolerant

(8-12 dSm-1)

Almond Corn Figs Barley Bean Grain Sorghum Oats Cotton Clover Lettuce Pomegranate Olive Onion Soybean Sunflower Rye Potato Tomato Wheat Wheatgrass

Source: Brady and Weil (2016)

9.9.3. Balanced fertilization

Salinity, sodicity and their combination induce unfavorable nutrient ratios in soils. Excess of Na+ and deficiency of many macro- and micro-nutrients are common in sodic and saline-sodic soils. The predominant factors responsible for low nutrient availability and mobility in sodic soils are high soil pH and poor soil physical conditions due to dispersed soil matrix because of Na+ dominance. For this reason, special fertilizer management practices are needed for optimum crop production.

Low organic matter coupled with deficiency of nitrogen is the basic feature of the salt affected soils. Nitrogen deficiency can be met by adopting the green manuring technique using sesbania species that also decrease the harms and hazards of salinity/sodicity. During the reclamation of the sodic soils part of the N may also leach down along with the other soluble salts and Na+. Some studies that were conducting in Pakistan (Murtaza 2011) as well as in India (Yaduvanshi and Dey 2009) reveal that application of higher dose of nitrogen than the requirement for the crops growing under saline/sodic conditions endow with more yield and production may be due to stimulation of dilution effect coupled with enhanced salt tolerance potential of plants (Woyema et al. 2012). Yaduvanshi and Dey (2009) and Murtaza et al. (2014) reviewed a series of experiments and recommended that rice and wheat crops grown in sodic soils should receive 25-30% N over and above the recommended rates for non-saline/sodic soils.

Sodic and saline-sodic soils usually have higher available phosphorus than the normal soils because higher concentrations of Na2CO3 results in the formation of soluble Na3PO4. On the basis of some studies, it has been proposed that the sodic soils after reclamation require less additional P fertilizer for some years. Similarly, it has been suggested that a 50% reduction in the recommended dose of P may be practiced for a rice-wheat rotation grown up to three years during reclamation without yield loss. Increasing sodicity nearly always results in a deficiency of Ca2+ concentration in the soil. Fertilizers containing Ca2+ (calcium nitrate, single superphosphate) or those producing physiological acidity (ammonium sulphate, urea) perform better than the equivalent rates of Ca-free or physiologically less acidic

materials like NH4NO3 etc. Generally, it is recommended that application of fertilizers, except P containing fertilizers, to the marginal salt-affected soils should be done at higher rates (15-30%) compared to their counterpart normal soils in any agro-ecological zone.

9.9.4. Planting techniques

Under field conditions, it is possible through the modification of planting practices to minimize the tendency of salts to accumulate around the seed and to improve the stand of crops those are sensitive to salts during germination. Seeds of a crop sprout only when they are placed so as to avoid excessive salt around them. The pattern of salt concentration changes with the shape of beds on which seeds are sown.

9.9.5. Saline agriculture

Saline agriculture is defined as the profitable and integrated use of genetic resources (plants, animals, fish, insects and microorganisms) and improved agricultural practices to obtain better use from saline land and saline irrigation water on a sustained basis. Saline agriculture presents a systematic approach for the utilization of salt-affected lands involving a combination of salt tolerant crops, crop genotypes and salt tolerant grasses, trees and shrubs. The components of this system are site-specific and are changed according to the farmer needs, land capability, locality, market availability and climatic conditions of the area. Salt-affected lands are mostly potentially productive although with a lot of spatial variability. Therefore, the potential of salt-affected land is evaluated and considered to select plants and other genetic resources for its utilization. Slightly salt-affected lands are used for salt tolerant varieties of different crops. Moderately salt-affected lands are used for salt-tolerant trees and grasses and the highly salt-affected lands are used for salt-tolerant shrubs and bushes.

In the world there are more than 1500 salt-tolerant plants species but in Pakistan less than 1% of these species are present. The major crops including rice, wheat, cotton and maize have different tolerance to salinity and associated problems. It has been observed that these major crops have little or no growth at ECe 15 dS m-1. However, there is genetic difference among the genotypes of each crop. Rice cultivars KS-282 and NIAB-6 are moderately salt-tolerant which produce about 30-35% more paddy than ordinary varieties. But rice is only crop that gives best results in water logged and sodic soil conditions. Salt-tolerant wheat varieties selected by Saline Agriculture Research Center at the University of Agriculture Faisalabad include SARC-I, SARC-II, SARC-III, SARC-IV, SARC-V and SARC-VI. Cotton crop is a salt tolerant crop but problems occur with the emergence in sodic or saline-sodic soils condition. NIAB-78 and MNH-93 are best salt tolerant cotton varieties.

Salt tolerant trees and grasses include date palm (Phoenix dactylifera), sugarbeet (Beta vulgaris), wheat and semidwarf (Triticum aestivum), bermuda grass (Cynodon

dactylon), kallar grass (Diplachne fusca), mesquite (Prosopis juliflora) and river salt bush (Atriplex amnicola). Many of the salt tolerant plants have the potential to rapidly grow at electrical conductivity ECe ≥30 dS m-1. These other salt-tolerant plants which can be used in saline agriculture include sugar beet (Beta vulgaris), fig (Ficus

carica), guar (Cyamopsis tetragonoloba), oats (Avena sativa), papaya (Carica

papaya), rape (Brassica napus), sorghum (Sorghum bicolar), soybean (Glycine max), Rhodes grass (Chloris gayana) and cynodon dactylon species (dela khabbal grass).

9.10. Economics of Soil Reclamation

Crop cultivation on stress soil is usually dejected because of the expensive soil reclamation process. While the success of any technology is dependent upon its cost: benefit ratio, economics is always considered a key factor for adoption by farmers. In most of the studies, economic evaluation of treatments is overlooked. If it is computed, then only on the basis of variable costs and produce only. The long term benefits, like appreciation in land value, improved environment, farm-level employment opportunities etc are not included in economic analysis. Multi-location research studies that were conducted on salt affected soils of Indus Basin in Pakistan comparing different amendments for the reclamation of saline sodic soils declare that gypsum has proved highly cost-effective than acids or acidulents for native soils. Acids and acid formers for the treatment of native salt-affected soils are not suitable because of clay mineralogy concerns since considerable chlorite is present in clay fraction. However, organic matter has no substitute regarding health of normal and salt-affected soils. The biological reclamation approach, although is cost-effective than the chemical amendments, but time and amount of irrigation water required to achieve soil reclamation make it impractical for most of the farmers except landlords. Small land holding (70% farmers own land <5 ha) is another issue to be considered while recommending reclamation technologies.

9.11. Conclusion

Soil and water salinity/sodicity are potential threat to irrigated agriculture. Salination and sodication of millions of hectares of land continues to severely reduce crop production in Pakistan and rest of the world. Salt-affected soils are classified into three major categories namely saline, saline-sodic, and sodic. Saline soils can easily be reclaimed through simple leaching with good quality water without any amendment even high EC water can serve the purpose during initial phase. For the reclamation of saline-sodic/sodic soils, there is a need of some Ca-amendment and gypsum is the most promising. Lower solubility of mined gypsum compared to other industrial sources is an additional advantage to sustain electrolyte concentration in these soils. Acids or acid formers can reclaim such soils relatively at a faster rate but at a much higher cost. Another way to combat the salinity/sodicity of soils is saline agriculture approach, i.e. cultivation of salt tolerant plants. Along with reclamation measures, various aspects related to agronomic management like mulching, tillage, green manuring and seed bed preparation do merit.

References

Ahmad, N.C. and M.R. Chaudhry (1997). Review of research on reclamation of salt-affected soils in Pakistan, Publication No. 175, IWASRI, WAPDA, Lahore

Anonymous (2011). Statistics of Pakistan. Ministry of Food, Agriculture and Livestock, Government of Pakistan, Islamabad, Pakistan.

Baig, M.B. and M.S. Zia (2006). Rehabilitation of problem soils through environmental friendly technologies - ii: role of sesbania (sesbania aculeata) and gypsum. Agric. Trop. Subtrop. 39: 26–33.

Chang, M.H. and Q.A. Sipio (2001). Reclamation of saline-sodic soils by rice husk. J. Drain. Water Manage. 5: 29–33.

Eaton, F.M. and V.P. Sokoloff (1935). Adsorbed sodium in soil as affected by the soil to water ratio. Soil Sci. 1:528–534.

Farid, M. (2000). Effectiveness of various gypsum particle sizes to reclaim a saline-sodic soil having different ECe : SAR ratios. M.Sc. Thesis, Dept. Soil Sci., Univ. Agri., Faisalabad, Pakistan

Ghafoor, A., M. Qadir and G. Murtaza (2004). Salt affected soils: principles of management. Allied Book Center, Urdu Bazar, Lahore, Pakistan.

Khan, G.S. (1998). Soil Salinity/Sodicity Status in Pakistan. Soil Survey of Pakistan, Lahore, Pakistan.

Latif, M. and A. Beg (2004). Hydrostaticaly issues, challenges and options in OIC member states.pp.1–14. In: Latif, S. Mahmood and M. M. Saeed (Eds), Training Workshop on Hydrostatically Abatement and Advance Techniques for Sustainable Irrigated Agriculture. September 20-25, 2004, Islamabad, Pakistan.

Murtaza, B., G. Murtaza M. Saqib and A. Khaliq (2014). Efficiency of nitrogen use in rice wheat cropping system in salt-affected soils with contrasting texture. Pak. J. Agric. Sci. 51: 431–441.

Murtaza, G. (2011). Technology transfer for using tube well water on salt affected soils for crop production. Final technology report for FDTTPC funded project (Jully, 2008 – June, 2011). Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan

Murtaza, G., A. Ghafoor, G. Owens, M. Qadir and U.Z. Kahloon (2009). Environmental and economic benefits of saline sodic soil reclamation using low quality water and soil amendments in conjugation with a rice wheat cropping system. J. Agron. Crop Sci. 1995: 124–136.

Murtaza, G., M.N. Tahir, A. Ghafoor and M. Qadir (1999). Calcium losses from gypsum and farm yard manure treated saline-sodic soilduring recalimation. Int. J. Agri. Biol. 1: 19–22.

Qadir M. and J.D. Oster (2004). Crop and irrigation management strategies for saline-sodic soils and waters aimed at environmentally sustainable agriculture. Sci. Total Environ. 323:1–19.

Qadir, M., B.R. Sharma, A. Bruggeman, R. Choukar-Allah and F. Karjah (2007). Non-conventional water resources and opportunities for water augmentation to achieve food security in water scarce countries. Agric. Water Manage. 87: 2–22.

Qadir, M., E. Quillérou, V. Nangia, G. Murtaza, M. Singh, R.J. Thomas, P. Drechsel and A.D. Noble (2014). Economics of salt-induced land degradation and restoration. Natural Resources Forum, A United Nations Sustainable Development Journal DOI: 10.1111/1477-8947.12054

Qadir, M., S. Schubert, A. Ghafoor and G. Murtaza (2001). Amelioration strategies for sodic soils: a review. Land Degrade. Dev. 12: 357–386.

Schoonover, W.R. (1952). Examination of soils for alkali. Extension Service, University of California, Berkley, California, Memeograph. USA.

Shainberg, I. and J. Letey (1984) Response of soils to sodic and saline conditions. Hilgardia 25: 1–57

Sharif, A. (2011). Technical adaptations for mechanized SRI production to achieve water saving and increased profitability in Punjab, Pakistan, Paddy Water Environ. 9: 111–119.

Tanji, K. K. (1990). The nature and extent of agricultural salinity problems. In: TANJI K.K. (ed.), Agricultural salinity assessment and management. ASCE Man. Rep. Engin. Prac. 71: 1–41.

WAPDA (Water and Power Development Authority) (2003). Salinity and reclamation department, SCARP Monitoring Organiztion, Lahore, Pakistan

Weil, R.R. and N.C. Brady (2016). The Nature and Properties of Soils, New Jersey, USA, Prentice Hall.

Woyema, A., G. Bultosa and A. Taa (2012) Effect of different nitrogen fertilizer rates on yield and yield related traits for seven Durum Wheat (Triticum turgidum L. var Durum) cultivars grown at Sinana, South Eastern Ethiopia. Afr. J. Food Agric. Nutr. Dev. 12: 6079-6094.

Yaduvanshi, N. and P. Dey (2009). Nutrient management in sodic soils. In: G. Singh, A. Qadar, N.P.S. Yaduvanshiand and P. Dey (eds.). Enhancing nutrient use efficiency in problem soils. Central Soil Salinity Res. Inst. Karnal, New Delhi, India. pp. 84–108.

Zinc-enriched farm yard manure improves grain yield and grain zinc concentration in rice grown on a saline-sodic soil. Int. J. Agric. Biol. 14: 787–792.

Chapter 10

Environmental Pollution and

Management

Muhammad Javed Akhtar, Muhammad Usman, Nabeel Khan

Niazi and Muhammad Ibrahim†

Abstract

Environmental pollution is one of the major problems being faced by the world posing significant health hazards. Due to their ubiquitous presence, serious health and environmental concerns, this chapter is dedicated to the commonly reported environmental pollutants. This chapter provides insights about various components of environment (biotic and abiotic), types of pollutants (inorganic as well as organic), their sources (point and non-point) in contaminated soil and water and their fate in environment that ultimately affects their persistence and toxicity. Effects of environmental pollution on human, animals and plants are also discussed, as environmental pollution is not only seriously affecting the human health but pose serious threats to animals and plants. Various concepts related to soil and water pollution are also being discussed including biological oxygen demand, chemical oxygen demand and eutrophication. This chapter also illustrates various techniques to remediate contaminated matrices including biological, physical and chemical strategies. A brief description of each remediation strategy is presented to highlight their merits and associated limitations for the removal of pollutants in contaminated soil and water. Risk assessment of environmental pollutants is an important

†Muhammad Javed Akhtar*, Muhammad Usman and Nabeel Khan Niazi

Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: drmjavedakhtar@yahoo.com Muhammad Ibrahim

Department of Environmental Sciences, Government College University, Faisalabad, Pakistan.

Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

218 M.J. Akhtar, M. Usman, N.K. Niazi and M. Ibrahim

component and is discussed in regard to plants, animals and humans as the last part of this chapter.

Keywords: Environmental pollution; Fate; Effects; Management; Remediation; Risk assessment

10.1. Introduction

The human population is increasing continuously by adding eight million persons per year and world’s population is project to become approximately 10 billion by the year 2050. Every individual creates his demands on natural resources which continuously tend to increase with the passage of time. The demands created by the individuals are resulting in rapid agricultural and chemical industrialization. These industries are releasing different organic and inorganic chemicals into natural ecosystem which are becoming a serious threat to human beings, animals and plants because the release of these chemicals has resulted in addition of a variety of pollutants into soil, water and air . The adverse effects for humans and other organisms are the acute toxicity, changes in genetics, carcinogenicity and birth problems. Many of these toxic pollutants are not degraded by biological, chemical and physical processes and result in a meaningful threat to environment

10.2. Components of Environment

The environment is comprised of different components like atmosphere, hydrosphere, lithosphere and biosphere. These described in British and American literature are describes below.

10.2.1. Components of environment as per British literature

In the British system the environmental components have been classified on the basis of life, i.e., as abiotic or biotic. The abiotic components are further listed as edaphic (land) and climatic (air, water) and the biotic components are classified as producers, consumers and decomposers. The study of structure of ecosystem was derived from this system of classification.

10.2.1.1. Abiotic components

The non-living entities are included in abiotic components i.e., atmosphere, hydrosphere and lithosphere, i.e., soil, air, water and rocks. The pollution in these components causes changes in climate resulting in global warming, affecting plants, animals and human beings.

10.2.1.2. Biotic components

The living entities in an ecosystem are included in biotic components of environment. These biotic components effect on other living organisms like animals who consume that organism, and the living food which is consumed by them. The producers, consumers and decomposers are the subcomponents of biotic factors.

Environmental Pollution and Management 219

i. Producers

These organisms convert energy into food, i.e. autotrophs like plants which convert sunlight, water and carbon dioxide into energy through photosynthesis.

ii. Consumers

These are the organisms which depend on other living organisms (producers) for their food, i.e., heterotrophs like animals.

iii. Decomposers

These are the organisms which breakdown the chemicals produced by the producers into simpler forms which can be reused. For example the feeding process of bacteria and fungi returns the nutrients present in the waste and dead organisms to the soil.

10.2.2. Components of environment as per American literature

The components of environment as described in the American literature are given below (Fig. 10.1).

Fig. 10.1 Components of environment as per American literature.

1) Atmosphere (Air)

2) Hydrosphere (Water)

3) Lithosphere (Land)

4) Biosphere (Flora/Fauna/Microbes)

5) Anthrosphere (man-made things)

10.2.2.1. Atmosphere

The atmosphere is the gaseous envelop which surrounds our planet, Earth. The major part of the atmosphere is found close to surface of the Earth and it is most dense at sea level and rapidly decreases with increasing altitude. Major components of atmosphere are about 79% nitrogen and 21% oxygen; the remaining part consists of carbon dioxide (0.03%) and other gasses such as neon, helium, krypton, xenon, water vapours, dust particles and hydrocarbons, etc.

10.2.2.2. Hydrosphere

All of the water present on or near the surface of the Earth is the part of hydrosphere. This includes the oceans, rivers, lakes, and even the moisture in the air. Almost 97% of the Earth's water is in the oceans. The remaining part of it is fresh water and about 2% of this fresh water is solid which exists as ice sheets.

10.2.2.3. Lithosphere

The lithosphere consists of the solid, rocky crust which covers the entire planet. This crust is composed of minerals and it is inorganic in nature. It covers the entire surface of the Earth from the top of Mount Everest to the bottom of the Mariana Trench.

10.2.2.4. Biosphere

All the living organisms which include plants, animals and microorganisms constitute the biosphere. Most of the planet's life is found from three meters below the ground to thirty meters above it and in the top 200 meters of the oceans and seas.

All spheres of earth interact with each other in a common place. For example, soil will have mineral material from the lithosphere; moisture from the hydrosphere, insects, plants and other organism from biosphere, gases in the soil pores from the atmosphere.

10.3. Environmental Quality

Environmental quality is defined as the set of characteristics and properties of the environment. The properties may be local or generalized which directly or indirectly affect human beings and different organisms. Environmental quality is a measure of the condition of an environment in relation to the requirements of one or more species and /or to any human need or purpose.

10.4. Soil Pollution

Soil pollution may be defined as any physical, chemical and biological adverse change in soils made by high concentrations of chemicals, which are out of their proper place and have adverse effects on organisms living on or in the soil.

10.5. Sources of Soil Pollution

10.5.1. Agricultural wastes

The agricultural wastes produced on the farms as a consequence of farming practices include a wide range of organic and inorganic contaminants. The pollutants contained in those wastes can be added into the soil through runoff or drainage ditches. The most important kinds of pollutants from agriculture are:

10.5.1.1. Artificial fertilizers

The increased use of fertilizers on global scale to supply nutrients to crops also pollutes the soil because of the presence of some impurities present in them. Most of these fertilizers are stable chemicals and remain as such in soil, but the nitrate plays an important role because of the possibility it’s leaching into groundwater and deteriorating its quality. The nitrate leaching can happen due to application of high rate of nitrogenous chemical fertilizers couple with high rainfall and/or heavy irrigations.

Some phosphatic fertilizers may contain varying concentrations of cadmium. Therefore, these fertilizers can contaminate soils with cadmium. The rock phosphate may contain varying levels of cadmium; therefore, the producers of phosphatic fertilizers select rock phosphate on the basis of cadmium content. The concentration of cadmium in phosphate fertilizers may vary from 0.14 mg kg-1

to 50.9 mg kg-1 in

mono-calcium phosphate fertilizer (Lugon-Moulin et al. 2006) because the rock phosphate used can contain up to 188 mg kg-1 cadmium.

10.5.1.2. Pesticides and herbicides

Indiscriminate use of pesticides to control insect pests and diseases in crops, like cotton and vegetables, is of great concern as a source of soil pollution because the residues of these pesticides, such as aldrin, dieldrin, endrin, may get sorbed onto the soil. Consequently, the crop plants grown in the contaminated soils absorb these toxic pesticides; thus, toxic pesticides may become part of the food chain. Also, we know that pesticides not only kill the living pests (i.e., insects and pathogens) present on the plants and on soil surface but through tilling and irrigation operations, pesticides may reach to greater depths of the soil and thus may kill useful living organisms there.

10.5.1.3. Industrial effluents

The economy of Pakistan is rapidly transferring towards industrialization. The industries such as electroplating, textile, tannery, are discharging a variety of untreated effluents into rivers and irrigation channels. The toxic heavy metals, such as arsenic (As), lead (Pb), chromium (Cr), nickel (Ni), copper (Cu), mercury (Hg), etc, are likely to be present in these effluents; thus, untreated industrial discharge is a major cause of eco-toxicological pollution in both soils and water. The fresh water resources are continuously depleting and creating acute shortage of water for agricultural sector. This situation forces the farmers to use industrial effluent as

valuable source to meet the crop water requirements. The irrigational usage of these effluents pollutes the soil and water bodies.

10.5.1.4. Petroleum spills

The petroleum products such as benzene, xylene and toluene can pollute the soils due to petroleum spills or leaks from fuel tanks, gas stations or some other human activities. Additionally, some underground spills may also make their way to the surface soil. Some of these chemicals, particularly volatiles, remain in the surface soil and are taken up by the plants growing in such contaminated soils. These pollutants ultimately enter into the food chain.

10.6. Point Source Pollution

The point source pollution is defined as “any single identifiable source of pollution from which pollutants are discharged, such as a pipe, ditch, ship or factory smoke stack (Hill 1997). Point source pollution is the pollutants released at a specific point from outfalls, pipes, conveyance channels from either wastewater treatment plants or individual waste treatment facilities. Point sources also include pollutants being contributed by tributaries to the main water stream or river.

10.6.1. Point sources

The sources of point source pollution include sewage treatment plants and factories. Different factories, i.e., of textile, electronics, chemicals, paper and pulp mills, discharge some specific type of pollutants in their effluents. These effluents are discharged either directly on soil or into water bodies from where these effluents are used for irrigation purposes which results in the addition of pollutants in soil.

The point source pollution can be monitored and controlled through permit system. The point source discharges can also be controlled if the factories and sewage treatment plants are bound to obtain permit from the Environmental Protection Department for the discharge of their effluents into water bodies. The point sources should also be advised to use advanced technologies for the treatment of their effluents before their discharge to minimize the amount of pollutants. The different types of pollutants discharged from different industries are given below in Table 10.1.

Table 10.1 Types of pollutants discharged by different industries

Type of industry Contaminants Reference

Petrochemicals and Refineries

Metals

Cd, Cr, Cu, Iron (Fe), Ni, Pb, Zn, Aluminium (Al), Barium (Ba), Molybdenum (Mo), Strontium (Sr) Organic/ inorganic matter and

parameter

Benzene, styrene, toluene, indene, Naphthalene, 1, 4-dioxane, Ethyl Benzene, Xylene, O&G

Ahmad et al. (2008)

Paper Industry Metals

Cd, Cu, Fe, Ni, Pb, Zn Organic/ inorganic matter and

parameter

BOD, COD, TDS, dissolved solids (DS) Chloride, sulphate, phosphate

Devi et al. (2011)

Cystine production industry

Organic/ inorganic matter and

parameter

Sodium, chloride, calcium, COD, BOD

Srivivasa Gowd & Kotaiah et al. (2000)

Pharmaceutical and food industries

Metals

Fe, Zn, Cu, Ni, Pb, Cr

Radić et al. (2010)

Chemicals, beverage manufacturing, tanneries, oil, soap, paint production, paper, and metal processing plants

Metals

Fe, Mn, Pb, Zn, Cu, Ni, Cr, Cd , Co Organic/inorganic matter and

parameter

DO, COD

Jonathan et al. (2008)

Dyeing and printing industries

Metals

Cu, Fe, Zn and Mn Organic/inorganic matter and

parameter

TDS, TSS, COD, BOD, chlorides, sulfates, carbonates and sodium, calcium and magnesium

Nepal Singh et al. (2000)

10.7. Non-Point Source Pollution

It is a factor, which contributes to water pollution and cannot be pointed or traced as a particular point. Non-point source pollution is contributed from urban water runoff, agricultural operations, runoff from construction sites, atmospheric deposition,

rainfall, etc. It means that non-point source pollution comes from different diffused sources.

10.7.1. Non-point sources

10.7.1.1. Urban and peri-urban areas

The urban and peri-urban areas are the major contributors of nonpoint source pollution from where the huge runoff is produced from the paved roads and other surfaces. The materials asphalt and concrete used for paving the roads and surfaces do not allow the penetration of water into them. When the water comes in contact with these paved surfaces, runoff will take place and it is discharged in the surrounding areas. Thus the pollutants carried by that runoff water are easily deposited in soils of the surrounding areas. The materials are easily eroded from the construction sites by rain, snow and hail. Additionally, debris discarded from these sites can also be transported by runoff water and deposited into water reservoirs.

There are the chemicals which are typically used for lawn care in the suburban areas. These chemicals enter into the runoff water and added up in the surrounding environment through storm drains of the city. These chemicals enter into the water bodies directly because the storm water is not treated before its entrance into drains.

10.7.1.2. Agricultural operations

Agricultural operations contribute a major part of nonpoint source pollution because of plowing of large fields, these soils are exposed and the soil is loosened. During rainfall, these fields become more susceptible to erosion. This run off also can enhance the quantity of fertilizers and pesticides transported to the nearby water reservoirs.

10.7.1.3. Atmosphere

The different pollutants can enter into the atmosphere from different sources e.g. factories, transport. The factories emit different pollutants into the atmosphere through smoke. These pollutants are added into soil through rainfall. The atmospheric pollutants can also become source of water pollution when added into water bodies in the form of rain or snow.

10.7.1.4. Forest cutting

Forest cutting reduces the number of trees in that area which results in reduced soil stability. The plant leaves and roots are added into the soil which after decomposition produces organic substances which help in aggregation of soil particles and reduction of soil erosion. Therefore, it is likely that when the trees are cut the addition of plant leaves and roots is minimized which results in soil erosion. Further when heavy machinery is used for this purpose, the risk of soil erosion is also increased.

10.7.1.5. Mining

The abandoned mines are also a source of nonpoint source pollution. The runoffs from abandoned mining operations carry the pollutant with it and contribute to nonpoint source pollution. The top of the mountain is removed in strip mining

operations, to expose the desired ore. The soil erosion from this area is noticed if it is not properly refilled after completion of the mining process. Moreover, some chemical reactions of air and newly exposed rock may occur resulting in runoff. The water which releases from the abandoned mines may be highly acidic in nature and discharged into the nearest water bodies and it can change the reaction of water.

10.8. Types of Soil Pollutants

The different types of soil pollutants are described below:

10.8.1. Organic pollutants

Domestic garbage, municipal sewage and industrial wastes are the major source of organic pollutants and, if not disposed off properly, these become serious threat to human health, plants and animals. The organic groups that contain carbon like polycyclic aromatic hydrocarbons (PAHs), dioxins and furans, phthalates and brominated flame retardants, detergents and pesticides are included in these pollutants.

10.8.1.1. Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are organic compounds are found in tar materials which have been used for many centuries to preserve wooden materials in ships and buildings. Some of these compounds have the potential to be carcinogenic even when present in small quantities. The burning/combustion of coal and fossil fuels also produce PAHs. These PAHs are spread on soil in urban and peri-urban areas which contribute towards soil pollution.

10.8.1.2. Dioxins and furans

Dioxins and furans are toxic compounds which are prone to degrade very slowly as they contain benzene rings. Different quantities of chlorine atoms attach with these benzene rings, which results in the production of different furan and dioxin compound of different levels of toxicity. The primary source of these compounds is the incineration of organic wastes. The required temperature range for these compounds is 300 to 500 ˚C. The increase of temperature above this range causes the destruction of these compounds. The extent of spreading these compounds depends upon the wind velocity as they spread into the atmosphere through smoke.

Their sources in urban areas are incinerators, industrial and domestic fuel burning, metal industry, transport and fires.

10.8.1.3. Phthalates

Phthalates are organic compounds used as plasticizers in huge quantities. The sources of phthalates may be industrial processes, addition of products, recycling and disposal of products. Their presence has been reported in most of the ecosystem parts as well as food. The different concentrations of phthalates can accumulate in organisms which disintegrate at a slower rate in sediments than in water. The diethyl phthalate (DEHP) is well known compound of phthalate.

10.8.1.4. Brominated flame retardants

These are characterized by the presence of organic compounds containing bromine which helps to retard flame. These compounds are emitted into soil, water and air from industrial production processes, by using different products and from waste management processes.

10.8.2. Inorganic pollutants

Inorganic pollutants include nitrates, phosphates, heavy metals and inorganic acids. Major toxic heavy metals are cadmium, chromium, lead, mercury, nickel, zinc, etc. The two main reasons for the transfer of heavy metals to soil are smelting and mining activities and the spreading of sewage sludge on soil. The point source emissions of large metal industries are causing large scale contamination of land. The local impact of mining activity on soils may be very high, metal concentration in soils with values of order of 1%. The other industries causing heavy metal contamination are chemical manufacturing, oil refining, metal processing and plating, tanneries and fertilizer manufactures. Major sources and toxic effects of metals on animals and human health are given in Table 9.2.

10.8.2.1. Radio-active pollutants

The radio-active contaminants arise from the wastes from mining of uranium (U) and thorium (Th) (both absent in Pakistan), from wastes of hospitals, medical and other research laboratories. Such materials mainly remain on soil surface. Rock phosphate is said to contain U, Th and Ra radio-active nucleates. But locally these are not mined or processed at a scale to cause pollution.

10.8.3. Microbial pollutants

In Pakistan, the urban soils regularly receive refuse and organic manures e.g. FYM), the amounts of which can be visualized from large number of domesticated animals. It is expected that such refuse may contain pathogens, such as typhoid, dysentery, tuberculosis, poliomyelitis and many spp. of anaerobes. However, these microorganisms are not always found in various wastes but sanitary aspects should not be ignored. In general, metabolites of microorganisms and plants accumulating in soils as well as the established relations among the many groups of soil micro-biocenosis stabilize it to some extent and tend to eliminate foreign microorganisms, i.e. a natural control. This is precisely what determines soil’s self-purification process.

10.9. Water Pollution

A body of water, such as a lake, stream, river, pond, ocean or even underground water can become polluted when it is contaminated by sewage leaks, agricultural runoff or chemical spills. Thus, it is unsafe for human consumption because the water contains dangerous or toxic substances and disease-causing microorganisms.

When water is polluted from point sources, this is pollution from a discrete location.

Table 10.2 Major sources and toxic effects of metals on animals and human health

Metal Source Toxic effects of metals

Ni, Mo, Pb, As, Hg

Industry Sb: Shortening of life in rats. Be: Most toxic, accumulates in lungs to cause beryliosis – a fetal disease. Bi: Damages kidney and liver if taken in large doses. Cd: Cardiovascular disease and hypertension, interferes with Zn and Cu metabolism, severe nausea, salivation, vomiting, diarrhea, lipid deposition in the arteries of heart, abdominal pain. Pb: Brain damage, convulsions, behavioral disorder, fatal, anemia, headache, dizziness, loss of memory. Hg: Nerve damage, fatal. Cu: Nausea, epigastric burning, diarrhea, gastrointestinal bleeding. Ni: Carcinogenic for animals and human beings, particularly if inhaled as carbonyl [Ni(CO)4], abdominal discomfort, vomiting, nausea, diarrhea, headache, cough. Sn: Low order of toxicity, decrease life span in rats and mice. As: High levels inhibit tissue oxidation and cholesterol, lipids and amino acids synthesis, and can cause precipitation of serum proteins. Cr: Lung cancer, gastrointestinal upset, ulcer, edema, dermatitis. Fe: Gastrointestinal hemorrhage, metabolic acidosis. Mn: Lethargy, increase in muscle tone tremor, mental disturbance. Zn: Dehydration, electrolyte imbalance, abdominal pain, nausea, lethargy, dizziness, muscular discoordination. Co: Loss of apatite, anaemia leading to death.

Cr, Na, Cl

Waste gases, water, residues, metal industry

Pb, Cr, Hg, As

Tanneries, POL burning

Cd, Pb, Ni, Cu, Cr, Sb

Dust from coal and petroleum combustion

Hg, Pb, Cd

Waste water from electro-plating units

Pb Waste water from plastic and battery industry and autovehicle exhaust

Cu, As Exhaust gases from automobiles

Cr, Cd, Na

Pesticides, rubber and soap industry

I, P, Cr, Cd, Na

Textile, rubber, dye and leather industries

Source: Sethi and Sethi (1991)

This discrete location could be a factory, a sewer pipe or a runoff from a single farm. The Deepwater Horizon oil spill in 2010 (also referred to as the BP oil spill, the BP oil disaster, the Gulf of Mexico oil spill, and the Macondo blowout) is an example of point source pollution, because the massive amount of oil leaked from a single point of origin.

The water pollution could also be from non-point sources, that is, when several points of contamination over a large area contribute to the pollution of a water body. For example, one water body may be contaminated by multiple sources like agricultural runoff, city street runoff, construction sites and residential lawns.

10.9.1. Surface water pollution

Surface water pollution is the pollution of aquatic systems that are above ground, such as streams, lakes and rivers. These waters become polluted when rainwater runoff carries pollutants into the water. The pollutants transported by runoff are things like salts and chemicals from city and highway roads and nutrients and fertilizers from farms and lawns.

When pollution is caused by nutrients and fertilizers, this is called nutrient pollution, and it leads to an over production of algae and other aquatic plants. This overabundance of plants and algae causes problems, because they cover the water surface and prevent sunlight from reaching the plants underwater. This then leads to less oxygen production, which causes harm to oxygen-breathing organisms in the water, like fish.

Surface water may also be polluted with pathogens and causes waterborne diseases, which is usually the result of sewage leaks and runoff from animal sheds. These viruses and bacteria that pollute the water may cause dangerous human health problems such as giardia, typhoid and hepatitis. Toxic chemicals may also lead to surface water pollution. These come from pesticides, synthetic chemicals such as petroleum products and other car fluids, mercury, lead and arsenic from mining site drainage.

10.9.2. Groundwater pollution

The water found underground in cracks and voids in soil, sand and rock is known as groundwater. The groundwater channels are generally formed with gravel, sand stone, sand or fractured rocks such as limestone. The groundwater table could be deep or shallow and it could increase or go-down in depth contingent on several aspects.

The regions where material above the aquifer is penetrable, contaminants could immediately leach into groundwater channels. Groundwater can be contaminated by underground gas tanks, septic tanks, landfills and due to the extensive use of pesticides and fertilizers. If groundwater is contaminated it becomes unfit for drinking.

Contamination of groundwater resources occurs when contaminated oil, gasoline, road salts and toxic chemicals are released into the groundwater and the water becomes unfit and unsafe for human use. In addition, the natural processes such as weathering of parent material, oxidation-reduction reaction of minerals can also result in the release of some toxic heavy metals and metalloids in the valuable groundwater reservoirs. For example, arsenic and fluoride are released in the groundwater systems due to weathering of minerals (containing these toxic elements), in many countries of the Southeast Asia region. The pesticides and

fertilizers can leach to the groundwater supplies over time. Toxic substances released from mining sites, road salts and used motor oil from garages also may leach into the groundwater. Additionally, it is also possible that untreated waste materials from septic tanks and extremely toxic chemicals from leaked landfills and underground storage tanks may contaminate groundwater.

10.9.3. Biological oxygen demand

The biological oxygen demand (BOD) is defined as the amount of O2 required by microorganisms for decomposition/breakdown of organic matter present in a given water sample at certain temperature over a specific time period. Biological oxygen demand is an important factor to determine the O2 demand of the water coming from various sources (Yadav et al. 2013).

The organic matter can be decomposed by various microorganisms present in the water body. Aerobic microorganisms (bacteria and fungi) use oxygen and break down the organic matter and the end products are water, carbon dioxide, nitrate and phosphate. As the microorganisms ingest O2, concentration of dissolved O2 in the water stream starts decreasing.

The microbial species intolerant of low oxygen levels are stressed when the dissolved oxygen level drops below 5 mg L–1. When O2 concentration in water body is below 2 mg-1 over a few hours, the aquatic life such as fish species can die. For the O2 levels below 1 mg L-1, anaerobic bacteria (living in oxygen deficient environments) take the place of aerobic bacteria (Jordan et al. 2013). With the degradation of organic material by anaerobic bacteria, rotten egg smelling hydrogen sulfide is produced.

The biological oxygen demand can be divided into two parts: (i) the carbonaceous bio-chemical oxygen demand; and (ii) nitrogenous oxygen demand. The carbonaceous biochemical oxygen demand (CBOD) is related to the degradation of organic materials (molecules), for example, cellulose- and sugar-based compounds into CO2 and water.

The nitrogenous oxygen demand (NOD) is related to the breakdown of protein-containing compounds. The nitrogen is degraded from a sugar-based compound, usually in ammonia (NH3) form, which is immediately transformed to nitrate in the water environment. On the release of nutrients mainly nitrate and phosphate into the water body, the growth of plant species in the water is enhanced. The result is an increased plant decay that enhances the microbial populations, and as a result higher levels of BOD, and high demand of oxygen from the photosynthetic organisms during the dark hours. This leads to a decline in dissolved oxygen levels, particularly in the early morning times.

10.9.4. Chemical oxygen demand

Chemical oxygen demand (COD) is a measure of the capability of water to utilize O2 during degradation of organic materials and oxidation of inorganic substances such as NH3 and nitrite. It is expressed in milligrams per liter (mg/L), which indicates the mass of O2 consumed per liter of solution. The chemical oxygen demand determination is usually made on samples of wastewaters or of natural waters

polluted by wastes from domestic or industrial sources. It is related to BOD that is used as a standard test to assay the oxygen-consuming strength of waters. However, BOD only estimates the amount of O2 required for microbial oxidation and is highly relevant to wastewaters containing high content of organic matter.

10.9.5. Eutrophication

The ingression of the water body by surplus amount of phosphate and nitrate nutrients is known as eutrophication and it could lead to an explosive growth of algae, which probably disturb the water environment (Fig. 10.2). The algae can reduce the light penetration and when algae ultimately die, the microorganisms in water degrade them. Relying on the environment (e.g., silent bay or rough oceans/seas) and the kind of algae species (microscopic or macroscopic), the algal bloom can be found in diverse forms, e.g., foam/green tide on the beach.

Fig. 10.2 Schematic diagram of the different pathways of nutrient deposition into waters and ensuing processes leading to eutrophication (algal blooms) and hypoxia (Figure provided by Dr. Hans Paerl and Alan Joyner, University of North Carolina at Chapel Hill, Institute of Marine Sciences)

The major harmful effects of eutrophication are:

• Nutrient pollution in water bodies caused as a result of eutrophication can affect the aquatic life and may cause their death.

• Pollutants can negatively affect the reproductive processes, development of aquatic life and can increase their susceptibility to diseases.

• Smaller life forms may consume chemical pollutants which can become a part of food chain when they are passed to larger animals.

• Pollutants can change the metabolic activities of microbes residing in surrounding areas which can lead to the destruction of some food chain layers.

• High biomass of aquatic plants leads to algal blooms.

• Hypoxic water environment (deficiency of dissolved oxygen in water).

• Increase in fish deaths.

• Unacceptable and bad taste, color and odour of water.

• Reduction in biodiversity of species.

10.10. Effects of Pollution

There are of course many effects of environmental pollution which vary widely. The excessive levels of pollutants in air, soil and water are serious threat to humans, animals and plants. The various types of pollutants present in the environment affect the organisms or group of organisms. Their effects on living organisms may vary from minor disorders to serious diseases like cancer and missing limbs.

10.10.1. Effects of pollution on human health

Since soil and water pollution are closely linked, therefore their effects appear to be similar. Prominent problems associated with them are listed below:

• Exposure to various diseases like cancer, damage to the DNA, typhoid, hepatitis, cardiovascular diseases, stomach disorders, etc.

• Hormonal problems.

• Damage to the nervous system.

• Disruption to the reproductive and development processes.

• Damage to the body organs (liver, kidney etc.).

• Respiratory problems.

• Irritation and general illness.

10.10.2. Effects of pollution on trees and plants

Plants have ability to absorb pollutants from contaminated soil and water which become a part of food chain and ultimately passed to animals and human.

Pollutants have the capacity to disrupt photosynthesis in plants.

It may change the plant metabolism, growth and yield.

Pollutants can also lead to the death of plants and thus affect whole ecosystem.

10.11. Management of Soil and Water Pollution

10.11.1. Physical Management

10.11.1.1. Incineration

The process which is used to burn hazardous substances in an “incinerator” at temperatures high enough to degrade pollutants is called incineration. Incinerators are specially designed furnace to burn hazardous substances through combustion. Incineration can treat various hazardous materials like soil, gases, liquid and sludge by applying heat from 871–1371°C. It has the capacity to degrade various hazardous chemicals like pesticides, solvents, PAHs, PCBs, etc. but incineration is ineffective to destroy heavy metals, such as lead and chromium. It requires excavation or pumping of materials to be treated into the containers before incineration. After necessary preparations (drying to remove excess water, removal of debris and large rocks, grinding, etc.), materials are exposed to very high temperatures in combustion chambers for specified period of time. Nature of waste and contaminants present within determine the treatment length and temperature.

Lot of fuel/energy is required for incineration but there is a possibility to use generated heat for the production of electricity. Moreover, it is a rapid process that can quickly clean the contaminated materials to avoid immediate harm to the people or targeted environment. Incineration decreases the amount of material to be disposed off.

10.11.1.2. Thermal desorption

Thermal desorption is used to remove organic contaminants from excavated soil, sediments and sludge by applying heat which causes the pollutants to evaporate. Evaporation separates the organic pollutants from solid phase. This technique has shown strong efficiency to remove volatile organic compounds and semi-volatile organic compounds. Thermal desorption is differentiated from incineration on the basis of temperature difference, remediation strategy and time of treatment. In thermal desorption, materials are heated to evaporate the organic pollutants while incineration burns the contaminated material and thus volume of treated material is reduced at the end of treatment in incineration. Moreover, temperature varies from 93–538°C (depending upon pollutant present) in thermal desorption while incineration is characterized by higher temperature ranging from 871–1371°C. Thermal desorption usually takes longer than incineration.

Contaminated vapors are formed as a result of thermal treatment and thus gas released during treatment should be collected for further treatment. Sometimes vapors of organic pollutants are condensed to change them into liquids. These newly formed liquid chemicals may be treated by incineration or recycled for reuse. Dust particles if formed should also be removed from the vapors. Vapors can also be emitted directly to the atmosphere without treatment if dust is not a problem and the concentrations of pollutants are low enough. Thermal desorption offers a rapid cleanup solution to remediate soils highly contaminated with organic pollutants at shallow depths.

10.11.1.3. Soil washing

Soil washing is an ex-situ remediation technique to remove organic and inorganic pollutants by applying liquids and mechanical processes. Soil washing is used to dissolve or suspend the target contaminants by applying liquids comprising of water and/or combined with solvents. Chemical wash additives (acids, caustics, surfactants and chelating agents, etc.) are chosen on the basis of nature and extent of pollution, their ability to enhance pollutant availability and environmental impacts. Physical processes are also employed to remove pollutants. It includes the physical separation of fine-grained soil particles (clay and silt) from the rest of the soil. Physical separation of fine soil particles is employed because most of the pollutants tend to sorb and accumulate onto the fine-grained soil particles. Separation of these silt and clay particles from the rest of the soil decreases the volume of polluted soil for further treatment. Other methods can be used to treat this smaller volume of soil (chemical oxidation, incineration, thermal desorption or bioremediation) if not disposed.

10.11.2. Chemical Management

10.11.2.1. Advanced chemical oxidation processes

Chemical oxidation is an important environmental remediation technique used to treat various organic pollutants from contaminated matrices like drinking water, wastewater and soil. Chemical oxidation involves rapid destruction or degradation of contaminants with no significant waste products. This process involves the use of chemical oxidants able to oxidize organic pollutants and to convert them into non-toxic or less toxic forms. Injection of oxidants into the aquifer or soil is followed by its mixing with polluted materials to accomplish chemical oxidation at a contaminated site. Various oxidants are used including permanganate (MnO4

-), ozone (O3), persulfate (S2O8

2-) and Fenton’s reagent [hydrogen peroxide (H2O2) and ferrous iron (Fe2+)].

The oxidants being used are readily available and length of treatment is usually in days (sometime months) instead of years and thus making chemical oxidation economically feasible. Moreover, chemical oxidation can be employed at highly contaminated sites where bioremediation seems ineffective due to extreme pollution or elevated toxicity enabling microbes to survive there. Chemical oxidation is also known for its non-selective nature and thus able to degrade variety of organic pollutants coming into contact with chemical oxidants.

The efficiency of chemical oxidation process depends upon the quantity of oxidant being used, nature of oxidant especially its residence time or stability, geological conditions and how effectively oxidants come into contact with the pollutants. Major drawback associated with chemical oxidation is natural oxidant demand caused by non-selective oxidation which degrades the non-target compounds also. Significant chemical losses occur because of non-target consumption of oxidant by soil rather than contaminants thus increasing overall cost of the remediation. Higher oxidant doses also affect the physical properties of soil e.g. clogging of soil pores which in return affect soil aeration and dissolved residual oxygen used by aerobic microorganisms to degrade contaminants. Moreover, chemical oxidation is limited

by efforts to adjust contact time, temperature and pH of reaction which are important to ensure the desired extent of oxidation.

10.11.2.2. Sorption

Sorption is described as the fixation of molecules from liquid phase to solid phase. Sorption includes both absorption and adsorption. Adsorption is the attachment of sorbate (pollutant) at the interface of an aqueous phase and sorbent (solid material on which pollutant is sorbed) while absorption is defined as the partitioning from the solution phase into the sorbent matrix. Contaminated matrices can be complex and heterogeneous where both adsorption and absorption may occur at the same time, and it is often impossible to differentiate between the two.

Sorption removes the pollutants from contaminated matrices by reducing their activity as well as availability and therefore sorbed molecules are less toxic to the ecosystem and human health. Moreover, due to reduced availability, biodegradation of organic pollutants is also decreased with sorption. Rate of sorption/desorption determines the efficiency of other remediation techniques like bioremediation due to its influence on bioavailability.

Sorption is an important process in contaminated environments as it controls transport, transformation and distribution of the contaminants. It is also an important process of natural attenuation for relying on natural processes to decontaminate the polluted sites. Other processes involved in natural attenuation include biodegradation, dilution, evaporation and chemical reactions with natural substances.

10.11.2.3. Electrokinetic remediation

Electrokinetic remediation is an efficient technique to remediate water and soils polluted by organic, inorganic and mixed contaminants. This technique is particularly effective in fine-grained soils having large surface area and low hydraulic conductivity. However, its efficiency is limited by sorption of pollutants onto the soil particles. Moreover, hydrogen ions and hydroxide ions produced at the electrodes also affect its efficiency in a negative way. Recovery of ionic contamination from groundwater is limited by the fact that soil is a powerful ion exchange media. A direct-current (dc) electric field is imposed on the polluted soil which causes the migration of the contaminants by the combined mechanisms of electro migration, electro osmosis and/or electrophoresis.

10.11.2.4. Chemical extraction

Chemical extraction is used to transfer pollutants (organic, inorganic and mixed) from contaminated matrices into an aqueous solution by using an extracting liquid containing a chemical reagent (surfactants, salts, chelating agents, redox agent or acids/bases). Chemical extraction is accomplished by using chemical solutions having ability to enhance the availability or solubility of targeted pollutant followed by their separation from contaminated matrices. Certain forms of the metal compounds are formed as a result of extractions which are more soluble (e.g., conversion to soluble metal salts by valence change). Efficiency of this process depends upon the geochemistry of polluted medium, characteristics and extent of

pollution, chemistry and dosage of extraction agent and extraction conditions (pH, solid/liquid ratio, reaction time, mode of extraction etc.).

10.11.3. Biological Management

10.11.3.1. Phytoremediation

Phytoremediation is defined as the use of various plant species to eliminate, stabilize, transfer or destroy pollutants in the soil and water. Various types of phytoremediation are described below:

Phytodegradation: The plant roots secrete natural substances, which are a good source of nutrients to microbes in the soil. As a result, microbial species increase the degradation process. Phytostabilization – the chemical compounds produced by the plants and/or plant roots immobilize and bind the contaminants, rather than degrading them. Phytoaccumulation or phytoextraction: The plant roots extract the contaminants, mainly heavy metal (loid)s, along with other nutrients and water. This method is used primarily for remediation of soils containing heavy metal(loid)s, e.g., lead (Pb), arsenic (As).

Phytovolatilization: In this process, plants extract water contaminated with organic contaminants and then the contaminants are evaporated into the atmosphere through their leaves.

10.11.3.2. Applications of phytoremediation

Phytoremediation is a useful technique for the treatment of metals, pesticides, radionuclides, fuels, explosives, semi-volatile organic compounds (SVOCs) and volatile organic compounds (VOCs).

Studies on phytoremediation have shown that there are about 400 plant species which are capable for hyperaccumulation of heavy metals (Brooks 1998; Baker et al. 2000). The plant species capable of hyperaccumulation of heavy metals are either high biomass producing plant e.g. willow (Landberg and Greger 1996) or low biomass producing plants having high hyperaccumulating traits e.g. Thlaspi and Arabidopsis species.

Further research is being conducted to enhance our understanding about the role of phytoremediation in remediating perchlorate, which is present in surface and groundwater bodies and is reported to have high persistent in the environment. For radioactive pollutants and heavy metals, use of chelating agents is very important to increase the uptake of contaminants by plants.

10.11.4. Bioremediation process and its types

Bioremediation is the process of using microorganisms, plants, or enzymes produced by microorganisms, plants or both to degrade or transform the contaminants in soil or other environmental matrices. Generally, bacteria are considered as vital for bioremediation because they degrade dead organic matter and also use the organic matrix as source of nutrients for their nourishment. Special type of contaminants like

chlorinated pesticides can easily be digested by bacteria. Similarly oil spills can also be cleaned by bacteria. However, bioremediation cannot destroy all kinds of contaminants; e.g., heavy metals such as arsenic, cadmium and lead cannot be decomposed by the action of microorganisms.

10.11.4.1. Strategies for bioremediation

A number of bioremediation strategies can restore the quality of degraded soils and the environment. For a given contaminant, one or more of the following strategies may be needed to ensure successful bioremediation.

i. Passive bioremediation

It is a natural bioremediation process of contaminated sites by indigenous microorganisms. The contaminated soils can be remediated through this process but the rate of degradation (breakdown) may be too slow for some situation. The challenge with this type of bioremediation is that it is very difficult to monitor and therefore it is very difficult to predict the degradation rate of the contaminants.

ii. Biostimulation

It is the process of bioremediation in which the indigenous microorganisms are supplied with nutrients, such as nitrogen and phosphorus, to stimulate their activity. The small quantities of the contaminant or an analogue can be added as stimulants which encourage the production of enzymes responsible for degradation.

iii. Bioventing

It is the process of biostimulation in which gaseous stimulants, such as oxygen (to improve aerobic conditions) and methane (as energy and carbon source to enhance cometabolism), are added in the soil to stimulate microbial activity. These stimulants are added into soil by pumping process.

iv. Bioaugmentation

It is the inoculation of a contaminated site with microorganisms that enhance the contaminants biodegradation process. The bacteria are the most common organisms which are usually used for bioaugmentation. A single species or consortium (group) of organisms can be used for biodegradation of contaminants. The inoculants may contain genetically changed or wild type of microorganisms as a single species or consortium of many species. In most of the cases the selected organisms having high potential for degradation of the contaminants are used.

10.12. Remediation of Contaminated Water

10.12.1. Rhizofiltration

It is similar to phytoaccumulation, but the plant species are grown in greenhouse having their roots submerged in water. This process can be applied for ex-situ

treatment of groundwater in which, groundwater is brought to the surface by pumping to give water to the plants. As the roots become saturated with contaminants, they are harvested.

10.12.2. Biosorption

It is a physico-chemical process in which contaminants bind and accumulate gradually onto the cellular structure of certain biomass. Biosorption is a process in which energy is not needed, and the concentration of contaminants such as heavy metals that are removed by sorbent relies on kinetic equilibrium as well as cellular structure of sorbent cellular. These contaminants become adsorbed on surface of the living material and this process is run metabolically with the energy produced by living organism and it needs respiration. Use of biomass for environmental remediation has been practiced since many years, researchers are trying to make this process as an economical alternative to remove toxic heavy metals from contaminated industrial water and help in environmental cleanup.

10.12.3. Bioaccumulation

Bioaccumulation happens when contaminants are adsorbed and transferred onto the surface and move towards the interior of the cellular surface. As compared to biosorption in the remediation of environment, bioaccumulation is much preferable due to its faster rate and ability to concentrate higher quantities of contaminants. Since heavy metals are adsorbed onto the surface of sorbents, biosorption is potentially a reversible phenomenon than the partially reversible process of bioaccumulation.

10.13. Environmental Risk Assessment

Environmental risk assessment (ERA) is important to understand the concept of hazard and risk prior to proceed to environmental risk assessmet. Hazard is the ability, nature or property of a substance or situation which has the potential to cause harm in terms of human injury, ill health or damage to property or the environment. The term risk in everyday language means "chance of harm" but when it is used in risk assessment process, it has specific definitions, the most commonly agreed "The combination of the probability, or frequency, of occurrence of a defined hazard and the magnitude of the consequences of the occurrence" (Royal Society 1992) whereas Aldenberg and Jaworska (2000) defined it as a structured process to estimate the likelihood and severity of risk with attendant uncertainty.

Hazard and risk can be differentiated with an example. Most of the chemicals are hazardous. Acids can cause corrosion or irritate to human beings. However, these acids can cause risk to humans only if they are exposed to them. The extent of harm will depend on the situation of exposure. If this contact occurs after heavy dilution of acids, there will be minimum harm to human beings but the hazardous property will not be changed. Therefore, it is clear that these terms have different meanings and may not be exchanged with each other.

Risk assessment is a process which estimates the risks qualitatively or quantitatively caused by potential hazards. If we take the example of chemicals, the risks can arise during their manufacturing, distribution, or disposal. The identification of inherent hazardous and risks caused by those hazardous are estimated during the process of risk assessment. Risk is measured by incorporating the measurement of the likelihood of the hazard which causes harm and an estimation of the extent of harm in terms of consequences on environment and human beings. The term environmental risk assessment does not normally cover the risks to individuals or the general public at large from consumer products or from exposure in the work place, where other specific legislation applies. In other words environmental risk assessment is the qualitative and quantitative evaluation of environmental status. ERA is comprised of human health risk assessment as well as ecological risk assessment (Cowan et al. 1995).

10.13.1. Stages in carrying out an environmental risk assessment

The environmental risk assessment can be conducted by different methods; however, the particular methodology and responsibility may vary to carry out the assessment. The major rules and important stages of these methods are basically the same in every case (Lindenschmidt et al. 2008). It is important to clearly identify the problem which is being addressed and the limits to make the decisions on environment be fixed prior to carry out the risk assessment. This phenomenon is called problem formulation and particularly can define the cause of risk, environmental part to be harmed, location and time (Lee-Steere, 2009). All that can also help to select the level and types of assessment methodology which are going to be used in environmental risk assessment itself.

The following stages may be followed for environmental risk assessment;

1) Identification of hazard: It is the identification of situation or property that can cause harm.

2) If the hazard occurred what were its consequences.

3) Evaluation of the extent of consequences in which temporal and spatial scale of consequences which occurred over the period of time.

4) Probability estimation of the consequences based on the following three components (Lee-Steere, 2009); a) hazard to be there, b) exposure probability of the receiving organism which was exposed to hazard, and (c) harm probability resulting from exposure to hazard.

Evaluation or characterization of risk importance is obviously the result of the likelihood of the hazard which is being considered and extent of consequences. The uncertainty associated with risk and hazard may also be considered in this part (Fig. 10.3).

Fig. 10.3 Steps in risk assessment

On completion of risk assessment process, the control measures in hand should be noted and additional measures may also be taken to minimize or remove the identified risks. Evaluation of significance is the final stage which involves placing it in a structure; for instance, with respect to environmental standards or other criterion defined in legislation, statutory or good guidance practice.

10.13.2. Qualitative risk analysis

It is the utilization of methods in order to place the identified risks with respect to their potential effect on project objectives (Sudirman and Hardjomuljadi 2011). This process prioritizes risks with respect to their potential effect on the objectives of the project. Further qualitative analysis is one way of mentioning the significance of addressing specific risks and it also guides the response of risk measures. The Probability–Impact matrix is a common and simplistic example of qualitative risk analysis. Risk probability and risk impact may be described in qualitative terms such as very high, high, moderate, low and very low. The sense of high, medium, low and very low can be mentioned in different ways; for instance, the descriptive or numerical scales used are usually based on the judgment of experts. On identification of risks, the matrix permits to easily determine the relative significance, so that the risk can be prioritized and appropriate strategy or program can be executed as shown in Fig. 10.4.

Identification of Hazards

Decide, who might be harmed and how

Evaluate the risk and decide precautions

Recoded your findings and impliment them

Review your assessment and update if necessary

Fig. 10.4 Risk management strategy

The use of quantitative risk assessment approaches may be more suitable in relatively more complicated cases. Further the pathway and consequences can be defined though these approaches by using modeling techniques. These modeling techniques also allow determining the extent of exposure of any receptor, consequences to the receptor in a better way. The probabilistic models can be used in order to evaluate the real probability of the occurring risk in some cases.

10.13.2.1. Human health risk assessment

Human health risk assessment (HHRA) involves the followings:

• hazard identification,

• dose-response assessment,

• exposure assessment, and

• risk characterization.

10.13.2.2. Ecological risk assessment

The ecological risk assessment determines the likelihood of the occurrence/non-occurrence of adverse ecological effects as a result of exposure to some stress factors.

10.13.3. Quantitative risk assessment

The quantitative risk assessment deals with the following:

• The quantity of toxic material in the inventory is hazardous.

• Over pressure in the storage tank (with failure of the relief valve) may lead to tank rupture.

• The combination of wind speed and atmospheric stability may lead to an estimated spatial and temporal distribution of toxic material concentration over time and space.

• The distribution of population based on night-time occurrence.

• The vulnerability of quantitative risk assessment models has been postulated.

• These have been based almost exclusively on animal test data. For example, the equation is:

Pr = At + Bt ln (Cnt)

where Pr = probability function, At, Bt, and n are constants, C is the concentration of pollutant to which exposure is made (in ppm v/v), and t is the duration of exposure to the pollutant (measured in minutes). These types of models have been derived from dose relationships and the probability of affecting a certain proportion of the exposed population.

10.14. Conclusion

The rapid industrialization and urbanization is causing increased environmental pollution which is resulting in deterioration of natural resources like soil and water. Ultimately this environmental pollution is imposing great stress to humans, animals and plants.

The activities carried out by humans e.g. agriculture, industrialization, health care, road transport and generation of energy are the major causes of pollution. It has been observed that the industries like petroleum, paper, textile, sugar, food, cement and chemical pollute soil, water and air.

The use of pesticides, fertilizers and industrial waste water pollute our fo od. The pesticides enter into food chain and accumulate in plants, human and animal bodies. These accumulated poisons cause health issues which range from simple headache to brain tumors.

Soil ecosystem has been imbalanced along with various ecological hazards in urban and rural areas due to soil pollution caused by different solid and liquid pollutants.

Transport is the major source of air pollution in urban areas. The vehicles release greenhouse gases like CO, SO2, NO2 which pollute air in urban areas.

Human health problems e.g. breathing, throat/lung cancer are caused are the result of air pollution in urban areas. The polluted water cases the human health problems like gastric, ulcer and formation of tumors.

Environmental pollution control has become imperative to save human health and natural resources. But the fast growing population has become a serious challenge for the control of environmental pollution.

References

Ahmad, M., A.S. Bajahlan and W.S. Hammad (2008). Industrial effluent quality, pollution monitoring and environmental management. Environ. Mon. Assess. 147: 297–306.

Aldenberg, T. and J. Jaworska (2000). Uncertainty of the hazardous concentration and fraction affected for normal species sensitivity distributions. Ecotoxicol. Environ. Saf. 46: 1–18.

Baker AJM, S.P. McGrath, R. D. Reeves (2000). Metal Hyperaccumulator Plants: A Review of the Ecology and Physiology of a Biological Resource for Phytoremediation of Metal-Polluted Soils. In: Terry N, Banuelos G, editors. Phytoremediation of Contaminated Soil and Water. Boca Raton: Lewis Publishers; 2000. pp. 85–108.

Brooks R. R. 1998). Plants that Hyperaccumulate Heavy Metals. CAN International, Wallington, USA. pp. 379.

Cowan, C.E., D.J. Versteeg, R.J. Larson and P.J. Kloepper-Sams (1995). Integrated approach for environmental assessment of new and existing substances. Regulat. Toxicol. Pharm. 21: 3–31.

Devi, N.L., I.C. Yadav, Q. Shihua, S. Sing and S. Belagali (2011). Physicochemical characteristics of paper industry effluents – a case study of South India Paper Mill (SIPM). Environ. Assess. 177: 23–33.

Hill, M.S. (1997). Understanding Environmental Pollution. Cambridge University Press, UK.

Jordan, M.A., D.T. Welsh, R. John, K. Catterall and P.R. Teasdale (2013). A sensitive ferricyanide-mediated biochemical oxygen demand assay for analysis of wastewater treatment plant influents and treated effluents. Water Res. 47: 841–849.

Jonathan, M., S. Srinivasalu, N. Thangadurai, T. Ayyamperumal, J. Armstrong-Altrin and V. Ram-Mohan (2008). Contamination of Uppanar River and coastal waters off Cuddalore, Southeast coast of India. Environ. Geol. 53:391–1404.

Landberg, T. and M. Greger (1996). Differences in uptake and tolerance to heavy metals in Salix from unpolluted and polluted areas. App. Geochem. 11:175–180.

Lee-Steere, C. (2009). Environmental Risk Assessment Guidance Manual for Industrial Chemicals. Australian Environment Agency Pvt. Ltd. Environment Protection and Heritage Council, Australia.

Lindenschmidt, K.E., S. Huang and M. Baborowski (2008). A quasi-2D flood modeling approach to simulate substance transport in polder systems for environment flood risk assessment. Sci. Total Environ. 397: 86–102.

Lugon-Moulin, N.L. Ryan, P. Donini, L. Rossi (2006). Cadmium content of phosphate fertilizers used for tobacco production. Agron. Sus. Dev. 26: 151–155.

Radić, S., D. Stipaničev, P. Cvjetko, I. Mikelić, M. Rajčić and S. Širac et al. (2010). Ecotoxicological assessment of industrial effluent using duckweed (Lemna

minor L.) as a test organism. Ecotoxicology 19: 216–222. Royal Society (1992). Risk Analysis, Perception and Management. The Royal

Society, London.

Sethi, M. S. and I. K. Sethis (1991). Understanding Our Environment. Common Wealth Publ., New Delhi, India.

Singh, N., B. Sharma and P. Bohra (2000). Impact assessment of industrial effluent of arid soils by using satellite imageries. J. Ind. Soc. Rem. Sens. 28: 79–92.

Srinivasa Gowd, S., Kotaiah, B. (2000). Groundwater pollution by Cystine manufacturing industrial effluent around the factory. Environ. Geol. 39: 679–682.

Sudirman, W.B., Hardjomuljadi, S. (2011). Project risk management in hydropower plant projects a case study from the state-owned electricity company of Indonesia. J. Infrastruc. Dev. 3: 171–186.

Yadav, A., S. Mukherji and A. Garg (2013). Removal of chemical oxygen demand and color from simulated textile wastewater using a combination of chemical/physicochemical processes. Ind. Engg. Chem. Res. 52: 10063–10071.

Chapter 11

Climate Change and Carbon

Sequestration

Muhammad Sanaullah, Muhammad Saqib and Abdul Wakeel†

Abstract

Climate change has adverse impact on all the spheres of life and is the most serious issue among all the global environmental challenges for human beings. Irregular rainfalls, severe droughts and seasonal disturbances in many areas of the world are the major consequences of climate change. The climate change has significant impact on agriculture and it is assumed that it will further impact the food production directly and indirectly. There is a diverse range of options to tackle emerging climate change related issues. The carbon sequestration is considered as the most suitable option, as a natural remedy to overcome the climate change. Soil carbon turnover is a predominantly weak link in our thoughtful of ecosystem reactions to climate change, such as the probable for carbon sequestration or release. One way to manage carbon is to use energy further professionally to decrease our need for main energy and carbon source-fossil fuel incineration. Additional way is to raise our use of low-carbon and carbon free fuels and machineries. A third way, is carbon sequestration, in which carbon is seized and stockpiled, thus alleviating carbon emanations. We can quickly and voluntarily amend prevailing management practices to raise carbon sequestration in our widespread forest, range, and croplands. Additionally, any measure that raises soil organic carbon content is probable to have helpful impacts on soil properties and functioning, along with mitigating climate changes.

†Muhammad Sanaullah*, Muhammad Saqib and Abdul Wakeel

Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: sanasial@gmail.com Yakov Kuzyakov

Soil Science of Temperate Ecosystems, Georg-August University of Göttingen, Germany. Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

246 M. Sanaullah, M. Saqib, A. Wakeel and Y. Kuzyakov

Keywords: Climate change; Global warming; Greenhouse gases; Carbon biosequestration; Soil organic matter

11.1. Introduction

Climate of the Earth is of great concern for mankind as it is directly related to the mankind’s prosperity (Florides et al. 2013). Climate change is the most serious issue among all the global environmental challenges for the inhabitants of this planet, especially the human beings. The climate changes severally affect all the spheres of life (FAO 2008). Pakistan is highly vulnerable to the adverse impacts of climate change but has done little to contribute to the problem. Pakistan is ranked 16th out of 170 countries in a recent Climate Change Vulnerability Index (Maplecroft 2011). The world food security, ecosystem balance and health of the people are at risk by the climate change. The irregular rainfalls, severe droughts and disturbance in the duration of summer and winter in many parts of the world are the major adverse consequences of climate change. The climate change is badly affecting agricultural productivity all over the world and poses a severe threat to world’s food security (Zia 2011). The extreme climate events in the form of elevated temperature and uncertain rainfall patterns are directly affecting social life. Droughts in Pakistan, Middle East and the Swahel region in Africa and intense flooding of low lying plains in Bangladesh, East Asia, Far East and present flooding in Pakistan provide the recent examples of climate change (Bhatti and Khan 2012). There is a diverse range of options to tackle the fast emerging climate change related issues. The carbon (C) sequestration is considered as the most effective option, as a natural remedy to overcome the climate change (Stephens 2006). Soil carbon sequestration is getting attention around the globe as the rapidly increasing CO2 concentration in the atmosphere needs to be reduced (IPCC 2011). The carbon in the atmosphere (mainly in the form of CO2) can be transformed into the stored soil carbon through the process of carbon sequestration. Plants have the ability to transform the carbon in the atmosphere through the process of photosynthesis into the relatively stable form and ultimately enhancing the soil organic carbon (SOC). The main advantage of carbon sequestration is the reduction of atmospheric concentration of carbon along with improvement in the quality and productivity of the soil (Stephens 2006). The goal of carbon sequestration can be accompanied by efficient utilization of natural resources in a manner which is less destructive to the environment and agricultural productivity (Raman et al. 2012).

11.2. Climate Change

The United Nations Framework on Conventions on Climate Change (UNFCCC) precisely defined the climate change as “A change of climate which is attributed, directly or indirectly to human activity that alters the composition of global atmosphere and which is in addition to natural climate variability observed over comparable time periods”. Simply the change in average weather conditions of a region or the changes in distribution of weather is termed as climate change (IPCC 2007).

Climate Change and Carbon Sequestration 247

The climate change has significant impact on agriculture (Lobell et al. 2011) and it is assumed that it will further impact the food production directly and/or indirectly. The changing climatic conditions like the change in rainfall sequence, hydrological cycle, rising sea levels and increased frequency of droughts have significant effects on agriculture, forestry and fisheries (Gornall 2010; IPCC 2007; Beddington et al. 2012). The intensity and periodicity of these factors determine their intensity. The uncertainty in rainfall patterns increases the chances of crop failure and threatens the raising of crops and livestock rearing. It is evident in many dryland areas where the climate change affects the delicate ecosystem balance. The magnitude of overall rainfall has declined as a result of climate change leading to more frequent and intense periods of drought. The climatic zones are shifting due to temperature extremes both hot and cold and this result in decreased length of growing seasons and a shift in the prevalence of pests and diseases in different areas. Climate models predict about 4°C rise in atmospheric temperature at the end of this century that will result in shortening of growing periods up to 20%. The dry land areas of the world are more threatened by climate change, posing severe threat to food security in these areas.

11.2.1. Causes of climate change

An important cause of climate change is the emission of greenhouse gasses which are directly or indirectly influencing the climate. The causes of climate change can be categorized into human induced and natural causes. The human induced causes may include the burning of fossil fuels, deforestation, agricultural practices and urban and industrial developmental activities. These all lead to the emission of greenhouse gasses which ultimately alter the natural climate. Among the natural causes, the changes in Earth orbital, intensity of sun and circulation of ocean and atmosphere are mainly responsible for climate change. The greenhouse gasses vary in their capability to cause the climate change. The most devastating effect on climate is caused by CO2, methane, nitrous oxide and chlorofluorocarbons. The aerosols and changes in land use patterns are also responsible for climate change. The extent and lifetime of clouds is also affected by changes in their physical and chemical properties resulting in unpredicted rains and heavy storms or severe droughts. The aerosols are produced by many anthropogenic activities like burning of fuels, exhaust emissions from auto mobiles and from various industrial processes. The agricultural sector being an important producer of greenhouse gasses is also responsible for climate change. The major greenhouse gasses in agricultural emissions are carbon dioxide, methane and nitrous oxide.

11.2.1. Global warming and greenhouse gases

The greenhouse gasses act as a blanket and trap heat energy, resulting in an increase in global atmospheric temperature. Global warming is the result of greenhouse effect and is directly related to the emission of greenhouse gasses. Only the carbon dioxide contributes around 50% to the total greenhouse effect. So the net increase in the concentration of carbon dioxide in the atmosphere is the main cause of global warming. If by any mean the concentration of greenhouse gasses particularly the

concentration of carbon dioxide is controlled in the atmosphere, the present global warming trend can be reduced.

11.2.2. Ozone depletion

Ozone depletion is an environmental issue and is directly related to global warming which, in turn, leads to climate change. As a result of ozone depletion, the global climate is more exposed to UV radiations. The ozone is known as the protective layer of Earth or sun screen that saves the Earth from the UV radiations. Ozone is present in stratosphere, about 15-20 km above the Earth surface. Ozone depletion is the breakdown of ozone molecules (O3) into O2 and atomic oxygen due to a number of compounds, like chloroflourocarbon (CFCs), carbon tetrachloride, methyl chloroform and Halons (brominated fluorocarbons) which are commonly known as ozone depleting substances (ODS). These ozone depleting substances are very stable for years in the troposphere and are transported to stratosphere where these substances are broken down by intense UV radiations resulting in the release of chlorine atoms which react with ozone and destroy ozone molecules. Approximately, one chlorine atom can destroy 100,000 ozone molecules. As a consequence of these reactions, there may be the holes in ozone layer or thinning of ozone layer. As a result, harmful radiations come to Earth and pose severe threat to environment and health related issues like reduction in efficiency of immune system, skin cancer and blindness due to damaging of eyes. Ozone depletion and climate change works side by side. The Montreal Protocol is the successful agreement in the last century and had a marked effect on climate change. Due to Montreal Protocol the radiative forcing of chlorofluorocarbons (CFCs) has been markedly reduced as these have been gradually phased out.

11.3. Factors affecting global climate change

11.3.1. Physical factors

11.3.1.1. Solar activity

The ultimate source of energy is sun that affects the weather and climate. The climate of Earth is ever changing and it is evident from geological and historical records. Any phenomenon that changes the incident radiations coming from sun can cause climate change. Under normal conditions the balance exists between the radiations coming from the sun and the radiations that the Earth reflects back into space. The radiations are mostly reflected in the form of long wave radiations attributing to the average temperature of the Earth. The increasing concentration of GHGs serves as a blanket and trap heat resulting in increased global temperature. The change in total irradiance of the sun is major cause of climate change. A change in solar activity, cloud cover and ocean circulation also affect global temperatures (Florides et al. 2013).

11.3.1.2. Orbital Variation

Earth’s orbit is the principle factor which is responsible for seasonal distribution, affecting the falling of sunlight on Earth and its distribution across the globe. A small

change in Earth’s orbit results in significant change in seasonal and geographical distribution of sun light as a consequence of which the climate of the globe changes. The Earth’s eccentricity, angle of Earth’s axis of rotation and precession of Earth’s axis are the major orbital variations responsible for Milankovitch cycles which have direct relation to glacial and interglacial periods (NASA, 2012). Florides et al. (2013) have argued that Earth’s orbital variation and the Milankovitch cycles have great effect on the Earth’s climate. The Milankovitch cycles also affect the glacial cycles followed by temperature change leading to the formation of deep oceans and rise in ocean temperature which increases the solubility of CO2 and also affects exchange of CO2 between the ocean and the atmosphere.

11.3.1.3. Solar output

The sun is the prime source of energy to this planet. The global climate may be affected by the variation in solar intensity and this has been extensively reviewed by Haigh (2007). Around four billion years ago, sun’s emitting power was 70% than today. Thus, here is a gradual increase in solar energy. The concentration of greenhouse gasses and atmospheric composition vary widely. It is assumed that approximately after five billion years, the emitting power of sun will become too high to make it a red giant and ultimately will lead to its destruction. For shorter periods of time, the solar output varies over the 11 years cycle and this variation is also contributing to climate change.

11.3.1.4. Atmospheric circulation

The radiations coming from the sun are the sole source of light and heat to Earth and the regions which are more exposed to these radiations are warmer than the regions receiving less exposure to these radiations. Obviously, it is true for the tropical regions which are more exposed to solar radiations and experience less seasonal variations. High moisture contents and warm air are the major characteristics of the tropical regions due to high temperature of these regions. The moist warm air rises up due to its less density, when it reaches the upper atmosphere it becomes cool and the moisture is transformed into clouds which ultimately fall back as rain. Due to high temperature at the equator the moist warmed air moves away, i.e., towards the poles, leaving behind its moisture in the form of rainfall.

11.3.2. Chemical factors

The greenhouse gases as chemical factors are among the primary causes of climate change. Consequent to rapid industrialization in different parts of the world, there has been a significant increase in the global atmospheric concentration of the greenhouse gases. There is a marked increase in the concentrations of carbon dioxide, methane and nitrous oxide in the global atmosphere due to different human activities. These concentrations are far higher than the pre-industrial atmospheric concentrations of these gases. The CO2 concentration has increased from 290 ppm at the beginning of 20th century to 385 ppm in 2009 (Ciattaglia et al. 2010). The increase in carbon dioxide concentration in the atmosphere is the consequence of burning of fossil fuels and changed land use, whereas higher concentrations of methane and nitrous oxide are due to agricultural activities.

11.3.3. Biological factors

The major biological factors affecting global climate change are the deforestation and modern crop production systems. Deforestation has led to less carbon storage in the soil which ultimately increases carbon level in the atmosphere in the form of CO2, which is a major greenhouse gas contributing in the global warming. Computer simulation studies show that a doubling of the atmospheric CO2 concentration will increase the temperature by about 1.5 to 4.5°C (IPCC, 2001). Deforestation not only decreases the carbon sequestration but also changes the land use systems leading to urbanization or industrialization. The urbanization or industrialization results in more utilization of fossil fuels which is the major contributor of greenhouse gasses. The radiations which are being absorbed by the plants and converted into biomass by the process of photosynthesis are reflected back as a result of deforestation and trapped by the greenhouse gases resultantly increases the global atmospheric temperature. Crop production is very vital activity to feed the ever increasing world population but it is also considered as major contributor of greenhouse gasses which are responsible for global warming. The carbon dioxide releases from combustion of fossil fuels, during land preparation and during transportation of agricultural goods while the methane is produced during the decaying of organic materials and due to anoxic conditions especially from rice fields. On the other hand, the nitrous oxide is produced by the fertilization of agricultural crops.

11.3.4. Social factors

The human activities have a considerable impact on Earth’s climate particularly through an increase in the greenhouse gases mainly as a result of the burning of fossil fuels in transportation, building heating and cooling and the manufacture of cement and other goods. The balance of the incoming solar radiation and the outgoing radiation has been disturbed by the greenhouse gases and aerosols which in turn have led to changed climatic conditions. A change in the atmospheric abundance of the greenhouse gases and aerosols as a result of different human activities either leads to warming or cooling of climate.

11.4. Global Carbon Cycle

Being most important constituent of all living things, carbon is the very definition of life, along with nitrogen, calcium, oxygen, hydrogen and phosphorus. Among these elements, carbon is the vital part of those compounds obligatory for life, such as fats, sugars, proteins, and starches. Overall, carbon accounts for approximately half of the total dry mass of living creatures. The movement of carbon, in its many forms, between the atmosphere, plants, soil and oceans, is described in the carbon cycle, illustrated in Fig. 11.1.

The carbon cycle, like every biogeochemical cycle, is a complex cycle consisting of different pools and stocks. Global C cycle can be divided into two components, inorganic and organic C cycle. Inorganic carbon cycle mainly consisted of dissolution of CO2 in rainwater forming carbonic acid which then reacts with basic cations to form secondary carbonates, or with calcium–magnesium silicate minerals

during the weathering process to release basic cations that then precipitate as carbonates (Hester and Harrison, 2010). This inorganic C cycle is mainly important in alkaline and sodic soils. Both organic and inorganic C cycle components are connected together via different fluxes like plant photosynthesis and soil respiration. The strong link between global C cycle and world climate is the reason why global C cycle gets such important attention from world’s scientific community.

In order to simulate C cycling and future changes in atmospheric CO2, it is important to know major carbon pools and the mechanisms for C storage and transformation from one pool to another, as shown in Fig. 11.1.

Fig. 11.1 Global Carbon Cycle (adapted from Hester and Harrison 2010)

Major C pools in global carbon cycle are:

11.4.1. Atmosphere

The atmosphere contains approximately 760 Pg C (1 Pg = 1015 g = Gt; t = tone; a = annum, or year), most of which is mainly in the form of CO2, with much smaller amounts of methane (CH4) and several other complexes. Atmospheric carbon is of dynamic rank because of its greenhouse effect and ultimately on climate. This C in the atmosphere is mainly exchanged with all other carbon pools like plants, soil, oceans as well as fossil fuels which contribute 7.0 pg C a-1 to the atmosphere.

11.4.2. Land plants

Land plants contain 560 pg C which is mainly taken up by plants from the atmosphere through photosynthesis (120 pg C a-1) and is release back into the atmosphere via respiration (60 pg C a-1). Deforestation also adds C to the atmosphere, and is contributing approximately 1.6 pg C a-1.

11.4.3. Soil organic matter

Carbon from land plant material (60 pg C a-1) in the form of plant litter, root exudates, dead roots, etc. can be transferred into soils, where it is stabilized in the form of soil organic matter (SOM). This carbon pool contains 1550 pg C which is two third C of total terrestrial ecosystem. Soil organic matter stocks are the result of balance between the inputs and outputs of carbon in the environment. This SOM is stabilized in the soil until being fragmented by soil microbes and released back to the atmosphere in the form of soil respiration (60 pg C a-1). Small part of SOM is eroded due to soil erosion and it contributes the transfer of 0.6 pg C a-1 towards oceans.

11.4.4. Oceans

Oceans are the major reservoir of global carbon, containing 38400 Pg C which is mainly in the form of highly recalcitrant dissolved inorganic carbon. Ocean carbon is in direct exchange with atmosphere through (i) physical processes, such as CO2 gas absorbed into the water, and (ii) biological processes, such as the growth, death and deterioration of plankton. But oceans are mainly the sink for carbon as uptake by oceans is 92.3 pg C a-1 while release is 90 pg C a-1.

11.4.5. Soil organic matter and terrestrial carbon cycle

In terrestrial ecosystems, carbon is present in the form of animals, plants, soils and microorganisms (bacteria and fungi etc.). Soils and plants are the main C reservoirs in terrestrial ecosystem, representing ~ 75% of the total land biosphere C. The word soil organic matter has been used in diverse ways to define the organic ingredients of soil. SOM is an innumerable of organic compounds made from organic material and resulting products after microbial decomposition.

Soil organic matter is not a single chemical entity but a complex range of compounds, of which the precise nature of many is unknown. Part of the SOM will consist of

newly added plant material and in many environments this will be under seasonal control. As this plant material undergoes decomposition, the more readily available and simple constituents, sugars, amino acids, nucleic acids, proteins, etc., are broken down first (Fig. 10.2). The structural polymers, pectin, hemicellulose and cellulose are then more slowly degraded. Finally, lignin is attacked once most other constituents have been exhausted. However, in much plant material, these chemical entities are not present singly, but rather in varying degrees of physical and chemical complexity. Hence, cellulose is often complexed (intimately mixed, probably with some covalent bonding) with lignin, and the lignin component then offers some resistance to the cellulose against decay. Some proteinaceous material may also survive longer when closely associated with lignin. Other complex plant components of SOM, such as tannins and cutins (waxes), can also offer some protection to otherwise rapidly decomposable substrates.

Fig. 11.2 Forms of carbon in plant residues and their decomposition rate

The soil microbes performing the mineralization of SOM use C as energy as well as assimilate it along with other nutrients as their DNA. The stabilization and mineralization of SOM is thus not exclusively dependent on inputs and outputs of C but on the availability of other nutrients as well.

11.5. Trends in Carbon Emission

Total CO2 emissions are being increased with time and this increase is causing real threat to the environment. It has been recorded that in 2011, this threatening increase in CO2 emission was highest, i.e., 3% increase and CO2 reached 34 billion tones in 2011 (Olivier et al. 2012). Top global CO2 emitting countries include China with 7.2 tones CO2, European Union with 7.5 tones and United States with 17.3 tones per capita emissions. Russian Federation and Japan are also included in top five CO2 emitting countries of the world. Continuous economic growth rate and expansion of infrastructure are the main reasons for this increase in CO2 emissions due to increased

fossil fuel consumption. Human activities including deforestation have been estimated for 420 billion tones of CO2 emissions. There is also small share of renewable energy sources including biofuels and solar and wind energy.

11.6. Carbon Capture and Storage

Carbon capture and storage (CCS) is the process of preventing the release of large quantities of CO2 into the atmosphere, mainly produced from large point sources, such as fossil fuels. These fossil fuels are mainly hydrocarbons and release CO2 on incineration. This CO2 and other greenhouse gases production is the major source of global warming (Anderson and Newell 2004; IPCC 2011).

11.7. Carbon Biosequestration

11.7.1. Introduction and concepts

As mentioned earlier, carbon sequestration in soils is the process whereby atmospheric carbon dioxide can be fixed into soil such that it is held there in a relatively permanent form. Biosequestration mentions to a kind of biological processes that absorb carbon dioxide, the primary greenhouse gas, from the atmosphere and encompass it in living organic matter, soil, or aquatic ecosystems. The prospects for growing biosequestration by fluctuating management and land-use practices are making debate among landowners, policy makers and the media. Other possibilities of enhancing natural carbon seizing processes may exist, but more study is desirable to regulate their probable for climate change modification (Bird et al. 1999; Bruce et al. 1999). Biosequestration occurs naturally in the global carbon cycle.

11.7.2. Terrestrial biosequestration

Terrestrial biosequestration is the fixing and storage of atmospheric CO2 by terrestrial vegetation in soil on long-term basis. Such kind of C sequestration is possible either by decreasing atmospheric CO2 or by reducing CO2 emissions from terrestrial ecosystems. Thus sequestration can be enhanced by decreasing decomposition of organic matter, increasing photosynthetic carbon fixation by vegetation and generating energy offsets using biomass for fuels and other products. The terrestrial biosphere is projected to sequester about 2 billion tones of carbon yearly.

In count to increasing plant carbon contributions, approaches for improving soil carbon sequestration consist of decreasing organic matter turnover and increasing its residence time in soils. Chemical alteration and physico-chemical protection can help in stabilizing soil organic carbon (SOC) and reduce SOC turnover (Hungate et al. 1997; Koch and Roy 1995). Stabilization of SOC can happen by its sorption to mineral or its physical protection in soil pores where decomposers and extracellular enzymes have not easy access. Soil structure is also very important variable that both

controls and indicates the soil organic carbon maintenance status of a soil (Read and Moreno 2003).

11.7.3. Role of soil enzymes and plants in C biosequestration

Soil carbon turnover is a predominantly weak link in our thoughtful of ecosystem reactions to climate change, such as the probable for carbon sequestration or release. Soil extra-cellular enzymes are specialized proteins produced by plants and/or microorganisms that combine with a specific substrate and act as catalysts in a biochemical reaction. These soil enzymes are very important drivers for nutrient cycling such as in carbon and other nutrient cycles.

Microbes take up simple forms of nutrients (C, N and P), which are used for their biomass growth or to produce extracellular enzymes in order to decay multifarious resources into available nutrients. The microbes enhance the magnitudes of diverse biomass mechanisms and enzyme assembly to exploit their growth rate. So, microorganisms release extracellular enzymes to decay organic materials (e.g., dead plants) into available nutrient elements: C, N and P. Extracellular enzymes are the tackles that microbes use to accomplish their roles as material recyclers in the global carbon and nutrient cycles. Increased N and P concentrations in plant materials stimulates enzyme production, which in turn escalate decay of organic materials and nutrient recycling. Addition of available carbon to microbes growing on new plant litter quashes enzyme production and decay. Addition of reachable carbon to old intractable material increases enzymatic decay, liberating nutrients and carbon through a grooming effect.

Following are the major factors which can influence enzyme activities in soil:

11.7.3.1. Soil Temperature

Soil enzymes are very specific to temperature and are only efficient at a certain optimum temperature range. Increase in temperature until optimum level can provide kinetic energy to the molecules and can increase the reaction rate of soil enzymes. Any increase in temperature above optimum can cause denaturing of soil enzymes. Denaturing of enzymes may result in the breakage of enzyme bonds holding molecules and enzyme active sites lose their shapes.

11.7.3.2. Soil pH

Similar to temperature, enzymes have an optimum pH requirement. Any change in soil pH can modify the chemical nature of the amino acids of enzymes which will result in a change in enzyme structures. The active site may be denatured resulting in denaturing of soil enzymes.

11.7.3.3. Change in concentration

Both enzyme and substrate concentrations can affect enzyme activities in soil.

i. Enzyme concentration

When enzyme concentrations are low, rate of reaction is low because of competition for the active sites. While with increasing enzyme

concentration, more free active sites are available which enhance reaction rate. Ultimately, increase in enzyme concentration above certain point may have no effect because at this point, substrate concentration becomes the limiting factor.

ii. Substrate concentration:

Similar to enzyme concentration, low substrate concentration will result in low reaction rate because there are many unoccupied active sites. With increase in substrate concentration up to a certain limit will result in higher reaction rates because enzyme-substrate complexes can be increased with the availability of active sites. Above that concentration limit, there will be no effect because active sites will be saturated so no more enzyme-substrate complexes can be formed.

11.8. Factors Influencing Carbon Sequestration in

Soil and crop management strategies that improve soil organic carbon pool in soil and thus carbon sequestration comprise of the following:

11.8.1. Climate

Climate plays a major role in determining C sequestration in soil and temperature and moisture are major determinants of the rate of decomposition since they directly affect the activity of the microbial biomass. Microbial activity and ultimately SOM decomposition is increased with increasing temperature which can cause reduction in C sequestration in soil. Soil moisture has also significant role in carbon sequestration, as under optimum moisture conditions, there will be less C sequestration in soil due to favorable conditions for microbial activity. While high moisture in the form of saturations may decrease decomposition rates due to anaerobic conditions and limited availability of oxygen. Such wet conditions help in the accumulation of C stocks in soils especially, peats and organic soils. Similarly, drought stress inhibits SOM decomposition (Sanaullah et al. 2011& 2012) and can help to increase C sequestration, although primary productivity tends to be very low.

11.8.2. Plant input

As mentioned in global C cycle, plant input has direct contribution towards increase in C stock in soil. However, this increase in C sequestration in soil due to plant input is highly dependent on type of vegetation, plant species and strains. Different type of plants input having different initial biochemical composition have different role in C sequestration in soil. Because plant material with higher N contents and lower recalcitrant phenolic compounds may have tendency for higher decomposition rates as compared with plant litter having higher recalcitrant compounds (Sanaullah et al. 2010).

11.8.3. Organic inputs and manuring

Regular application of livestock manure can induce substantial changes in soil organic carbon over the course of few years. And it was reported that continuous application of animal manure, soil organic carbon contents were increased. The reality of smallholder resource availability indicates that the movement in soil organic matter contents lead to offset losses for soil organic carbon accumulated prior to land management rather than to lead to a net increase in soil carbon (Bajracharya et al. 1997; Dormaar and Carefoot 1998).

11.8.4. Soil tillage

Overall, conventional tillage practices can cause losses in soil organic carbon pools. Disturbance of the soil exposes SOM that may have been physically protected, particularly by the soil mineral components, against further decomposition. Better soil aeration due to tillage operations and changes in temperature and moisture may also result in C loss because of stimulation of microbial biomass activity in soil. Though, the welfares of no till on soil organic carbon sequestration may be soil or site specific, and the development in soil organic carbon may be unpredictable in fine textured and poorly drained soils (Compton and Boone 2000).

11.8.5. Soil erosion

Soil carbon loss occurs both as a result of mineralization as well as through soil erosion and makes it often more difficult to interpret soil carbon responses to management practices in long term basis. However, the disruptive forces of soil erosion are similar to those of tillage and will probably promote increased decomposition. However, the deposited C after soil erosion may be protected from decomposition when eroded SOM becomes part of ocean sediments. Indirectly, decrease in plant yields and biomass due to soil erosion can cause decrease in plant return to soil and ultimately, to lower carbon sequestration.

11.9. Role of Carbon Sequestration in Climate

Change Mitigation

Human deeds, particularly the scorching of fossil fuels such as coal, oil, and gas, have initiated a considerable proliferation in the absorption of carbon dioxide in the atmosphere. This intensification in atmospheric CO2 from about 280 to more than 380 ppm over the last 250 years is producing quantifiable global warming. Probable antagonistic effects contain sea-level rise; enlarged incidence and strength of wildfires, floods, droughts, tropical storms; fluctuations in the extent, timing, and circulation of rain, snow and runoff and disruption of coastal marine and other ecologies (Marland et al. 2003; Olivier et al. 2012).

According to the Inter-governmental Panel on Climate Change (IPCC) agriculture presently accounts for 10-12% of global greenhouse gas (GHG) releases and is predictable to increase further. GHGs ascribed to agriculture by the IPCC comprise

releases from soils, enteric fermentation, rice production, biomass scorching and ma-nure management (Houghton 1996). There are additional unintended sources of GHG releases that are not accounted for by the IPCC under agriculture such as those made from land-use changes, use of fossil fuels for modernization, transport and agro-chemical and fertilizer production.

Carbon releases and atmospheric concentrations are predictable to continue throughout the next century if main changes are made in the way carbon is coped. Managing carbon has arisen as a persistent national energy and environmental need that will drive national policies and treaties through the coming decades. One way to manage carbon is to use energy further professionally to decrease our need for main energy and carbon source-fossil fuel incineration (Houghton 1996). Additional way is to raise our use of low-carbon and carbon free fuels and machineries. A third way, is carbon sequestration, in which carbon is seized and stockpiled, thus alleviating carbon emanations.

Sequestration of carbon in the terrestrial biosphere has arisen as the standard means by which the world will meet its near-term economic necessities for dropping net carbon releases. Terrestrial carbon pools and fluxes are of adequate extent to efficiently mitigate global carbon emanations (Lal and Bruce 1999; Liang et al. 2008). We can quickly and voluntarily amend prevailing management practices to raise carbon sequestration in our widespread forest, range, and croplands. Additionally, any measure that raises soil organic carbon content is probable to have helpful impacts on soil properties and functioning, along with mitigating climate changes.

11.10. Conclusion

Climate change has adverse impact on all the spheres of life and is the most serious issue among all the global environmental challenges for human beings. Irregular rainfalls, severe droughts and seasonal disturbances in many areas of the world are the major consequences of climate change. The climate change has significant impact on agriculture and it is assumed that it will further impact the food production directly and indirectly. There is a diverse range of options to tackle emerging climate change related issues. The carbon sequestration is considered as the most suitable option, as a natural remedy to overcome the climate change. Soil carbon turnover is a predominantly weak link in our thoughtful of ecosystem reactions to climate change, such as the probable for carbon sequestration or release. One way to manage carbon is to use energy further professionally to decrease our need for main energy and carbon source-fossil fuel incineration. Additional way is to raise our use of low-carbon and carbon free fuels and machineries. A third way, is carbon sequestration, in which carbon is seized and stockpiled, thus alleviating carbon emanations. We can quickly and voluntarily amend prevailing management practices to raise carbon sequestration in our widespread forest, range, and croplands. Additionally, any measure that raises soil organic carbon content is probable to have helpful impacts on soil properties and functioning, along with mitigating climate changes.

References

Anderson, S. and R. Newell (2004). Prospects for carbon capture and storage technologies. Annu. Rev. Environ. Resour. 29: 109–142.

Bajracharya, R.M., R. Lal, and J.M. Kimble (1997). Soil organic carbon distribution in aggregates and primary particle fractions as influenced by erosion phases and landscape position CRC Press. Boca Raton, USA.

Beddington, J., M. Asaduzzaman, M. Clark, A. Fernandez, M.Guillou, M. Jahn, L. Erda, T. Mamo, N. Van Bo, C.A. Nobre, R. Scholes, R. Sharma and J. Wakhungu (2012). Achieving food security in the face of climate change. Final report from the Commission on Sustainable Agriculture and Climate Change. Copenhagen, CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS). http://ccafs.cgiar.org/commission

Bhatti, A.U. and M.M. Khan (2012). Soil management in mitigating the adverse effects of climate change. Soil Environ. 31: 1–10.

Bird, M.I., C. Moyo, E.M. Veenendaal, J. Lloyd, and P. Frost (1999). Stability of elemental carbon in a savanna soil. Global Biogeochem. Cy. 13: 923–932.

Bruce, J.P., M. Frome, E. Haites, H. Janzen, R. Lal, and K. Paustian (1999). Carbon sequestration in soils. J. Soil Water Conserv. 54: 382–389.

Ciattaglia, L. R., C. Rodriguez, H. Araujo, J. J. Station, Antarctica (2010). Monthly and annual atmospheric CO2 record from continuous measurements. Atmospheric CO2 mixing ratios expressed in ppmv, relative to the NOAA/GMD standards. http://cdiac.ornl.gov/ftp/trends/co2/Jubany_2009_Monthly.txt

Compton, J.E., and R.D. Boone (2000). Long-term impacts of agriculture on soil carbon and nitrogen in New England forests. Ecology 81: 2314–2330.

Dormaar, J.F., and J.M. Carefoot (1998). Effect of straw management and nitrogen fertilizer on selected soil properties as potential soil quality indicators of an irrigated Dark Brown Chernozemic soil. Can. J. Soil Sci. 78: 511–517.

FAO (2008). Climate change and food security: a framework document. Rome, Italy. http://www.fao.org/forestry/15538-079b31d45081fe9c3dbc6ff34de4807e4.pdf.

Florides, G.A., P. Christodoulides and V. Messaritis (2013). Reviewing the effect of CO2 and the sun on global climate. Renew. Sust. Energ. Rev. 26: 639–651.

Gornall, J., R. Betts, E. Burke, R. Clark, J. Camp, K. Willett, and A. Wiltshire (2010). Implications of climate change for agricultural productivity in the early twenty-first century. Philos. T. Roy. Soc. B. 365: 2973–2989.

Haigh, J.D. (2007). The sun and the Earth's climate. Living Reviews in Solar Physics. 4: 5–59. http://www.livingreviews.org/lrsp-2007-2

Houghton, J.T. (1996). Climate change 1995: The science of climate change: contribution of working group I to the second assessment report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK.

Hungate, B.A., E.A. Holland, R.B. Jackson, F.S. Chapin, H.A. Mooney, and C.B. Field (1997). The fate of carbon in grasslands under carbon dioxide enrichment. Nature 388: 576–579.

Hester, R.E. and R.M. Harrison (2010). Carbon capture: sequestration and storage. RSC publishing, Cambridge, UK.

IPCC (2001). Third Assessment Report – Climate Change: Working Group I: The Scientific Basis. F. 3 Projections of Future Changes in Temperature.

http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/031.html

IPCC (2007). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (Eds.)]. Cambridge University Press, Cambridge, UK.

IPCC (2011). IPCC Special Report Carbon Dioxide Capture and Storage Summary for Policymakers". Intergovernmental Panel on Climate Change. http://www.ipcc.ch/pdf/special-reports/srccs/srccs_summaryforpolicymakers.pdf

Koch, G.W., and J. Roy (1995). Carbon dioxide and terrestrial ecosystems. Academic Press, London, UK.

Lal, R. and J.P. Bruce (1999). The potential of world cropland soils to sequester C and mitigate the greenhouse effect. Environ. Sci. Policy 2: 177–185.

Liang, B., J. Lehmann, D. Solomon, S. Sohi, J.E. Thies, J.O. Skjemstad, F.J. Luizao, M.H. Engelhard, E.G. Neves, and S. Wirick (2008). Stability of biomass-derived black carbon in soils. Geochim. Cosmochim. Ac. 72: 6069–6078.

Lobell, D., W. Schlenker and J. Costa-Roberts (2011). Climate trends and global crop production since 1980. Science 333: 616–620.

Maplecroft (2011). Climate Change Vulnerability Index 2011, Maplecroft, Bath, London, UK.

Marland, G., T.A. Boden, and R.J. Andres (2008). Global, Regional, and National Fossil Fuel CO2 Emissions. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., USA.

NASA. 2012. Milankovitch. http://earthobservatory.nasa.gov/Features/Milankovitch

Olivier, G.J., G. Janssens-Maenhout and A.H.W. Peters (2012). Trends in global CO2 emissions. PBL, Environmental Assessment Agency, Netherlands.

Raman, S.V., S. Iniyan and R. Goic (2012). A review of climate change, mitigation and adaptation. Renew. Sust. Ener. Rev. 16: 878–897.

Read, D.J. and J. P. Moreno (2003). Mycorrhizas and nutrient cycling in ecosystems-a journey towards relevance? New Phytol. 157: 475–492.

Sanaullah, M., A. Chabbi, G. Lemaire, X. Charrier and C. Rumpel (2010). How does plant leaf senescence of grassland species influence decomposition kinetics and litter compounds dynamics? Nutr. Cycl. Agroecosys. 88: 159–171.

Sanaullah, M., E. Blagodatskaya, A. Chabbi, C. Rumpel and Y. Kuzyakov (2011). Drought effects on microbial biomass and enzyme activities in the rhizosphere of grasses depending on plant community composition. App. Soil Ecol. 48: 38-44.

Sanaullah, M., A. Chabbi, X. Charrier and C. Rumpel (2012). How does drought stress influence the decomposition of plant litter with contrasting quality in a grassland ecosystem? Plant Soil 353: 277–288.

Stephens, J. (2006). Growing interest in carbon capture and storage (CCS) for climate change mitigation. Sustain. Sci. Prac. Polic. 2: 4–13.

Zia, M. (2011). Climate change and its impact with special focus in Pakistan. In: Symposium on “Changing Environmental Pattern and its impact with Special Focus on Pakistan” organized by Pakistan Engineering Congress, Lahore, Pakistan.

Chapter 12

Soil and Water Conservation

Safdar Bashir, Atif Javed, Irshad Bibi and Niaz Ahmad†

Abstract

Conservation of soil and water resources is important for sustainability of agriculture and environment. Soil and water resources are under immense pressure due to ever increasing population thereby ensuing growing demand for food, fiber and shelter. Soil and water resources are being deteriorated due to different anthropogenic and natural factors. Soil erosion is one of the several major deteriorative processes which results in deterioration of the soil. Soil erosion is removal of soil due to movement of water and/or air. Soil erosion may lead to the significant loss of soil productivity and thus may lead to the desertification under sever conditions. Water and wind are the major agencies which are responsible of soil erosion. Deforestation, over-grazing, mismanagement of cultivated soils, intensive cultivation and intensive urbanization are major factors triggering the soil erosion. For sustainable agriculture and environment, it is pertinent for the protection of soil resources against erosion. Different control measures should be adopted to protect the soil resources against erosion. The concept of soil conservation cannot be materialized without conserving and efficient use of water resources. It is therefore pre-requisite that soil conservation practices should be adopted. Soil conservation practice include soil management, crop management, engineering, range management and forestry operation.

Keywords: Soil, Water, Erosion, Conservation, Management

†Safdar Bashir*, Atif Javed and Irshad Bibi

Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan. *Corresponding author’s e-mail: safdar.bashir@uaf.edu.pk Niaz Ahmad

Department of Soil Science, Faculty of Agricultural Sciences and Technology, Bahauddin Zakariya University, Multan, Pakistan.

Managing editors: Iqrar Ahmad Khan and Muhammad Farooq Editors: Muhammad Sabir, Javaid Akhtar and Khalid Rehman Hakeem University of Agriculture, Faisalabad, Pakistan.

264 S. Bashir, A. Javed, I. Bibi and N. Ahmad

12.1. Introduction

Soil is the most fundamental resource to fulfill basic requirements of food, fiber and shelter of human race. The basis of all terrestrial life is soil although it is perceived as something of insignificant value and it is considered as dirt but humans cannot survive without soil. Soil provides a wide range of ecosystem services which are summarized in Table 12.1. Soil erosion is detachment and dislocation of soil due to the action of water or wind. Soil loss due to erosion has great consequences because it leads to loss of its productivity. Soil erosion occurs though out the world but it is a very common feature and more serious problem in dry areas. Soil erosion disturbs agricultural, environmental and ecological functions performed by the soil. Soil erosion results in depletion of soil fertility, decreased moisture storage capacity and consequently in decreased crop productivity. In addition to loss of soil fertility and crop yields, soil erosion also increases environmental pollution, increasing the sediment load in streams and rivers, thereby disturbing the aquatic life, particularly fish. In the long run, soil erosion affects socio-economic conditions of the society by causing floods, silting up of water reservoirs and disruption of communication systems. The soil covering the Earth surface has taken millions of years to develop. The rate of soil formation is very slow (during every 100 to 400 years, only 1 cm soil is formed) and the enough soil depth is formed in 3000 to 12000 years to have a productive land. Thus, when soil, a non-renewable natural resource is ruined then it will be lost entirely (Pimentel et al. 1995; Lal 2001). Globally, out of 22% of the land suitable for sustaining agricultural productivity, around 5 to 7 Mha are being lost annually due to land degradation, consequently, threatening food security of the world. Soil and water resources conservation and management is important for the welfare of the people.

Table 12.1 Ecosystem services provided by soils

Food security,

biodiversity and

urbanization

Water quality

control

Climate change

mitigation

Energy resource

Fiber Food Housing Recreation Infrastructure Disposal of waste Diversity of microbes Flora and fauna Conservation

Pollutants filtration Purification of water Retention of sediments and chemicals Chemicals buffering and transformation

CH4 and CO2 sinks Sequestration of Carbon in soil and biota Nitrification inhibitors Deposition and burial of carbon enriched sediments

Bioenergy crops (warm season grasses and short rotation woody crops Prairie grasses)

Source: Blanco and Lal (2008)

The demands of growing world population force the wise use and management of resources to meet the high demands of food production. If we will not realize the importance then there will be a time when we will not have enough soil left to support

Soil and Water Conservation 265

life on this planet, as the soil is an essential resource to support plants for producing food, and to provide shelter to insects and animals. Thus, it is important to consider soil as a living object. Soil and water conservation and sustainable use of these resources are not only crucially important to farmers but to the entire mankind for their sustenance. Sustainable agriculture, therefore, is dependent on conservation of water and soil resources through a variety of methods.

12.2. Soil Erosion

In agriculture, soil erosion refers to the removal of topsoil by the natural physical forces of water and wind at a greater rate than it is formed or through forces associated with farming activities such as tillage. Erosion removes the topsoil first and once this nutrient-rich layer is lost, the potential of soil to sustain plants is reduced. Without soil and plants the land becomes desert like and unable to support life. Soil erosion is a naturally occurring process that affects all landforms.

Soil erosion can be classified into two major types, i.e., accelerated and geological erosion. The normal process of weathering is geological erosion that usually happens as a part of natural soil-forming mechanisms at low rates in all soils. It is not affected by human activities as well as it happens at the period of long geological time. The processes influenced by the slow but constant geological erosion are the development and disintegration of rocks. On the contrary, in accelerated erosion, soil erosion becomes a main anxiety and a specific threshold level is exceeded by the erosion rate and soil loss through erosion exceeds the soil formation through pedogenic processes. Anthropogenic activities such as slash-and burn agriculture, intensive and uncontrolled grazing, deforestation and burning of biomass and intensive plowing are main factors which trigger accelerated soil erosion. The soil becomes less productive after the loss of fertile topsoil even by applying the same farm inputs. So, the control and management of soil erosion are essential. Although soil erosion cannot be eliminated but there are ways to minimize excessive erosion and its adverse effects on agricultural production. The extent and the effects of soil erosion on yield depend on soil profile development, terrain, soil management and climatic conditions.

When we consider the scenario of soil erosion in Pakistan (Table 12.2), the soil in the territory of Indus River are comparatively young and under the process of development and mountains in the region have sharp and long slopes in the world. Heavy rainfalls in the summer and snow melting in hilly areas aid to the soil erosion hazards. The major factors linked to soil and water erosion are management practices, vegetation type, soil type and soil structure. The northern hilly areas having steep slopes are less prone to water erosion due to presence of forests with permanently closed canopy but arable crops sown in steep slopes are more vulnerable to erosion. In Pakistan, water erosion affect the total area about 11 million hectares. Because of soil erosion in the upstream, land and water use efficiency decreases by the sedimentation in canal irrigation system in the plains. It is estimated that about 40 million tonnes of soil is brought into the Indus basin every year because of accelerated soil erosion in mountainous and sloppy areas and thus life span of major water reservoirs and their efficiency have been shortened. The productivity of area

has been declined due to removal of top soil and destruction of upstream riverside infrastructure. In downstream, efficiencies of irrigation system and hydropower generation system have been reduced due to sedimentation.

Considering wind erosion in Pakistan, the sandy deserts of Tharparkar, Cholistan, Thal and sandy areas along Maekran coastal areas in Baluchistan are commonly degraded by the wind erosion. The areas near populated areas and watering points with free access to livestock are more vulnerable to wind erosion. The major factor of degradation in these areas is the over exploitation of rangelands by deforestation and livestock grazing. The worldwide effect of wind erosion is more dominant in the areas where sand dunes are leveled off for crop irrigation. Movements of sand dunes at a height of 0.5-4 meters are known to occur which possess threat to infrastructure and cultivated land. Wind erosion affected about the 3-5 million hectares of land. Only wind contributed the 28% of total soil loss in this area. Deposition of thick sand layers on roads, severe movement of sand dunes, croplands, railway tracks resulted by the fast moving wind storms that ultimately threaten the rural life as well as the communication systems. Detailed description of wind and water erosion is given below.

12.2.1. Water erosion

On global level, most severe type of soil erosion is water erosion. Detachment of soil particles from its original place due to movement of water is called water erosion. Water from runoff, rain, irrigation and snowmelt may contribute to soil erosion but rainwater is the major factor which causes the movement and detachment of soil particles. The transportation of soil organic and inorganic particles with the water flowing along the slope is subsequently deposited in surface water bodies and at lower landscape positions in water erosion. The new soil reservoirs, streams or simply fill lakes are formed from these transported materials. In humid and sub-humid areas of world which are characterized by repeated rainstorms, the dominant form of erosion is wind erosion. The same problem is noticed in the soil that is bare and have no vegetation like in the arid and semiarid regions that have limited precipitation in the form of intense storms (torrential rain). There are many types of water erosion: inter rill, splash, rill, gully, stream bank, and tunnel erosion. Inter rill erosion is also known as sheet and splash erosion, but these two differ in the underlying fluvial processes (Blanco and Lal 2008).

12.2.1.1. Raindrop or splash erosion

Raindrops strike the soil surface, scatter and then splash the soil by displacing particles from their original location. Splash erosion is initiated by hitting of the soil surface by the falling raindrops (Fig.12.1). Soil particles displace from their original position after the striking of raindrops that scatter and splash the soil. Falling drops initiate the splash erosion by hitting the soil surface. Splash of soil particles, depression formation, raindrop impacts are included in the process of splash erosion. (Ghadiri 2004). A raindrop-soil particle momentum is formed after the hitting of raindrops to the soil surface before discharging their energy in the form of splash. These raindrops form holes or cavities after hitting the soil like small bombs of different shapes and sizes. A function of raindrop size, shape and velocity is

penetration and the depth of raindrop energy is equal to the holes depth (Blanco and Lal 2008).

Table 12.2 Area Affected by Wind and Water Erosion (000’ ha)

Degree and Type

of Erosion

Punjab Sindh KPK +

Baluchistan Gilgit-

Baltistan

Pakistan

Wind erosion

Slight 2251.4 295.0 13.1 36.0 2595.5 Moderate 279.1 70.2 3.8 143.6 496.7 Severe to very severe

1274.0 273.8 19.6 100.9 1668.3

Total 3804.5 639.0 36.5 280.5 4760.5 Water erosion

Slight Sheet or rill erosion

61.2 156.3 180.5 398.0

Moderate Sheet or rill erosion

896.8 853.8 1805.0 25.8 3581.4

Severe Rill, Gully and/or stream bank erosion

588.1 58.9 1765.1 829.6 504.2 3754.9

Very severe Gully, Pipe and Pinnacle erosion

357.9 1517.0 1571.6 3446.5

Total 1904.0 58.9 4292.2 2634.6 2282.1 11171.8

Source: Ahmad et al. (1998)

12.2.1.2. Sheet/ inter-rill erosion

Immediately after the sheet/inter-rill erosion starts, runoff quickly forms small rills and part of the runoff flowing in between these rills is called sheet or inter-rill erosion. Shallow flow of water is the main reason of such type of erosion. Some soil particles in form of a thin sheet are moved away with the runoff and some settle in these small rills. The most common type of soil erosion is the sheet or inter rill erosion. About 70% of total soil is contributed by splash and inter-rill erosion and occur simultaneously with the splash erosion dominating during the initial process.

A function of rainfall intensity, field slope and particle detachment is inter-rill erosion. The gradual removal of entire field surface in more or less uniform way starts the sheet erosion. It is a gradual process and it is not immediately obvious that the soil is being lost.

Fig.12.1 (a) Raindrop falling on the surface (b) Splash impact of raindrop (c) Process of water erosion (modified from Stitcher 2010)

12.2.1.3. Rill erosion (channel erosion)

The erosion occurs in small channels or rills is rill erosion (Fig. 12.2). It is due to rigorous rather than shallow flow. The soil is eroded more quickly in small channels by the runoff water than inter-rill erosion. The soil particles creeping and flow velocity along the rill bed widen the rills. The second most common form of soil erosion is rill erosion. The tillage operations can easily manage these rills but large soil erosion might be caused especially under heavy rains. In Pakistan, erosion found in the regions of Pothwar Plateau and western hilly areas is visible rill erosion.

12.2.1.4. Gully erosion

Formation of V- or U-shaped channels takes place in gully erosion (Fig. 12.3). These gullies are formed in form of small channels with 0.3 m depth and 0.3 m width. The concentrated runoff which is joined in lower slopes is the primary mechanism of formation of these gullies in the field. Concentrated flow erosion is a term that is used to describe the erosion occurring in these channels. When the water moves down the slopes in small channels, the uneven fields demonstrate the concentrated runoff in natural swales. The entire soil profile can also be removed in confined segments by continuous gully. Increase in gully growth increases sediment transport. Gully erosion can be permanent and ephemeral. The normal tillage practices can easily remove the ephemeral gullies that contain shallow channels. On the other hand, permanent gullies require expensive means of reclamation and control as these are too large to be corrected by regular tillage. The most common locations for gully erosion in Pakistan is Pothwar Plateau especially on loess soil.

Fig.12.2 Global Water erosion vulnerability (USDA-NRCS 2003)

12.2.1.5. Tunnel erosion

The lands in arid and semiarid areas are highly erodible and sodic B horizon but have a stable A horizon and it is also known as pipe erosion. Tunnel erosion is initiated by the runoff in natural cracks and channels produced due to the movement of burrowing animals in subsoil layers. Due to tunnel erosion, geo-morphological and hydrological characteristics of the area are affected. These tunnels can be cured by deep ripping, repacking of soil surface, contouring, reduction of runoff ponding and diversion of heavy runoff. This type of erosion is also reduced by the revegetation that include tree and deep rooted grass species.

12.2.1.6. Stream bank erosion

In this type of erosion, breakdown of banks along streams, creeks, and rivers occurs due to the erosive power of runoff from uplands fields. Pedestals formation with fresh vertical cuts along streams is the reason of stream bank erosion. Exhaustive cultivation, grazing and traffic along streams, and presence of bare land accelerate stream bank erosion. This type of erosion can be reduced by planting grasses and trees, establishing engineering structures, mulching stream borders with rocks and woody materials, geo-textile fencing, and diverting runoff (Blanco and Lal 2008).

12.2.2. Wind erosion

Wind erosion occurs mainly in dry areas where soil surface is left bare. In dry regions, because of low rainfall, soil is too dry and flat to allow the wind to carry the soil away over several consecutive days. Mostly the material carried by winds contains silt-sized particles.

Accumulation of this material is named as “loess”. Normally, the areas where loess deposits are converted into soils are very fertile with deep soils. The thickness of

recorded loess deposits ranges between 20 and 30 m, but it can be as thick as 335 m. Animals also play major role to cause erosion i.e. the upper part of soil is disturbed by the hooves of animals and as well as plant protective cover is removed when animals graze in land. The bare arable lands are also major problem leading to erosion.

Soil mismanagement is the key factor which results in excessive wind erosion and resulted in barren land in many arid regions. Anthropogenic activities such as deforestation and excessive tillage also lead to severe wind erosion. In arid and semiarid regions, the major factors of wind erosion are fast moving winds, low rainfall (≤ 300 mm annually), high evapo-transpiration, low vegetation and undeveloped soils. Rates of wind erosion in arid to humid areas of the world are in the order of: arid > semiarid > dry sub-humid areas > humid areas. Contrasting water, wind has the capability to transport soil particles up- and down-slope and can contaminate both air and water (Blanco and Lal 2008). Wind erosion is not only disturbing the properties and the processes of eroding soils but also is severely affecting the neighboring soils and landscapes where the deposition occurs. One of the dominant sign of wind erosion is the formation of sand dunes and some time these can be as high as 200 meters in deserts. Wind erosion can be classified into different types based on movement of soil particles.

Fig. 12.3 Types of water erosion (a) Sheet erosion (b) Rill erosion (c) gully erosion Modified from Kilders (2015)

12.2.2.1. Suspension

The fine particles that are pushed upward into the atmosphere by strong wind and moved parallel to the soil surface have size of 0.1mm. This is exceptional erosion process because of which the fine soil particles can be conveyed high into the atmosphere and settle down again when the wind speed diminishes or brought around precipitation. The suspended fine particles can move to the hundreds of miles by wind.

12.2.2.2. Saltation

Soil particles dislodged with every impact and these moves along the surface of the ground by a series of short bounces. Some bouncing particles remain within 30 cm of ground surface mostly have the size of 0.1-0.5 mm. the 50 to 90% of the total soil movement by wind is accounted for this process that depend on the wind movement.

12.2.2.3. Soil creep

The soil particles along the surface of ground roll and slide. The bouncing effect of saltating particles is responsible for the movement of these particles. The total soil movement by wind is accounted 5 to 25% and soil creep having size of 0.5 to 1 mm in diameter can move comparatively large particles.

12.3. Causes of Soil Erosion

Soil erosion is influenced by political, economic, social conditions, climate, land use and management and topography. Poverty level directly relates with soil erosion in developing countries. There is no way to measure conservation practices for poor farmers that have limited or lacked resources. The risk of soil erosion is decreased by the elimination of implementing conservation practices and for year after year food production on small agriculture farms (0.5-2 ha) compels farmers to use over exploiting practices by Subsistence farming.

12.3.1. Deforestation

Energy fluxes, erosion control, moderation of climate and ecosystem stabilization are the essential ecosystem services provided by forest. Medications, wood, numerous other wood-based items and sustenance is also provided by wood. The major causes of denudation are urbanization, unnecessary logging and clear-cutting, construction of roads and highways, frequent fires and expansion of farming to marginal lands. As the human population continues to increase, there is a clear need for more food. In addition, the increases demand of agricultural products has created incentives to convert forests to farmland and pastures. Once a forest is converted to agriculture, usually gone forever, along with many of the plants and animals that once lived there. The land availability for agriculture or other uses is done by deforestation that causes the permanent destruction of forests. The land is swept into river by erosion without vegetative cover. So, the cycle of soil loss continues by the movement of farmers in the forest, clearing more forest as well as soil fertility is also lost.

12.3.2. Intensive cultivation

Industrial agriculture that is also termed by the intensive farming or cultivation is attributed by maximum use of inputs such as low fallow ratio, labor and capital per unit land. Higher yields are produced with the use of less land and less labor that capable the farmer by more intensive agriculture. But, blessing is not unmixed for the agriculture intensification. Human health and farm productivity is affected by increased environmental impacts by the potential degradation of water and soil resources. Even when there is not excessive soil erosion, soil quality can also be reduced by depletion in organic matter and natural supplies of trace elements as the result of intensive cropping. Wide range of plant and animal species maintain the fertility of soil with the diverse contributions and recycling of nutrients in natural ecosystem. When no fertilizers are used than some trace elements are depleted as a result of no diversity and rotation is replaced year after year by a single species grown. The organic content of the soil also decreased if there is no replacement of consumed nutrient over time when no crop residues or organic matter is added.

12.3.3. Overgrazing

In many livestock farms, the same piece of land for a long time is mostly concentrated by the herds of cattle and sheep. Soil displacement during traffic, repeated crushing or trampling and overgrazing is resulted by this confinement. Soil erosion on steeps slope or hillsides is increased when the protective cover is reduced by removing or thinning of grasses. Acceleration of water and wind erosion, degradation of soil structure and reduction in organic matter content of soil is resulted by overgrazing. Reduction in root proliferation and growth, soil compaction, drainage and water infiltration rate decreases by cattle trampling. Soil erosion in heavily grazing areas increase runoff by increasing stocking rate. Soil erosion is increased in wet and clayey soils by surface runoff and compaction on overgrazing lands. Siltation and sediment-related pollution of downstream water bodies also increase the soil erosion of pasture lands. Wind erosion is susceptible to increase soil erosion in surface soils that disintegrate the particles by animal traffic in dry regions. Flowing water and wind preferentially removed the detached fine particles of surface sand. Loss of topsoil and nutrients by the conversion of natural ecosystem that caused higher rates of erosion by increasing continuous grazing that initially damage the land. Wind and rain enabling erosion, ground cover and compaction of soil by overgrazing.so, water penetration and plant growth is reduced that is harmful to soil microbes and lead to soil erosion.

12.3.4. Cultivation of steep slopes

Raindrop is absorbed into soil pore spaces as it falls on the soil. When all the pore spaces are filled with water soil becomes saturated and extra water will either stand on surface or flow down as runoff. The moving water will flow soil particles away and starts the process of erosion. As the intensity of rain increases, the runoff increases and the force exerted on soil particles also increases. As the slope steepness increases, the velocity of runoff and force on soil particles also increases. The soils which have less or no vegetation on the surface are more vulnerable to erosion caused

by flowing water. Amount of rainfall, slope steepness, vegetation and soil type are the major factors causing slope erosion. Terracing on the slopes decreases the erosion by decreasing speed of runoff and crops which require heavy irrigation i.e. rice can be grown on these terraces.

12.3.5. Soil mismanagement

The common cause of soil erosion is the expansion of agriculture on poor quality water irrigation, indiscriminate chemical input, and no vegetation degrade soils. Crop residues are removed for fodder and biofuel and industrial uses, this practice leaves the soil bared from protective cover below a critical level and soil becomes vulnerable to erosion. Runoff is increased by intensive cultivation causing soil erosion, and ultimately transporting nutrients and pesticides off-site and water and soil quality is reduced. When a eroded soil is left fallow to recover and new land is brought into cultivation, the erosion problem is worsened as during fallow period amount of dense vegetative cover is reduced.

12.3.6. Urbanization

There is significant effect of urbanization because most of the productive agricultural land near cities has been converted into residential and commercial area. As a result, agricultural area is decreasing which ultimately affects the farmer’s income as the natural resources are also decreased. Despite decrease in agricultural land, the limited land is used intensively for cultivation which results in decreased soil fertility over the time.

12.4. Factors Determining Soil Erosion

Shear stress of runoff water and the critical shear stress of soil are the two major factors affecting gully erosion. Soil materials from the base and sides of channels by the shear stress of runoff are removed and transported to the small channels. Some of the important factors affecting soil erodibility are:

12.4.1. Slope

Slope is the major factor to control soil erosion. Length and steepness of slope are the main factors that affect soil erosion. As the steepness increases the erosion increases similarly, as the length of slope increases the eroded effect of running water increases. The water conservation practices such as terraces and buffer strips reduce the intensity of flowing water by reducing the slope. Runoff velocity of water and discharge is more from channels that have relatively more smooth surfaces. On the other hand, construction of water catchments and minimizing the soil slope reduces water runoff and thus decrease the erosion.

12.4.2. Soil structure

The arrangement or aggregation of soil particles is termed as soil structure. Intensive cultivation and large compaction results in deterioration of soil structure and particles

binding and thus make them susceptible to erosion. Soil structure results from of symmetrical arrangements of soil particles, which keep pore spaces, micro and macro-organisms, and different sized aggregates, shapes and stability within a limit. The resilience of soil to erosion is largely depends upon its structure. The soils with poor structure more are weekly aggregated, easily compacted and have high runoff with low infiltration. The quantitative measurement of soil structure is difficult therefore water infiltration, air permeability, and soil organic matter dynamics are usually related to soil structure development. Measurement of properties of aggregate is also a helpful way if soil structural stability at the aggregate level determines the macroscale structural attributes of the whole soil to withstand erosion. There are numerous techniques for characterization and modeling of soil structure. Advanced techniques for soil structure modeling aim to capture the heterogeneity of soil structure and correlate these quantifications with various processes such as erosion. The focus on soil-based techniques, coupled with the characterization of aggregates, can provide additional insight into soil structure dynamics. Current technologies include tomography, neural networks and fractals. Tomography allows the investigation of soil interior architectural design and allows for three-dimensional visualization of soil structures. By using this method, the geometry and distribution of macro pores and microporous networks in the soil can be examined, which facilitates the flow of air and water. The use of neural networks is another way to observe the structural properties of the soil to conserve water, store organic matter and resist erosion. Soil debris and its sensitivity to soil erosion are controlled by fractal theory in the process of cultivation. This theory involves the study of the complexity of soil particle arrangement, tortuosity and soil pore abundance, which is the key to explain the process of water flow through the soil. These relatively new technologies can help quantify the structural properties of the soil.

12.4.3. Organic matter

The cementing agent that binds the soil particles together is the organic matter. Organic matter plays important role in soil erosion prevention. The fundamental source of energy for soil organisms is organic matter. It is both of animal and plant origin. Soil protection from compaction and erosion, improvement in soil structure, water and nutrient holding capacity increases and healthy communities of soil organisms are supported with the frequent addition of organic matter. Crop rotations that contains high plants residues, leaving crop residues in the field growing cover crops, using low or no tillage systems, mulching, growing perennial forage crops, using optimum nutrient and water management strategies for healthy plants production with large number of residues and roots, growing cover crops and applying compost or manure are the practices that increase organic matter addition in soils.

12.4.4. Vegetation cover

Loss of protective vegetation through fire makes, ploughing and overgrazing makes soil susceptible to being wash away by water and wind and to reduce erosion losses, the vegetative cover provides natural measure. The water is slowed down by the plants as it flows over the land and ground is soaked by allowing much of the rain to

do this. Soil is prevented from being swept or blown away by the plant roots that hold the soil in place. Soil’s ability to erode is reduced by plants that protect the soil from the abrasive effect of raindrops. The flow of water is slowed down by the plants in wetland and on the banks of river and the roots prevent erosion by binding the soil.

12.4.5. Land use

The best soil protector against soil erosion is grass due to its highly dense cover. The considerable obstruction to surface wash is small grains such as wheat. During the early stages of growth, little cover is provided by the row crops such as potatoes, maize that also encourage erosion. The areas that are most subjected to erosion are fallowed areas where entire residue has been incorporated into the soil and no crop is grown.

12.5. Soil Erosion Prediction

Soil losses from cultivated fields by sheet and rill erosion are predicting by Universal soil loss equation (USLE) developed by Wischmeir and Smith (1978). USLE considers all the variables as the soil erosion is influenced by several factors. Soil management by erosion losses reduce to permissible limits is done by information of USLE equation variables. In Europe and USA, this equation was successfully applied and validated in various fields. The equation is as follows

A=R×K×LS×C×P

A = soil loss in metric tonnes per hectare (t ha-1)

R = rainfall and runoff factor or rainfall erosivity (j ha-1)

LS = slope length and steepness factor (compared to reference values of 22.6 m and 9%), dimensionless.

C = crop management factor – a ratio which compares soil loss from an experimental field with that from a field with standard treatment, dimensionless.

The soil loss in t ha-1 is obtained by multiplying all the variables

12.6. Soil Conservation Measures

Soil erosion is prevented by several agronomic and biological properties. Crop rotations, agro-forestry and soil synthetic conditioners, reduced tillage, riparian buffers, cover crops, vegetative filter strips, residue, canopy cover management and no-till are important among these. This Chapter discusses the importance of (1) soil amendments (e.g. manures) (2) soil conditioners (e.g., polymers) (3) crop residues (4) cover crops for soil erosion reduction. There are differences among these biological practices in relation to their mechanisms of erosion control. Biological measures such as buffers or thin films (e.g., conditioners), conditioner application in direct contact with the soil surface, crop residues using manure protect the soil from

erosion. The raindrop is intercepted above the soil surface by the protective effect of canopy cover and standing vegetation reduces soil erosion. Growing vegetation produce the mulching effect.

12.6.1. Agricultural conservation measures

Contour cultivation, manuring, mulching and mixed cropping are included in these practices.

12.6.1.1. Crop management

Soil fertility is improved and wind and water reduce soil erosion by good crop management practices. Keep soil covered is fundamental principle of conservation agriculture. Soil protection from erosion by leaving crop residues on soil surface after harvesting is also helpful approach.

12.6.1.2. Crop selection

If the gap is too long between harvesting one crop and sowing of the next crop than the additional cover crops may be required. The stability of the conservation agriculture system is increased by cover crops and erosion impacts are reducing by the improvement of soil properties and this biodiversity in the agro-ecosystem are promoted for their capacity. The more effective crops in soil erosion are perennials than annual crops. The most effective are sugar cane, fodder grasses, sweet potatoes and tea.

12.6.1.3. Early planting

The protection of the ground against raindrop impact is ensured by the crop shoots from the ground within one or two weeks after the onset of the rains.

12.6.1.4. Crop rotation

The practice of growing a series of dissimilar types of crops in the same space in sequential seasons is crop sequencing or crop rotation for benefits such as such as avoiding pathogen and pest buildup that occurs when one species is continuously cropped. Soil nutrient depletion is avoided by the crop rotation that balance the nutrient demand of various crops. The replenishment of nitrogen with the use of green manure and legumes in sequence with cereals and other crops is a traditional component of crop rotation. Soil structure and fertility by alternating shallow-rooted and deep-rooted plants can also be improved by crop rotation. The multi-species cover crops between commercial crops is also another technique. The advantages of intensive farming with polyculture and continuous cover are combined by these techniques. So, soil fertility, reduction of diseases and pests, addition of humus and control of erosion is ensured by crop rotation.

12.6.1.5. Inter-cropping

The impact of raindrops is reduced with the soil cover by the fast-growing legumes such as cowpeas and beans early in the season before a canopy is developed by cotton or maize to shield the soil.

12.6.1.6. Cover cropping

The practice of growing crops to cover the soil surface to reduce wind and water erosion is called cover cropping. This practice creates a favorable habitat for microorganisms by regulating the soil heat and temperature. These also sources of organic matter in soil as the fallen are decomposed.

12.6.1.7. Strip cropping

This is the practice of growing different crops in alternate strips in the same field. It helps minimizing wind and water erosion. Crop rotation and minimum tillage in addition to contour strip cropping has proven to be best method to conserve soil and water.

12.6.2. Soil management

Soil conditions are often changed by the inappropriate land use practices which ultimately result in soil erosion. Optimum soil management aims to provide favorable conditions for plant growth through improved soil nutrient availability and aggregation. Optimum soil management practices improve infiltration of water and improve soil capacity to hold water and in result reduce runoff and erosion.

12.6.2.1. Use appropriate tillage practices

Optimum soil physical conditions for better crop production are the main objectives of tillage. It also ensures timely seedbed preparation, planting and weed control.

Tillage practices should be adopted by keeping in mind that;

• Soil is neither too fine nor powdery; and

• It breaks up the hardpan if necessary.

The main tillage methods are slash and burn, hand hoeing, ploughing and harrowing, conservation or minimum tillage, deep tillage.

12.6.2.2. Applying organic manures and mineral fertilizers

Application of manure and fertilizers provide essential plant nutrients in the soil for better crop growth. The crops with fast growth cover the soil quickly and give higher yields. Essential plant nutrients such as nitrogen, phosphorus, potassium, and sometimes Sulphur required by plants are provided by inorganic fertilizers. There is no substitute of inorganic fertilizers therefore integrated use of organic and inorganic fertilizers should be adopted. Farmyard manure, green manure and composts etc. are the main sources of organic fertilizers.

12.6.2.3. Mulching and the use of crop residues

Spreading on the bare soil surface or placement of plant materials such as dry grass, straw, dry leaves, banana leaves, sugar cane trash, and other crop residues around the stem of the plants is helpful in controlling soil erosion and moisture conservation.

12.6.3. Agro-forestry

Planting of trees or shrubs or protecting the naturally sustaining trees is called agroforestry. Trees decrease the magnitude of splash erosion by reducing the raindrops impacts on the soil. They regulate soil temperature by shading the soil thus reducing the water evaporation. They also minimize the wind erosion by acting as wind breaks. They also play important role in nutrient recycling in the deep soil; leguminous trees fix nitrogen that benefits food crops.

12.6.4. Contour farming practices

Cultivation across the slope rather than up and down is called contour farming. Soil loss as much as 50% has been reported to be reduced by contour farming on gentle. The main objective of contour ridges in semi-arid areas is water harvesting and in humid areas potato cultivation. Plant residues are placed in lines along the contour for construction of trash-lines. These trash-lines slow down the runoff and trap the eroded soil. Grass barrier strips of Napier or other fodder grasses are planted along the contour.

12.6.5. Physical soil conservation measures

Physical soil conservation structures are permanent features made of Earth, stones or masonry, designed to protect the soil from uncontrolled runoff and erosion and retain water where needed.

• Selection and design of structures depend on:

• Climate and the need to retain or discharge the runoff

• Farm sizes

• Soil characteristics (texture, drainage, and depth)

• Availability of an outlet or waterway

• Labour availability and cost

• Adequacy of existing agronomic or vegetative conservation measures.

Below are some of the physical conservation measures:

12.6.5.1. Cut-off drains

Cut-off drains are made across a slope for intercepting the surface runoff and carrying it safely to an outlet such as a canal or stream. Their main purpose is the protection of cultivated land, compounds, and roads from uncontrolled runoff, and to divert water from gully heads.

12.6.5.2. Retention ditches

These are made along the contours to capture and retain incoming runoff water and hold it until it seeps into the ground. They are alternate to cut-off drains when there is no channel to discharge the water nearby. Sometimes these are for water harvesting in semiarid areas.

12.6.5.3. Infiltration ditches

The structure used to harvest water from roads or other sources of runoff is infiltration ditches. They comprise dug along the contour, upslope from a crop field and a ditch of 0.7-1.5m deep. Water is blocked at the other end when it is diverted from the roadside into ditch and seep into soil after it is being trapped.

12.6.5.4. Water-retaining pits

Water-retaining pits allow runoff water to seep into soil after by trapping the water. The runoff normally occurs into a series of pits which are dug into ground. Banks around the pits are made by the soil from the pit. Excessive water carry from one pit to next by furrows. The amount of runoff determines the size of pit and its typical size is 2 m square and 1 m deep.

12.6.5.5. Broad beds and furrows

The runoff water is diverted into field furrows (30 cm wide and 30 cm deep) in a broad bed and furrow system. The lower end of field furrows is blocked. The water backs up into the head furrow after the filling of one furrow and flows into the next field furrow. Crops are grown on the broad bed furrows of about 170 cm wide between the fields.

12.7. Water Loss

Water loss to the atmosphere by two natural processes that are evaporation and transpiration. Water loss from soil surface by evaporation and from leaf surface by transpiration. Evaporation is the combination of evaporation and transpiration.

Physical characteristics of soil and water affect the evapotranspiration as well as density, type and rate of plant growth in the field. Some other factors like wind speed, solar radiation, rooting depth, season of year, availability of soil moisture and land surface characteristics. Evapotranspiration is mainly dependent on the solar energy that is utilized to vaporize the water when the water is available.

12.8. Water Conservation

12.8.1. Plant wind breaks

Wind velocity is reduced by windbreaks. Reduced wind velocity reduces the evapotranspiration rate in the area that is directly downstream of the barrier. 50 % porosity should present in wind breaks to slowed down the wind that passes through windbreaks as well as it is not deflected over the wind break. The direction of predominant wind is necessary to plant the wind breaks. A diversity of adapted species decreases the soil erosion by stabilizing the slope, decreases the risk of pest and disease problems and it provide shade for livestock. So, these provide better results. Minimized competition for water around the surrounding crops and irrigation would not be needed as much by drought tolerant species.

12.8.2. Keeping plant residues on the field

Wind speed over the soil surface is reduced by crop residues that also decreases evapotranspiration; soil temperature and moisture is also reduced as it provides shades by conserving soil moisture. So, crop residues act as wind breaks over the soil surface. The residues that are not break down or decompose readily are best. Crop residues should be partially removed for fodder or fuel use and if possible, it should be left standing in the field after harvest. To retain much of the plant residues in the field, the stubbles should be cut at high point in the plant during harvesting time. The soil loss and impact of raindrop also reduces by keeping crop residues in the field.

12.8.3. Choosing water conserving species

Too much high water utilizing crops like alfalfa cannot conserve water in the landscape. Generally, low biomass producing annual legumes like chickpeas and lentils are the crops that transpire the least amount of water.

12.9. Environmental and Agricultural Consequences

of Soil Erosion

The valuable top soil is the most productive part of soil profile for agricultural purposes is removed by soil erosion. Production cost will be high and yield will decrease resulted by the loss of this top soil. Gullies and rills make the cultivation of paddocks impossible and these are produced by the erosion when the top soil is removed.

The long-term impacts of erosion on cropping lands include:

• Top soil that is rich in organic matter and nutrients is removed

• The depth of soil that is available for water storage for crop growth and for rooting is reduced.

• Increase runoff by reducing the infiltration of water into soil.

Short-term loss and increased costs can result from:

• Seedlings, fertilizers and pesticides, need to repeat field operations and loss of seeds.

• Erosion of soil from the roots

• Wind erosion blasted the young plants with sand

• Extra cultivations are needed to level out the eroded surfaces

Damage to the off-farm environment includes:

• Sediment deposition onto roads, in roadside drains and on neighbouring properties.

• Excess inputs of phosphorus, pesticides and nitrogen damage the quality of lakes, coastal water and watercourses.

• Spawning grounds of fish damage by sediments in rivers.

• Greater flood hazard downstream is caused by the deposition of sediment and increased runoff.

12.10. Recent Developments in Soil and Water

Conservation

Substantial development has been made in emerging conservation techniques against erosion. A better understanding of factors, processes of soil erosion, causes and the related process are being investigated by the middle of 20th century. The magnitude of soil erosion risk is determined by the better understanding of the factors that establish more effective control practices in many regions of the world. The extent of soil erosion remains high in spite of these technological advances. The on- and off-site severe effects of soil erosion are stressed by conservation policies from 1980’s. Soil erosion is reduced by the conservation practices as well as by adopting no-till farming. No-till farming is a practice of growing crops without turning soil. Better soil management are due to these efforts. The major problem is a water pollution with chemicals and sediments. USA and other developed countries have achieved significant improvements in soil and water conservation and these efforts are not reflected in those parts of the world where erosion possesses a great threat to food security. The integrated economic, political, social and agronomic approach are base to counteract the soil erosion by the more difficult measures of soil conservation. Soil erosion is a potential threat to environmental and agriculture sustainability and economically feasible, environmentally sound practices of soil conservation are the base of farming system. Soil management, type, climatic characteristics and ecoregion varies with the rate and magnitude of soil erosion. Soil erosion data is highly limited in less developed regions and estimates are particularly crude in degraded and erosion-prone areas. That’s why some people think that soil erosion issue is exaggerated while according to others views soil erosion is a severe problem and possesses great threat to the stability of agricultural production. Implications of erosion are either under- or over-estimated when credible data on the rate of erosion and its impact are non-existent or limited (Blanco and Lal 2008).

12.11. Soil and Water conservation practices in

Pakistan

Water conservation and reduction in water logging is managed by the by the consumptive use of water (CWU) for 19 crops in agro-ecological zones of Pakistan. Water scarce areas like DG Khan, Bahawalpur, Kohat, Balochistan and Attock had trickle irrigation systems. Rainfed ecologies were introduced with drip and sprinkler irrigation.

Increased use of ground water decreases the water logging and it also helps in lowering the groundwater table, much of the salts are leached down below the root zone as well as increased evapotranspiration. Crop yields and income increased with the installation of diesel and tube-well engine for the use of ground water by the

farmers that were the drivers of this revolution. On-Farm Water Management Programme (OFWM) is to promote the efficiency in the distribution and use of water to some extent that have been operating in Pakistan from 1976. There are 86,000 watercourses remain to be attended and 21,000 have been brought under OFWM. The work through Water Users’ Associations (WUAs) is the major achievement of OFWM. Farming community is totally involved in the renovation, post improvement maintenance of water courses as well as in their constructions. Trained and untrained labor with technical help is provided by the members of WUAs from the OFWM staff. Some of the costs involved is also borne by them. Sprinkler irrigation, lifting devices, mountainous regions and hydra dams are the water storage tanks that are constructed intimately for water management. The siltation decreases from 30 to 7 tonnes per acre foot of run-off in the Kanshi basin by the plantation of 5% of watershed with long rooted trees and grasses, construction of 350,000 mansonry check dams and 2,500 silt traps by Mangla Watershed Management Project that is under the Water and Power Development Authority (WAPDA). The life of Mangla Dam prolonged to 70 years by this project. Developed by PARC in collaboration with the International Center for Integrated Mountain Development (ICIMOD), the Sloping Agricultural Land Technology (SALT) for the Himalayan foothills of Pakistan was developed. Preservation of the soil fertility was the aim of this technology. Pakistan Agricultural Research Council (PARC) set up a model in the Pothowar Plateau of northern Punjab at Mungial (near Fatehjang) to develop the integrated land and water conservation approach.

According to the land capability for pasture, fruit trees, crops and other tree planting, the land area was used. The land was used to produce pastures, grasses forest trees and crops while its 4% of total area had grassed waterways of ponds and gullies. Without removing or disturbing the soil, the minimum required land-development operations were carried out. Erosion has been fully removed as well as gully have been completely reclaimed after 10 years. An appreciable wood is a source of handsome return that is produced by the harvesting of forest trees. The continuous source of income is fruit trees. Other farmers of the area inspired and got confidence from this model. The farmers can raise their income reasonably as well as make best use of degraded land by adopting this model.

The bed planting technologies for the major cotton wheat and rice wheat system of the country as well as zero tillage was developed by the Pakistan Agricultural Research Council (PARC) in collaboration with provincial research and extension system. Water resource conservations, crop stand establishment, fertilizer use efficiency enhance and germinating issues resolve by these technologies that significantly contributed in improving crop productivity.

The technologies related to sustainable high value crop production, increasing cropping intensity and moisture conservation were identified by the development of an integrated land use for Barani areas. The better crop production in rain-fed Pothwar by soil moisture conservation and land protection from water erosion is obtained from the low-cost water conservation structure technologies that was developed by the Soil and Water Conservation Research Institute. Other watershed management related projects presently under implementation in rain-fed areas consider these technologies in the up-scaling phase.

Water distribution structures for combating effects of cyclical droughts, for flood water diversions and also for increasing moisture availability were developed for rod-Kohi system agriculture (2 million ha). Reseeding of grasses like Symbo and Chryso species in Punjab and Balochistan provinces, V-shape plants rehabilitations structures, as ridge formation for shrubs establishments, the plantation of drought tolerant shrubs for winter grazing by the development of fodder reserves (Atriplex and Acacia species) are the micro-catchment water harvesting technologies that is included in the rangeland development technologies. The spread of these rangeland development technologies requires planned participatory efforts through social mobilization and community involvements and these are also still limited.

12.12. Conclusions

The growth of agriculture sector and rural livelihood depends on important natural resources like soil and water. High productivity goals and intergenerational food security is achieved critically by the conserving these vital natural resources. Overall improvement in ecological environment and sustained availability of the basic human needs for shelter, food and fiber is ensured by the optimal use of these resources. The basic factors causing soil erosion-induced degradation are wind and water erosion. Acidification, compaction and salinization are some other causes of soil degradation. The main causes of enhanced soil erosion are intensive cultivation, urbanization, overgrazing, poor management of arable soils and deforestation. Soil deserves more attention as it is being eroded faster than its formation. Agricultural productivity and environmental quality is sustained by managing and alleviating the off-site and on-site impacts of accelerated soil erosion. The livelihood of all inhabitants particularly in poor regions of the world is affected by the high cost of erosion. The global climate is affected by soil and soil maintains water resources clean as well as providing food security. Soil erosion is a major issue but the medium to store carbon globally and buffers water pollutant is soil. The regions where farmers are poor and the soil erosion is the major risk, the proper conservation policies implementation and the technologies must be done. Soil erosion is reduced and effectively stabilized in developed countries by the implementation of adequate conservation policies and programs but there is needed much more to be done. The poor farmers that do not have adequate resources to implement erosion control practices and mitigate the threat of soil erosion in developing countries require greater needs.

References

Ahmad, B., M. Ahmad, Z. A. Gill and Z. H. Rana (1998). Restoration of soil health for achieving sustainable growth in agriculture. Pak. Dev. Rev. 37:997-1015.

Blanco, H. and R. Lal (2008). Soil and water conservation. Principles of soil conservation and management, Springer, the Netherlands.

Ghadiri, H. (2004) Crater formation in soils by raindrop impact. Earth Surf. Proc. Landforms 29:77-89.

Lal, R. (2001). Soil degradation by erosion. Land Degrad. Dev. 12:519-539.

Kilders, L. (2015). http://conservationdistrict.org/2015/the-power-of-a-raindrop.html.Accessed on 7 May 2016

Stitcher, P. (2010). http://restoringutopia.blogspot.com/2010/07/like-hollow-point-bullets-from-sky.html. Accessed on 05 April 2016.

Pimentel, D., C. Harvey, P. Resosudarmo, K. Sinclair, D. Kurz, M. McNair and R. Blair (1995). Environmental and economic costs of soil erosion and conservation benefits. Science 267:1117-1122.

USDA-NRCS (2003). Soil map and soil climate map. Soil Science Division, World Soil Resources, Washington DC, USA.

Wischmeier, W.H. and D.D. Smith (1978). Predicting rainfall erosion losses—a guide to conservation planning. U.S. Department of Agriculture, Agriculture Handbook No. 537.

Adenosine, 149, 150 Aeration, 33, 49, 99, 105, 109, 118,

119, 123, 124, 128, 129, 131, 132, 134, 135, 152, 153, 177, 178, 183, 198, 235, 259

Ammonia, 103, 110, 111, 152, 231 Assessment, 124, 136, 174, 218, 219,

220, 239, 240, 241, 242, 243, 244, 245, 261

Bacterial, 116, 185 Biochar, 141, 170, 181, 182, 189,

191 Biofortification, 169 Bioremediation, 103, 115, 116, 117,

235, 236, 237, 238 Buffering, 77, 98, 99, 266 Capability, 71, 74, 215, 232, 249,

272, 284 Cell, 108, 111, 148, 150 Chemistry, 1, 5, 6, 13, 14, 77, 78, 89,

91, 92, 103, 237 Class, 7, 47, 71, 73, 124, 125, 149 Classification, 2, 5, 6, 7, 12, 18, 34,

39, 42, 43, 47, 48, 71, 74, 75, 98, 124, 131, 220

Climate change, 4, 103, 104, 119, 163, 182, 247, 248, 249, 250, 251, 252, 256, 257, 260, 261, 262, 263

Colloids, 26, 28, 29, 77, 78, 79, 80, 85, 88, 91, 92, 94, 95, 96

Complementary, 94 Composting, 115, 118, 120, 141,

165, 170, 171, 180, 181, 189 Conduction, 133, 209 Conservation, 6, 72, 139, 142, 168,

265, 266, 267, 273, 275, 278, 279, 280, 283, 284, 285, 286

Consistence, 49, 53, 124, 131 Convection, 133

Creep, 270, 273 Crystalline, 7, 8, 20, 80, 85, 102 Cycle, 10, 11, 14, 106, 112, 113,

144, 149, 151, 153, 154, 155, 190, 249, 251, 252, 253, 254, 256, 258, 274

Cycling, 1, 3, 32, 104, 105, 109, 123, 138, 151, 188, 253, 257, 262

Deforestation, 182, 249, 252, 255, 267, 268, 272, 273, 285

Density, 39, 44, 46, 86, 89, 94, 95, 119, 124, 126, 128, 129, 130, 136, 137, 140, 179, 204, 251, 281

Deposits, 12, 24, 25, 51, 53, 55, 114, 196, 271

Diffuse double layer, 91 Dilution, 94, 208, 214, 236, 239 Diversity, 51, 103, 104, 106, 107,

108, 109, 119, 121, 169, 182, 274, 282

Dynamics, 5, 107, 142, 155, 174, 177, 178, 189, 190, 262, 276

Earth, 2, 18, 38, 92, 124, 125, 186, 222

Ecological, 4, 11, 77, 166, 167, 215, 240, 242, 243, 266, 283, 285

Ecology, 1, 6, 13, 104, 120 Effluents, 223, 224, 244 Emission, 5, 105, 187, 249, 255 Enzymes, 3, 104, 107, 109, 112, 113,

115, 117, 149, 150, 151, 178, 183, 237, 238, 256, 257

Equations, 100 Equilibria, 5, 101 Eutrophication, 4, 138, 219, 232 Family, 7, 43, 47, 49 Fauna, 105, 111, 115, 266

286 Index

Fixation, 30, 104, 110, 111, 112, 115, 119, 151, 152, 168, 174, 236, 256

Functional groups, 85, 86, 87, 88 Genesis, 1, 5, 6, 17, 18, 25, 30, 39,

127, 197, 198 Green manure, 109, 112, 145, 167,

180, 207, 209, 212, 278, 279 Greenhouse, 105, 133, 135, 138,

139, 158, 186, 187, 190, 238, 243, 249, 251, 252, 254, 256, 259, 262

Growth regulators, 104, 107, 113, 115

Halophytic, 193 Horizons, 2, 14, 17, 18, 26, 28, 29,

30, 31, 32, 33, 34, 35, 36, 37, 38, 41, 43, 44, 46, 48, 51, 53, 54, 55, 62, 65, 74, 126, 127, 197

Hydration, 22, 26, 28, 30, 91, 138 Hydraulic, 124, 137, 138, 193, 198,

204, 236 Hydrolysis, 22, 26, 28, 30, 91, 149,

150 Implications, 190 Indicators, 3, 49, 123, 124, 136, 261 Infiltration, 4, 33, 123, 124, 128,

137, 138, 157, 188, 202, 204, 208, 274, 276, 279, 281, 282, 286

Integrated, 13, 17, 18, 123, 136, 139, 168, 170, 173, 215, 279, 283, 284, 285

Interactions, 4, 13, 18, 89, 103, 104, 114, 118, 119, 120, 151, 167, 174

Interception, 147 Ion exchange, 77, 91, 92, 93, 236 Ionization, 87 Land use, 6, 34, 124, 125, 136, 194,

249, 251, 252, 273, 279, 285 Ligands, 80, 90 Medium, 1, 2, 4, 14, 88, 98, 124,

157, 178, 200, 208, 236, 241, 285 Membrane, 147, 148, 149, 150 Metals, 90, 92, 100, 115, 116, 117,

119, 138, 188, 195, 223, 228, 229, 231, 234, 237, 238, 239, 244

Mobility, 31, 78, 102, 108, 118, 156, 158, 170, 171, 214

Molarity, 79, 89 Morphological, 17, 27, 33, 34, 43,

49, 54, 104, 147, 271 Morphology, 17, 33, 35 Nitrification, 112, 113, 152, 170 Nodule, 110, 111 Nomenclature, 35, 47, 131 Non-point, 219, 225, 230 Nutrition, 95, 141, 142, 143, 144,

162, 168, 169, 170, 208 Order, 9, 31, 34, 42, 43, 47, 48, 49,

51, 53, 54, 62, 94, 116, 132, 177, 213, 228, 229, 241, 242, 253, 257, 272

Oxidation, 5, 22, 26, 77, 78, 99, 106, 113, 118, 132, 135, 149, 186, 229, 231, 232, 235

Ozone, 10, 235, 250 Parent material, 14, 17, 18, 23, 24,

25, 26, 27, 29, 30, 31, 36, 37, 39, 41, 52, 56, 68, 74, 77, 78, 89, 126, 163, 195, 197, 231

Pedogenic, 18, 31, 36, 39, 77, 78, 267

Perspectives, 14, 142, 168 Phases, 17, 94, 102, 124, 200, 261 Phosphorus, 3, 50, 78, 103, 137, 145,

147, 154, 159, 164, 170, 174, 178, 183, 185, 187, 214, 238, 252, 279, 283

Phytoremediation, 115, 117, 118, 237

Planting, 131, 133, 136, 188, 212, 215, 271, 278, 279, 284

Pore space, 101, 109, 124, 178, 274, 276

Pore-size, 124, 128, 129, 130, 131, 137, 138

Potassium, 3, 27, 33, 94, 137, 146, 155, 158, 159, 164, 174, 279

Precipitation, 4, 5, 27, 32, 77, 89, 92, 93, 151, 153, 154, 157, 197, 198, 203, 212, 213, 229, 268, 273

Principles, 5, 12, 14, 163, 217 Products, 19, 22, 30, 93, 101, 112,

113, 115, 119, 169, 183, 195, 196,

Index 287

207, 224, 227, 228, 230, 231, 235, 240, 254, 256, 273

Profile, 4, 14, 17, 20, 21, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 37, 38, 41, 44, 54, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 74, 105, 126, 129, 130, 135, 138, 139, 189, 196, 204, 205, 210, 214, 267, 270, 282

Reactions, 5, 22, 26, 28, 30, 77, 78, 79, 89, 91, 92, 93, 94, 99, 100, 101, 132, 149, 150, 151, 152, 156, 183, 200, 207, 212, 226, 236, 247, 250, 257, 260

Recalcitrant, 103, 254, 258 Reclamation, 95, 194, 203, 204, 205,

206, 207, 208, 209, 214, 216, 217, 218, 270

Residues, 28, 30, 36, 109, 111, 112, 127, 132, 133, 136, 167, 169, 177, 178, 180, 183, 184, 185, 186, 188, 223, 229, 255, 274, 275, 276, 278, 280, 282

Retention, 4, 44, 46, 49, 78, 91, 92, 95, 102, 112, 128, 130, 156, 179, 182, 188

Rocks, 1, 2, 6, 10, 12, 13, 14, 17, 18, 19, 20, 21, 22, 23, 26, 27, 28, 34, 39, 78, 84, 151, 194, 221, 230, 234, 267, 271

Rodex potential, 99 Root zone, 31, 33, 60, 113, 156, 187,

197, 201, 202, 203, 204, 205, 212, 213, 284

Rotation, 3, 98, 166, 214, 251, 266, 274, 278, 279

Saline, 11, 31, 33, 62, 70, 96, 100, 174, 193, 195, 196, 198, 199, 203, 204, 205, 206, 207, 208, 209, 213, 214, 215, 216, 217, 218

Salinity, 33, 72, 73, 113, 119, 158, 188, 194, 195, 196, 197, 199, 201, 202, 203, 204, 208, 209, 212, 213, 214, 215, 216, 217, 218

Saturation, 4, 44, 45, 77, 98, 213

Sequestration, 107, 119, 123, 139, 247, 248, 252, 256, 257, 258, 259, 260, 261

Series, 9, 31, 32, 33, 34, 42, 43, 47, 48, 49, 54, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 74, 80, 94, 126, 204, 214, 273, 278, 281

Significance, 35, 111, 128, 129, 131, 154, 165, 173, 241

Silicate clays, 7, 22, 30, 36, 38, 78, 80, 81, 82, 83, 84, 85, 86, 87, 88, 92, 95

Silicon, 6, 80, 85, 88, 146 Slopes, 28, 52, 72, 73, 267, 270, 274,

275 Sodic, 33, 62, 96, 100, 174, 193,

198, 199, 203, 204, 205, 206, 207, 208, 209, 214, 215, 216, 217, 218, 253, 271

Sodication, 33, 195, 197, 216 Specific heat, 132, 133 Spheres, 1, 222, 247, 248, 260 Splash, 268, 269, 280 Storage, 4, 72, 120, 139, 150, 157,

165, 179, 180, 202, 211, 212, 231, 242, 252, 253, 256, 261, 263, 266, 282, 284

Substitution, 26, 82, 84, 87 Sulfur, 3, 6, 99, 103, 146, 159, 178,

183, 185, 187 Symbiotic, 109, 110, 114, 119, 152 Symptoms, 141, 158, 159, 160, 161,

162 Synergistic, 116, 179 Taxonomy, 42, 43, 47, 48, 74 Texture, 2, 4, 10, 24, 25, 27, 31, 43,

45, 46, 49, 54, 123, 124, 128, 129, 131, 134, 137, 138, 145, 156, 184, 205, 217, 280

Tillage, 72, 73, 109, 125, 128, 129, 131, 133, 136, 137, 139, 185, 187, 188, 203, 216, 259, 267, 270, 272, 276, 277, 279, 284

Tilth, 128, 138, 157 Transformations, 2, 3, 6, 104, 106,

112, 156, 167, 180

288 Index

Weathering, 6, 17, 18, 19, 20, 21, 22, 23, 25, 26, 27, 28, 30, 38, 39, 50, 86, 89, 100, 153, 194, 195, 196, 231, 253, 267

Wind breaks, 280, 281, 282 Xenobiotic, 103, 115

Soil Science - Concepts and Applications - HandoutsEt

Documents

Transcript of Soil Science - Concepts and Applications - HandoutsEt

Landscape complexity and soil moisture variation in south Georgia, USA, for remote sensing applications

NESTED INTERVALS AND SETS: CONCEPTS, RELATIONS TO FUZZY SETS, AND APPLICATIONS

Module 4: ACL Concepts

SOIL MECHANICS-CH2-Soil Formation

1 Basic Concepts

Managerial Accounting Concepts and Applications - Services

Lecture 1 SOIL RESOURCE INVENTORY CONCEPTS What ...

CK 12 PreCalculus Concepts

Ambient Intelligence Concepts and Applications

Civil Disobedience Concepts

Database Concepts

Soil Organic Matter and Soil Humus

Gaseous nitrogen emission from soil aggregates as affected by clay mineralogy and repeated urine applications

Political concepts

Stability concepts and their applications

Sequence Stratigraphy Concepts and Applications - CiteSeerX

Object Oriented Concepts

Networking Concepts

Fundamental Concepts - Fluid Mechanics

Power Center Basic Concepts