Soil degradation: I. Basic processes

LAND DEGRADATION & REHABILITATION, VOL. 1 , 51d9 (1989)

SOIL DEGRADATION: I. BASIC PROCESSES*

R. LAL, G. F. HALL, AND F. P. MILLER Department of Agronomy, The Ohio State University, Columbus, Ohio, 43210-1086 USA

ABSTRACT

This paper examines soil and land degradation. I t describes basic processes and factors responsible for degradation, illustrates the cause-effect relationships and differentiates between natural and man-induced regressive effects. The ‘critical limit’ concept is described in terms of properties beyond which the soil will not support an economically-viable agriculture. This paper is not an exhaustive literature review but emphasizes the scientific principles involved and highlights natural against man-induced processes. Important natural processes are: laterization, hard-setting, fragipan and clay pan formation, and geologic erosion. In comparison, man-induced processes consist of: soil compaction, accelerated erosion, desertification, salt accumulation and leaching and acidification. One of the principal constraints is the problem of data reliability. A reliable database and precise criteria are definitely lacking and hinder the assessment of the extent, type and degree of soil degradation and establishing the cause-effect scenario. Improving our database is, therefore, of a high priority if we are to adopt land use policy for sustainable soil management and long-range resource management. Also outlined, are vital research and development strategies. Judicious resource management policy should emphasize managing prime agricultural land to produce to its maximum potential so that there is no need to cultivate marginal and easily-degraded fragile ecosystems. A strict code of conduct is needed for utilizing marginal/fragile lands. Methods of restoring the productivity of degraded lands must be researched so as to minimize the need to clear and develop new lands.

KEY WOKIIS Soil erosion Compaction Latcrization Lcaching Acidification Dcscrtification Rcstoration

INTRODUCTION

Soil is the most basic of all resources and the primary medium for food production. I t is also non-renewable over the human time-scale. Consequently, there is an increasing concern among soil scientists, agriculturalists, environmentalists anu policy-makers as to whether soil resources will always be enough to feed, clothe and shelter the expected 10-5 billion (10’) inhabitants of the Earth by the year 21 10 (Population Research Bureau, 1986). It is estimated that with present technology the minimum per capita arable land needed for an adequate diet and acceptable standard of living is 0-5 ha. However, the available land resources are gradually diminishing below the minimum requirement. Assuming that there is no further reduction in arable land area due to degradation or conversion to non-agricultural uses, the per capita arable land area of 0.33 ha in 1986 will progressively decline to around 0.23 ha in AD 2000, to 0.15 ha AD 2055 and 0.14ha in AD 2110. The problem is compounded by the fact that on the global-scale land resources and population are unevenly distributed. Globally-speaking, human needs can only be met from the finite land resources by: (i) using new technological innovations that may alleviate production constraints and bring about quantum jumps in production; (ii) restoration of productivity of those lands that are now out of production but which were once biologically productive; (iii) halting or significantly reducing the conversion of prime land to non-agricultural uses and (iv) prevention of further decline in soil quality and productivity.

*Part 11: Soil Surface Management for Prevention of Soil Degradation and Rehabilitation of Degraded Lands will appear in a forthcoming issue.

0898-5812/89/010051-19$09.50 0 1989 by John Wiley & Sons, Ltd.

Received 28 November 1988 Revised 3 January 1989

52 K. LAL. G. F. HALL A N D F. 1'. MILLER

soil erosion later izotion leoching acidification solt occurnulotion blologicol degradotion hord -setting

,

- . . -. - - . -. . -. . L-1 degrodotion I'- policies I ornendments

deforestation inappropriate lond use high population density urbonization industriol woste lond tenure

ent

I plonted fflows I

\ 2 woter quality 3'Greenhouse Effect' 4economic and

political instability

Figure 1. C;iuscs, el'rccts and processes of soil degradation in relation to possiblc rcstorativc measures

Soil, the uppermost-part of Earth's crust, is teeming with life, dynamic and can be considered as virtually a living organism. Comparison of soil with a living body is also justified on the basis of its self-regulating ability to supply nutrients, buffer acid and base reactions, destroy and absorb pathogens, detoxify and attenuate xenobiotic and inorganic compounds, and its capacity for self-restoration through soil formation. The latter is, however, a slow process and a substantial amount of soil formation can occur only over a geologic time-scale. Soil misuses and extremes of conditions can upset these self-regulating attributes, and cause a soil to regress from a higher to a lower state of usefulness, and/or drastically diminish its productivity.

In this paper soil degrudution is defined as: diminution of soil quality (and thereby its current and potential productivity), and/or a reduction in its ability to be a multi-purpose resource due both to natural and man-induced causes. Decrease in soil quality implies there have been changes in soil properties and processes that have an adverse effect on that soil's life-support processes. In the agricultural sense, soil degradation leads to loss of sustainable production (Figure 1). Soil degradation may be caused by unfavorable alterations in one or all of a soil's physical, chemical and biological properties and processes. Processes that lead to soil degradation include: accelerated erosion, increasing wetness and poor drainage, laterization, salinization, nutrient imbalance, decline in soil organic matter, and reduction in activity and species diversity of soil fauna and flora. The effects of degradation may be local or global (Figure 1).

The term soil degradation is used vaguely. In order to be precise, it is important to identify the critical limits of soil properties and processes that constrain various uses. In the agricultural sense, this critical limit is the point where the soil will not support either an economically-viable or subsistance agriculture. For non-agricultural uses, the critical limit is reached when the soil begins to be economically-unsuitable for such purposes as: waste-attenuation, load-bearing, etc. The generalized, three-phased soil degradation pattern (in relation to crop productivity) is shown in Figure 2. In Figure 2 soil property refers to: rooting depth, soil organic matter content, plant-available water reserves, nutrient capital, etc. Phase I, (the range A-B on the x-axis of Figure 2) , represents degradation in the none-to-slight category. Soil

SOIL DEGRADATION: I BASIC PROCESSES 53

productivity within this range is influenced by exogenous factors, e.g., climate, other resource inputs, cropping system, etc. Phase I1 (range B-C) represents a rapid rate of soil degradation, whereby the crop yields decline drastically as the value of the soil property decreases from B to C. Points B and C represent critical limits within which productivity decline accelerates and eventually decelerates. The limit C is the point-of-no-return, at which the soil is so degraded that it does not deteriorate any further and from which it cannot be restored for crop production. Within the range C-D the soil may be irreversibly degraded or at best, degraded beyond practical utility for the most desired crops or other uses. The range and magnitude of these limits, therefore, vary among soils, crops, cropping systems and agro-ecological regions. Likewise, these limits vary for non-agricultural uses. The upper- and lower-limits of key soil properties (in relation to crop growth) are not known for any of the important soils of the world. For example, we do not know precisely the lower-limit of soil organic matter content (biomass-carbon) below which a favourable level of soil structure cannot be maintained. Furthermore, both quantity and quality of organic matter necessary to maintain an adequate structural condition vary for different soils and environments and crucial values are not known. There is also a paucity of basic research information on crop/animal growth (critical growth stages, etc) in relation to: the degree of soil degradation, reduction of effective rooting-depth, reduction of plant-available water reserves. Also unknown are the index properties that govern the susceptibility to degradation. In some soils the critical property may be the rooting-depth, in others organic matter content. The critical property is the one that cannot be easily and economically restored.

It is not the authors intent in this paper to provide an exhaustive literature review (for that, readers are referred to other sources e.g.: Boels, et al. , 1982; Chisholm and Dumsday, 1987; FAO, 1978; FAO/UNEP, 1983; UNEP, 1982). This paper emphasizes the scientific principles involved and highlights the comparative role of natural versus man-induced processes.

FACTORS versus PROCESSES OF SOIL DEGRADATION

Table I differentiates between processes and factors of soil degradation. Soil degradation processes include chemical, physical and biological actions and interactions that affect a soil’s capacity for self-regulation and its productivity. Factors of soil degradation are natural and man-induced agents and catalysts that set in motion those processes that lead to changes in a soil properties and its life-support attributes.

Physical processes of soil degradation lead to changes in soil physical, mechanical, hydrological and rheological properties which have a negative effect on crop and animal production, farm income and environmental quality. Change in soil structure is a principal effect of physical degradation; it may be manifest as: crusting; compaction; impeded-drainage and poor aeration, or high runoff and accelerated erosion. Eluviation of soil colloids (clay and humus), from surface to subsoil, is another important physical degradation process. Deforestation and intensive cultivation can lead to to transfer of clay to the subsoil (Roose, 1977; Lal, 1986a, b). Factors responsible for physical degradation are: deforestation, intensive row-cropping, excessive wheeled traffic, plowing and physical soil manipulation.

Chemical degradation processes include changes in soil’s chemical properties that regulate nutrient activity and capacity; or which maintain a favourable balance among principal nutrient elements, and the accumulation of substances possibly to toxic concentrations. Soil chemical degradation leads to a reduction in a soil’s ability to inactivate toxic compounds. Depletion of major plant nutrients, accumulation of salts and heavy metals in concentrations toxic to plant growth, leaching of bases and accumulation of A13+, Mn3+ on the exchange complex, are the principal processes of soil chemical degradation. Factors responsible for chemical degradation are intensive cropping with no-, or low-nutrient input; disposal of industrial, human and animal wastes; irrigation in aridhemiarid regions using water of poor-quality , and accumulation of industrial by-products and gaseous emissions through aerial deposition by ‘acid rain’ and atmospheric circulation. Principal processes leading to soil chemical degradation include: leaching, acidification and salinization.

Causative factors responsible for reduction in soil organic matter content are: intensive row-cropping:

Tabl

e 1.

Fact

ors

and

proc

esse

s of

soi

l de

grad

atio

n

soil

degr

adat

ion

proc

esse

s (a

ctio

ns a

nd in

tera

ctio

ns)

fact

ors

(age

nts

and

cata

lyst

s)

phys

ical

a de

teri

orat

ion

in

soil

stru

ctur

e le

adin

g to

: i.

com

pact

ion

ii. cr

ustin

g iii

. ac

cele

rate

d

iv.

hard

-set

ting

eros

ion

b im

bala

nce

in w

ater

iair

ratio

: i.

wet

ness

ii.

dro

ught

c ex

trem

es o

f te

mpe

ratu

re:

i. pe

rmaf

rost

ii.

sup

ra-o

ptim

al

chem

ical

a le

achi

ng

b fe

rtilit

y de

plet

ion

c so

dica

tion

d la

teri

zatio

n

e to

xific

atio

n (A

l+',

Mn4

3,

heav

y m

etal

s)

biol

ogic

al

a de

clin

e in

bi

omas

s ca

rbon

b re

duct

ion

in

orga

nic

mat

ter

cont

ent

c de

crea

se i

n po

pula

tion

activ

ity a

nd

spec

ies

dive

rsity

of

soi

l fau

na

and

flora

d al

tera

tions

in

biol

ogic

al

proc

esse

s fr

om

favo

rabl

e to

un-

fa

vora

ble

tren

ds

agric

ultu

ral

activ

ities

in

dust

rial

ac

tiviti

es

urba

niza

tion

a de

fore

stat

ion

a w

aste

a

appl

icat

ions

of

b ex

cess

ive

and

disp

osal

ci

ty w

aste

untim

ely

plow

ing

b 'a

cid

rain

' b

conv

ersi

on of

land

to n

on-

c in

tens

ive

row

- ag

ricul

tura

l cr

oppi

ng a

nd m

ono-

us

es

cultu

re

d in

disc

rimin

ate

and

exce

ssiv

e us

e of

ch

emic

als

e ex

cess

ive

graz

ing

and

high

sto

ckin

g ra

te


plowing and other operations leading to mechanical soil disturbance; accelerated soil erosion; excessive applications and indiscriminate use of pesticides, and/or contamination with industrial wastes. Soil and crop management that leads to extremes of soil temperature and/or soil moisture conditions, also cause biological degradation of soils. The prevalence of high soil temperatures (Ghuman and Lal, 1987; La1 and Cummings, 1979) is a factor responsible for a rapid decline in soil organic matter content. Other factors remaining the same, the rate of mineralization of organic carbon is about four times as high in tropical than temperate region soils (Jenkins and Ayanaba, 1979).

NATURAL PROCESSES

Mineral material is exposed at the Earth’s surface as a result of: erosion, aeolian deposition, fluvial deposition, colluviation, volcanism, glaciation, or a combination of these physical processes. Upon exposure to the atmosphere many chemical and physical processes begin to alter the mineral material. Weathering breaks down the minerals, water dissolves weathering products, organic material accumulates at, and close to, the surface, and numerous other chemical and physical processes take place, all of which are considered part of the broad concept of pedogenesis. The result of these processes are often distinctive soil layers or horizons which have been considered ‘diagnostic’ (Soil Survey Staff, 1975).

Some of the initial processes can be considered as desirable for plant growth but the majority of the processes lead to less-desirable physical and chemical conditions and thus may be considered to cause degradation of the soil (Hall, et al . , 1982).

Some of the main degradation processes are discussed in the following section:

Laterization Laterization is a general term to describe the process of iron accumulation in soils. The term is derived

from ‘laterite’, a name originally applied to an iron-rich soil material used to make bricks. Other terms that have been introduced for the iron-rich ‘laterite’ materials are ferricrete and plinthite. Ferricrete is used almost synonomously with laterite. Plinthite is more specific, referring to a soft, iron-rich soil material that hardens irreversibly when exposed to several cycles of wetting and drying.

In general, laterization is a process which, at its extreme, involves intense weathering (resulting in a breakdown of all minerals except quartz) and intense leaching of the soil which removes all the soluble salts, much of the silica and some of the iron and aluminum. Kaolinite is the dominant clay mineral formed as a result of the process. The iron and aluminum in lateritic material has traditionally been considered to be the result of residual accumulation from the original parent material. Recent studies have, however, indicated that at least a portion of the material may have been transported vertically or laterally. Groundwater fluctuation is suggested as a mechanism for such transport, particularly in the case of plinthite.

Some consider laterite to be a very thick B-horizon of an intensely weathered soil, for laterites and plinthites are associated with very old, stable surfaces and a wet tropical climate. The weathering zone in these landscapes is commonly many meters thick. As a result of intense weathering such soils are very low in nutrients needed for plant growth. The high iron and aluminum content results in complex fertility management problems, and where the iron has hardened into ironstone, roots are restricted to the upper portion of the soil.

Hard-setting Soils with dense, sometimes weakly-cemented surface horizons, are identified as hard-setting. In

Australia where this soil condition has been most extensively observed and studied, hard-setting is associated with deeply-weathered soils that are low in phosphorus (Thompson, et al . , 1982). This low fertility has resulted in limited biological activity and soils low in organic matter. These soils are characterized by massive, single grain or weak pedality of the surfaces which crust. Soil Taxonomy does not identify a surface horizon with weakly-cemented characteristics, thus the hard-setting surface would

56 R. LAL, G . F. HALL AND F. P. MILLER

be classified as an ochric epipedon (Soil Survey Staff, 1975). Little is known about the formation of this condition (CSIRO, 1986). This dispersion of the clay fraction often associated with these soils is attributed to sodium (sodium on the exchange complex hydrates the clay particles, diffuses the double layer and disperses the clay). The role of sodium in causing hard-setting is supported by the occurrence of the hard-setting properties in many soils classified as solodized, solonetz and solodic (i.e. rich in sodium). This association is, however, not universal, because hard-setting surface horizons are also found in some podzolic soils which lack apparent sodium accumulation .

Establishment of plants on hard-setting soils is difficult because of the low-porositiy, particularly with respect to macropores, and the presence of fine laminations in the upper few centimeters. The low porosity and limited vegetation result in high runoff of precipitation. It has been estimated that in one area under natural vegetation more than one-third of the annual rainfall is unavailable to plants due to this condition (Thompson, et al., 1982). Hard-setting soils often have a textural contrast, with a loamy surface and high-clay content in the subsoil. The presence of a high-clay content subsoil adds to the problem of vegetation establishment.

When cultivated, the hard-setting characteristics can be destroyed temporarily but usually reappear after as little as a single wet season. It has been speculated that one of the contributing factors to the formation of the low-porosity layer is a low phosphorus status. In Australia, the application of superphosphate and establishment of grasses and legumes has led to reduction of the crusting (Thompson, et al . , 1982). Gypsum has also been found to be an effective ameliorant for some of these soils.

Although hard-setting occurs under native vegetation, it is frequently associated with cultivation. Intensive cultivation or over-stocking of pastures may deplete organic matter and lead to the loss of soil structure and dispersion characteristic of hard-setting.

Frugipan formation Fragipans are pedogenic subsurface horizons characterized by medium textures with a high content of

silt with very fine sand and low-to-moderate clay content, high bulk density (relative to the horizons above), very low organic matter content, and brittleness when moist and hardness when dry. Because of their high bulk density, fragipans have very low hydraulic conductivities. The combination of high bulk density and low hydraulic conductivity causes soils containing fragipans to have limited rooting zones and so, in dry and wet growing seasons, crops suffer more severe climate-induced stresses than soils without such horizons.

Most fragipans have abrupt or clean upper-boundaries at depths usually ranging from 33 to lOOcm below the original soil surface (Soil Survey Staff, 1975). Since fragipans occur almost exclusively in forested regions, where precipitation exceeds evaporation for at least part of the year, plant roots are commonly stressed because of saturated conditions above the fragipan in wet years and high water tensions in dry years, since the root-restricting pan precludes a deeper rooting-zone (Grossman and Carlisle, 1969). Thus, crops produced from soils containing fragipans are not as buffered against drought or excessive wetness of climate as crops produced from better-drained soils with deeper rooting-zones. Erosion of soils containing fragipans is more likely to affect crop yields than similar rates of erosion on soils with deeper rooting-zones (Olson and Nizlcyimana, 1988 have characterized the impact of erosion on soils with shallow rooting-zones above fragipans).

The genesis of fragipans is obscure, theories range from the weight of glaciers imparting fragipan characteristics to parent materials, which in turn are inherited by the soil, to relict morphology induced by permafrost (Grossman and Carlisle, 1969). Induration or cementation by various agents such as Si, oxyhydroxides of Fe and Al, and clay-bridging of individual sand and silt grains have been suggested as causes (Norton, et al., 1984).

The occurrence of fragipans is primarily confined to regions of humid mixed deciduous and coniferous forest in both cold and warm areas. Features common to parent materials from which pans form include a loamy texture, little-or-no carbonates, and appreciable amounts of silt or very fine sand (Soil Survey Staff, 1975). Fragipans roughly parallel the soil surface, maintain a relatively uniform range of depth and


thickness characteristic of the soils and areas where they occur, share a relatively similar morphology, nevertheless, they occur under a variety of horizons and in a range of parent materials.

The hardness of fragipans when dry is attributed to the close packing of their textural components and clay-bridging or binding. This mechanism does not explain, however, the fragipans’ brittleness when moist. Cementation by various agents, it has been suggested, is the cause of brittleness; this assertion is based on experiemnts where the brittleness has been reduced or destroyed by subjecting the pan to selective dissolution trials which extract Fe, Al, Si and amorphous materials (Grossman and Carlisle, 1969; Norton, et af., 1984; Soil Survey Staff, 1975).

The genesis of most fragipans seems to preclude a simple origin. The combination of textural, chemical, mineralogical, and climatic conditions common to fragipans suggests a complex genesis, combining induration processes, imposed upon an already dense matrix.

There are millions of hectares of these subsurface pans and they clearly restrict the use of the land where they occur. Since these and similar pedogenic pans restrict root growth, hydraulic conductivity and affect other soil characteristics, pan formation is a form of natural soil degradation which restricts the potential of the planet’s soils.

Clay-pan formation Translocation of clay, particularly fine clay, to the subsoil (B-horizon) or in situ formation of clay in the

B-horizon, results in a physical barrier to roots and to the movement of water. This barrier is particularly restrictive if the subsoil is lacking in well-developed soil structural units. The argillic horizon recognized by soil taxonomy can be considered equivalent to a clay-pan in most cases (Soil Survey Staff, 1975).

Several processes have been suggested for the formation of a clay-pan or argillic horizon. The most commonly-accepted theory is that clay, particularly fine clay, moves from the surface (A- and E-horizons) and accumulates in the subsoil (B- or argillic-horizon). In order for the clay to migrate in this way, alternate wetting and drying cycles are required. The presence of clay-enriched horizons in well-drained soils and their absence where the soils are saturated year-round supports this hypothesis. I n many cases loss of clay from the surface cannot account for the amount of clay in the subsoil. This discrepancy leads to the hypothesis, that much, if not all, of the clay is formed in situ from a re-synthesis of weathering products derived from the A-horizon.

It is probable that both of these processes, translocation and re-synthesis, are responsible for the clay-pan or argillic-horizon. A third hypothesis for the clay differential is that the surface and subsurface represent different depositional phases. However, the uniformity of the depth to the clay increase over large areas or regions makes this hypothesis less-acceptable than the first two (except for local situations).

The relative increase and total amount of clay seems to reflect the age of the soil. One indicator of soil age is the B:A clay ratio. As the soil becomes older the B:A ratio increases. A ratio of at least 1:2 is considered as one of the diagnostic properties of an argillic-horizon.

In general the degree of degradation attributable to a clay-pan soil is a function of the B:A clay ratio and the distance over which the increase in clay takes place. A ratio of at least 1:2 in a soil with well-developed structure is considered to indicate limited degradation. Soils which have a clay content that doubles in less than 7.5cm (abrupt textural change according to Soil Taxonomy) are considered degraded. The abrupt increase in clay would limit rooting, particularly where the soil lacks strong structure. The presence of sodium in the clay-pan or argillic-horizon of the soil creates an even more limiting environment for rooting, and is restrictive to water movement because of dispersion of the clay.

MAN-INDUCED PROCESSES

Physical degradation Deterioration in soil structure sets in-motion the process of physical soil degradation. ‘Soil structure’

refers to the total- and macro-porosity, stability and continuity of macropores, and to the functional attributes of soil pores to transmit and retain fluids and facilitate root growth. Deterioration of these

58 I< . LAL, G . F. H A L L A N D F . P. MILLER

structural attributes is manifested in different types of commonly observed problems: (i) capping and surface crusting, (ii) hard-setting, (iii) reduction in soil elasticity, (iv) decrease in bearing capacity and trafficability. (v) soil compaction leading to impeded root growth, and (vi) water imbalance causing poor drainage, frequent drought, excessive overland flow and accelerated erosion. These types of physical degradation are interrelated and lead from one problem to another. Two principal forms of degradation of economic importance are soil compaction and accelerated erosion.

Soil compaction. Root-zone compaction is a principal form of physical degradation observed on intensively-managed croplands and pastures. Structurally-inert soils, containing low organic matter and predominantly low-activity clays, are most prone to compaction. The structural stability of these soils is extremely vulnerable to the mechanical forces involved in normal farm operations. Compaction modifies the pore volume and poor size distribution and can be expressed in terms of dry bulk density (p,,), total U, and macroporosity (fill), permeability (KJ, diffusivity (Do). soil strength (a), cone resistance (R), soil surface and sub-surface deformation, and aggregate size distribution. The degree of soil compaction is usually expressed in terms of relative density:

PI, - Pmin

Pinax - Pmin Pr = Equation (1)

where pr is the relative bulk density, pI, is the actual bulk density, and pmin and pmilx represent minimum and maximum dry hulk density, respectively. No single parameter, however, has been found adequate to define soil compaction. Many researchers have suggested that new structural indices be developed to express soil compaction in terms of crop response (Kayombo and Lal, 1986; Soane, 1985; Soane and Boone, 1986). It is a difficult task indeed, because the level of compaction at which the optimum yield is obtained depends on antecedent soil properties, cropping system and weather conditions. With the exception of laterization and other forms of chemically-related compaction, it is rare that a soil is irretrievably degraded due to compaction. Soil compaction, however, leads to other processes that accelerate the pace of degradation. Notable among these are: increase in the volume and rate of overland flow; accelerated erosion; increased wetness and poor aeration, and decrease in biomass production.

Soil erosion: Soil erosion is a severe global problem and a major environmental concern. In extreme situations, excessive erosion can cause an irreversable degradation of soil, especially where substrata are exposed. Erosion is in fact a complete form of degradation because it depletes nutrient capital, decreases effective rooting volume, and reduces plant-available water reserves. Erosion encompasses chemical, physical and biological degradation. Despite the voluminous literature, quantitative and reliable data on the magnitude of the problem are few. Most of the available data on the global extent of the problem are either based on reconnaissance surveys that lack a strong database or on measurements of sediment transport in major rivers (Figure 3). The latter are often obsolete and obtained by unstandardized methods. Relating sediment yield to erosion is constrained by several factors: (i) lack of information on delivery ratio, (ii) temporal discontinuity in sediment transport, and (iii) the problem of relating sediment delivered downstream to the exact source upstream. Consequently, estimates of denudation rates, based on sediment transport in rivers and on reconnaissance surveys, often differ by several orders of magnitude (Walling, 1988) (Table I1 and Figure 4).

In addition to the global estimates, some field data on erosion measurements are available for different ecological regions (see Table 111 and IV). The data obtained from small plots are often not comparable because of unstandarized methodologies and non-uniform experimental conditions. Misinterpretation and erroneous conclusions are major worries when using such data for designing conservation measures, planning landuse, and implementing resource management policies.

Wind erosion can also be a serious problem in arid and semiarid regions. Arid lands comprise about 36 per cent of the world’s total land area and are subject to varying degrees of wind erosion. However, quantifying the extent of damage caused by wind erosion is even more difficult than quantifying that caused by water erosion. The United Nations Environment Program (UNEP, 1977) has estimated that


I Phase I I Phase Il I Phasem

I I I I I I

A 8 C D x soil degradation t

Figure 2. Stages of degradation in relation to soil productivity. The index property and the range of critical limits B and C differ among soils

globally 80 per cent of 3 700 million ha of rangeland; 60 per cent of the 570 million ha of cropland, and 30 per cent of 131 million ha of irrigated land are affected by wind erosion. Estimates of wind erosion have been made for Africa by LeHouerou (1977a; 1977b) and Prosper0 and Carlson (1972), for the USSR by Kovda (1980), for the USA by Beasley (1973) and Brown (1981), and worldwide by UNEP (1977).

While there is no denying the fact that excessive erosion has in many instances caused severe degradation of soil resources, it is difficult to establish the direct cause-effect relationship between soil erosion and crop productivity. In fact, the severity of the effects of erosion on crop yields is a controversial issue.The controversy is caused by the fact that although the visual effects of soil erosion can be spectacular the effects of erosion on crop yields are hard to quantify. Effects on crop yields are

Table 11. Comparison of measured and suspended sediment yields for African rivers and estimated rates of contemporary soil loss estimated by FA0 (1978)

river country

Watari Bunsuru Senegal Falema Hammam Kebir Ouest Mesanu

Nigeria Nigeria Mali Mali Algeria Algeria Ethiopia

catchment area

(km2)

1450 5900

157 400 15 OOO

485 1130

150

suspended sediment FA0 estimates

yield of soil loss (t/km2/yr) (t/km2/yr)

483 a* 1000- 5000 438 a 1000- 5000

40 b 1000-5OOO 198 c 100- S O 0 0 92 c 1OOO-5000

1680 d 1000-5OOO 5000-20000

14.6b 100- 5000

Sources: Author; Walling, 1988 Notes: *Figures followed by the same letter (a,b or c) are statistically the same. The letters refer to Duncan’s Range Test for statistical analysis of the data.

60 K. LAL, G. F. HALL A N D F. P. MILLEK

9 b 3

ti


Table 111. The magnitude of soil erosion from croplands in various countries

country rate of erosion

(tihalyr)

Argentina, Paraguay, and Brazil Belgium Benin Burkina Faso China Ecuador Ethiopia Guatemala Guinea India Ivory Coast Jamaica Kenya Lesotho Madagascar (Malagasy Republic) Nepal Niger Nigeria Papua New Guinea Peru Senegal United States Tanzania Zimbabwe

18.8 10.0- 25.0 17.0- 28.0 10.0- 20.0 11.0-251.0

2 10.0-564.0 34.0

5.0- 35-0 17.9- 24.5 75.0 60.0-570 90.0

5.0- 47.1 40.0 25.0-250.0 40.0 35.0- 70.0 14.4 6.0-320.0

15.0 14.9- 55.0 9-6

10.1- 92.8 50.0

Sources: Barber (1983); Fournier (1967); Humphreys (1984); La1 (1976a; 1976b; in press); Ngatun- ga, er at. (1984); Roose (1977); World Resources Institute (1986)

cumulative, and may only be observed long after the erosion has occurred. They are also greatly influenced by management and weather. The mechanisms of erosion-induced alteration in crop yield also differ among soils. The direct effects on yield are due to loss in crop stand by wash-off or burial. The indirect effects are due to alterations in soil properties, loss of nutrients and water reserves, and the loss of rooting depth. Of course ther are many other hidden losses that are hard to perceive. The magnitude of effect also depends on the antecedent soil conditions. The loss in yield may range from as low as 5 to 10 per cent over 50 years (Larson, et a/. ,1983) for soils with deep rooting depth and high inherent fertility, to complete crop failure even over a short period of, say, 10 years for shallow soils of low inherent fertility (Lal, 1987a; 1987b; Mbagwu, et al . , 1984). Depending on soil and climatic conditions, erosion may enhance yields, have no effects on yields, cause slight yield reductions or lead to a complete crop failure (see Tables V and VI). It is thus understandable that there are no reliable estimates of the worldwide loss of productivity due to erosion.

Desertification: similar to erosion, desertification is a major environmental concern. The term is used qualitatively and vaguely, and it is hard to separate emotions from facts. Desertification or ‘desertization’ has been defined as the spread of desert-like conditions. The UNEP (1977) defined it as the ‘ .. . impoverishment of arid, semiarid and sub-humid ecosystems by the impact of man’s activities . . . This process leads to reduced productivity of desirable plants, alterations in the biomass and in the diversity of life forms, accelerated soil degradation and hazards for human occupancy’.

62 K. LAL, G . F. HALL A N D F. P MILLER

Table IV. The annual soil loss from various crops in different regions of the USA ~~~~~~ ~

crophotation location slope soil loss (%) (tonslha)

corn (continuous) corn (continuous) corn corn corn (plow-disk-harrow) corn (plow-disk-harrow) corn (conventional) corn (conventional) corn (continuous) corn (contour) corn (contour) corn (contour) cotton cotton wheat wheat (black fallow) wheat wheat (pea rotation) wheat (following fallow) bermuda grass native grass forest forest

Missouri (Columbia) Wisconsin (La Crosse) Mississippi (northern) Iowa (Clarinde) Indiana (Russell-Wea) Ohio (Canfield) Ohio (Coshocton) South Dakota (eastern) Missouri (Kingdom City) Iowa (southwestern) Iowa (western) Missouri (northwestern) Georgia (Watkinsville) Georgia (Watkinsville) Missouri (Columbia) Nebraska (Alliance)

3.68 48.7 16.0 219.8

53-8 9-0 69-9

51.6 30.1 6.9

5.8 6.7 3.0 51.9 2.0 to 13.0 52.9

59.3 59.3

2.0 to 10.0 47.2 50.4

3.68 24.5 4.0 15-6

Pacific Northwest (Pullman) 24.8 Pacific Northwest (Pullman) 13.8 Washington (Pullman) 17.0 to 24.5 Texas (Temple) 4.0 0.07 Kansas (Hays) 5.0 0.07 North Carolina (Statesville) 10.0 0.005 Central New Hampshire 20.0 0.02

Source: modified from Pimental, et al . , 1976

Estimates of areas subject to desertification vary widely, and changes from year to year are hard to validate. The data in Table VII show that the global area subject to desertification is about 38 million km2 (Mabbutt, 1978). The UNEP (1984) estimated that the area of land irretrievably lost or degraded by desertification each year is about six million hectares, and that the total world population severely affected by desertification is about 135 million.

The effects of desertification are especially severe in the West African Sahel (which includes Cape Verde, Senegal, the Gambia, Mauritania, Mali, Burkina Faso, Niger and Chad), (National Research Council, 1984; World Bank, 1985). In northern Africa, the process of desertification is most marked in areas where the average annual rainfall is less than the 500 mm (Floret and Floch, 1973; Floret, et al., 1977). In Western India, it is believed that the Rajasthan Desert has been spreading outward about one

Table V. The effects of erosion on crop yield may range from: increase in yield, no effect, slight decrease or severe decrease

% difference in yield from severely eroded versus uneroded soil crop

grain stover

rye +0.4 -14.5

barley -3.3 - 3.6 winter wheat +4-7 + 10.9 winter wheat +5,7 + 3-2

barley -1.7 + 10.5

Source: Grosse, 1967

64 K . L A L , G . F. H A L L A N D F. 1’. MILLER

Table VI. Effect of soil removal depth on cassava yield from a tropical Alfisol

depth of soil removed cassava tuber yield (tiha)

with fertilizer without fertilizer (cm)

Control 36.0 (100.0) 39.5 (100.0) 10-0 21.4 (59.4) 12-7 (32.2) 20.0 17.1 (47.5) 7.8 (19.7)

Source: La1 (1987b) Note: metric tons

(figures in parenthesis) indicate relative values in relation to the control as 100

Table VII. Estimates of area subject to desertification

arid semiarid sub-humid

class of hazard area % of zone area % of zone area % of zone total area (million km’) (million km’) (million km’)

very high 1-1 6.4 2.2 12.1 0.2 1.2 3.5 high 13.4 77.3 2.4 13.6 0.6 4.3 16.4 moderate 2.1 12.1 12.5 694 3.2 23.3 17.8 total 16.6 95.8 17.1 95.1 4.0 28.8 37.7

Source: Mabbutt, 1978 Note: metric tons

kilometer per year for some considerable time (Singh, 1977). In the USA, two million hectares of land have been identified where the risk of desertification is extremely high (Dregne, 1983).

Chemical degradation Salt accumulation: Accumulation of excessive soluble salts in the root-zone in concentrations toxic to

plant growth commonly occurs in arid and semiarid regions. The problem is particularly severe where the mean annual evaporation (PET) significantly exceeds the precipitation (P), typically where P/PET is less than 0.75. The accumulated salts usually include: chlorides, sulfates, and carbonates of sodium, magnesium and calcium. Soils with high concentration of salt in the root-zone are called saline soils if the sodium absorption ratio (SAR) is less than 15, Where:

SAR = Na/(Ca + Mg)”’ 2

Equation (2)

the electrical conductivity of saturation extracts of saline soils is generally more than 4 dS/m at 25°C. Soils are called alkaline if the predominant cation accumulated is sodium and the SAR exceeds 15. Soils with neutral soluble salts have saturation paste pH 4 . 2 . Alkaline soils have saturation paste pH >8.2, and the electrical conductivity of saturation paste usually exceeds eight dS/m at 25°C. Salt-affected soils are widely distributed. The data in Table VIII show that about 900 million hectares are presently affected by excessive salts. Salt-affected soils are especially common in irrigated regions of Asia, Australia and western USA.


Table VIII. Global distribution of salt-affected soils ~ ~

area in 1000 hectares

continent saline alkali total

North America Mexico and Central America South America Africa South Asia North and Central Asia South East Asia Australasia

6 191 1965

69 410 53 492 83 312 91 621 19 983 17 359

9 564

59 573 26 946

1798 120 065

339 971

-

-

15755 1965

129 163 80 438 85 110

211 686 19 983

357 330

total 343 333 558 097 901 430

Source: Gupta and Abrol, in press

The loss in productivity of salt-affected soils varies widely. It is possible to restore the productivity of some salt-affected soils provided that good quality irrigation water is available to flush the salt out of the root-zone and that terrain and soil conditions permit safe discharge of drainage water out of the affected ecosystem. The latter is, however, often difficult to achieve. Because salts are easily recycled into the root-zone, relamation is a continuous process.

Soil degradation related to salt accumulation in the root-zone is likely to increase with global increases in irrigated cropland. At present, only 15-20 per cent of the world’s arable land is irrigated. Further expansion of irrigation in semiarid and arid regions is likely to accentuate the problem. The irrigated land area in the world increased from eight million hectares in ~ ~ 1 8 0 0 to 48 million hectares in 1900, to 235 million hectares in 1980 (Szabolcs, 1986). The irrigated cropland area is expected to reach 300 million hectares in the year 2000. Furthermore, 75 per cent of the cultivable land area lies in arid, semiarid and sub-humid regions (Fukuda, 1976). The high productivity expected in these regions depends on supplemental irrigation. The dangers of excessive salt accumulation in the root-zone of irrigated soils in these regions can be minimized through proper soil surface management, maintaining favourable salt and water balance through proper drainage, and adopting appropriate cropping/farming systems. Although the basic principles of preventing further degradation and reclaiming salt-affected soils are known, we have a long way to go toward developing and validating site-specific technologies and locally-adapted farming systems.

Leaching and acidification: There are many soils that are naturally acidic even without interference by man. Acid-sulfate soils and planosols are examples of naturally acidic soils. Acid-sulfate soils are believed to occupy a total area of 12.6 million hectares of which 3.7 million hectares are in Africa, 6.7 million hectares in Asia and 2-1 million hectares in Latin America (Beek, et al., 1980). In addition, planosols and other acid soils occur in seasonally-wet regions and occupy about 150 million hectares (Brinkman, 1980).

Some soils become acidified due to cultivation and intense cropping. Such man-induced acidification is characterized by rapid oxidation of soil organic matter content (Jenkins and Ayanaba, 1979), and excessive leaching of bases out of the root-zone - a process which is the reverse of salt accumulation. Leaching and acidification can occur in very humid conditions, in soils of predominantly low chemical activity clays and containing low-levels of soil organic matter. Excessive leaching leads to the loss of bases such as Ca2+, Mg2+, K+ and Na+ and accumulation of A13+ and Mn2+. In some cases the loss of bases is also associated with loss of clay fraction. The loss of bases and clay alters soils’ physical and chemical properties.

In the humid and sub-humid tropics, loss of bases and acidification are accentuated by deforestation and intensive row cropping. The data in Table IX show drastic decline in pH and the concentration of exchangeable cations. Over a period of six years soil pH declined by 1.7 units. The rate of decline in

66 K. LAL, G. F. HALL AND F. P . MILLER

Table IX. Loss of bases and acidification of a tropical Alfisol cropped to corn (maize) for 12 consecutive years

property precultivations 2-years

pH (1 : 1 in H20) 6.1 f 0.4 5.8 f 0.3 exchangeable cations (m mol (+) kg-') Ca+2 79.0 k 18.0 71.0 k 18.0 Mg+2 20.2 f 7.0 19.0 f 5.0 K + 8.0 i 2.0 10-0 k 4.0 Na+ 1.0 f 0.3 0.9 f 0.3 M R + ~ 0.5 f 0.2 0.3 k 0.2 Total acidity - 0.5 f 0.3

4-years 5-years 6-years

5.8 k 0.3 4.7 L 0.3 4.4 f 0.2

52.0 -t 13.0 19.0 f 3.0 19.0 f 4.0 6.0 k 2.0 3.0 f 1.0 1.0 k 0.3 4.0 -+ 1.0 2 f 0.5 2.0 f 0.5 1.0 k 0.3 0.4 f 0.1 2.1 f 0.2 - 1.2 k 0.6 3.0 k 1.1

2.1 f 1.7 9.5 f 4.5 - ~~

Source: Lal, 1985

exchangeable bases was 10.0, 3.2 and 1.0m mol (+) kg-' yr-' for Ca2+, Mg2+ and K + , respectively. In contrast, the concentration of MnZ+ and total acidity increased at the rate of 0.4 and 2.25 m mol (+) kg-' yr-', respectively. The rate of decline of exchangeable cations depends on antecedent soil properties and on soil and crop management. The rate of decline is less with mulch farming, conservation tillage, with frequent use of legumes and grasses and planted fallows, and with agro-forestry systems. Some fertilizers, such as ammonium sulfate increase the rate of acidification. Nitrogeneous fertilizers cause acidification through nitrification of NH3 to NO;.Thus H+ is a by-product of resource input.

Leaching is also one of the principal mechanisms of inorganic N-loss in tropical soils. However, there is a paucity of reliable data on leaching losses for benchmark soils monitored over a long period using monolithic lysimeters. Land use, cropping system, tillage method, rate and type of fertilizer use, method and time of fertilizer application and the stage of crop growth affect leaching and the quality of percolating water. The effects of biopores (worm holes and root channels) on leaching losses have not been quantified.

THE PROBLEM OF DATA RELIABILITY

It is often argued that two billion hectares of once biologically-productive land have been rendered unproductive through irreversible degradation (UNEP, 1986). The present rate of soil degradation is estimated to be between 5 and 7 million hectares per year implying that 0.3 to 0.5 per cent of the world's arable land area is being lost every year through soil degradation (Dudel, 1982; FAO/UNEP, 1983). The projected loss by the year 2000 is ten million hectares annually. In addition, about 654 million hectares are considered to be problem soils that require special management skills (Table X).

Table X. Problem soils of the world (millions of hectares)

region Vertisols peat soils acid sulfate Planosols soils

Africa 105.0 12.2 Near- and Middle-East 5.7 0-0 Asia and Far-East 57.8 23.5 Latin America 26.9 7.4 Australia 48.0 4.1 North America 10.0 117.8 Europe 5.4 75.0

World Total 258.8 240.0

3.7 0.0 6.7 2.1 0.0 0.1 0.0

12.6

15.9 0.0 2.7

67.2 49.3 12.3 4.0

151.4 ~ ~~~

Source: Beek, et al., 1980

SOIL DEGRADATION: I BASIC PKOCESSES 67

As has already been pointed out, the critical limits of soil properties in relation to degradation are different for various soils and are not well-defined. If these critical limits for organic matter content, water and nutrient status, porosity and compaction, effective rooting depth, etc., are not known for major soils and crops, it is obviously difficult to decide whether a soil is degraded, and if so, to what degree. If these critical levels are not known nor the response of different crops to these levels for different soil and management conditions, then how are we to establish the magnitude and trends in soil degradation and assess whether there is a genuine threat to humankind?

Obviously these serious questions can only be resolved through coordinated and well-planned, long-term research efforts. We cannot afford to compromise the most basic of our resources-soil. Improving our database is a high priority in any attempt to adapt sustainable soil management and long-range resource management strategies. While the threat of soil degradation is widely recognized, a reliable database and precise criteria are definitely lacking, and without them guidelines for effective resource management strategies will be difficult to establish.

RESEARCH AND DEVELOPMENT STRATEGIES FOR RESTORATION OF DEGRADED LAND

The world’s soil resources are adequate to feed its growing population and to maintain the minimum required standard of living for each of its expected 10.5 billion inhabitants. This is achievable, however, only if we adopt a suitable resource management policy. This resource management policy must have the following attributes:

1.

11.

... 111.

I t must manage the prime agricultural land to produce to its maximum potential. These lands should be managed with judicious application of all inputs so that there is no need to cultivate marginal and fragile lands that are prone to severe degradation. Low-input agriculture as currently practiced in much of the world, is not sustainable. Research efforts must, therefore, be directed toward continuously developing/upgrading technologies that will increase productivity from these lands. The low-input systems, often being suggested today, are irrelevant to better quality land. With an improved economic situation, farmers will be eager to invest in better quality, more productive land.. Researchers must thus be ready to provide information on components, sub-systems, and systems of technology, on inputs required and risks faced. Although coercive measures have rarely been successful, a strict code of conduct will have to be adopted for utilizing marginal/fragile lands in a manner comensurate with their capability or carrying-capacity (UNEP, 1982). Appropriate land use systems must be researched, validated and adopted for these lands. It is estimated that there are some 800 million hectares of potentially-arable land reserves in the humid tropical forest region of South America, Central and Western Africa and in South East Asia and Oceania. These lands should be developed only if it is absolutely necessary to do so, because tropical rainforest is a fragile ecosystem and its productive potential is limited by soil and environmental constraints. Appropriate land use and soil and crop management systems will have to be adapted to use these land resources. Assessing the extent, degree, and type of soil degradation is an important priority for characterizing degraded ecosystems. It is essential to obtain a reliable database on the soils that have already been degraded. Research is needed to establish the cause-effect relationship so that methods can be developed to restore the productivity of degraded lands.

Restoration of eroded and degraded lands deserves a high priority particularly to reduce the need to clear and develop new lands. Methods must, therefore, be developed to restore and rehabilitate eroded and degraded soils. Land evaluation criteria should indicate the time when soil should be taken out of production and put under a restorative and ameliorative regime so that it does not reach the point-of-no-return. Knowing the critical limits of soil properties for different levels of management is crucial in this endeavor.

68 K. LAL, G. F. HALL AND F. P. MILLER

REFEKENCES

Barber, R. G. 1983. ‘The magnitude and sources of soil erosion in some humid and semi-arid parts of Kenya, and the significance of soil loss tolerance values in soil conservation in Kenya’, pp. 20-46 in: D. S. Thomas and W. M. Senga. (eds), Soil and Water Conservation in Kenya, Occasional Paper No. 42, University of Nairobi, Kenya.

Beasley, R. P. 1973. Erosion and Sediment Pollution Control, Iowa State University Press, Ames, Iowa. Beek, K. J . , Blokhuis, W. A,, Driessen, P. M., Van Breeman, N., Brinkman, R. and J . Pons. 1980. Problem Soils: theirreclamation

Boels, D., Davies, D. B. and Johnston, A. E. 1982. Soil Degradation, A.A. Balkema, Rotterdam, 280 pp. Brinkman, R. 1980. ‘Saline and sodic soils’, pp. 62-68 in Problem Soils: their reclamation and managemenf, ISRIC, Wageningen,

Brown, L. R. 1981. ‘World population growth, soil erosion and food security’, Science, 214, 995-1002. Chisholm, A. and Dumsday, R. (eds) 1987. Land Degradation: problems and policies, Cambridge University Press, Cambridge. CSIRO, 1986. Research Report, CSIRO Division of Soils, Canberra. Dregene, H. E. 1983. ‘Desertification: man’s abuse of the land’, Journal of Soil and Water Conservation, 33, 11-14. Dudal, R. 1982. ‘Land degradation in a world perspective’, Journal of Soil and Water Conservafion, 37, 245-247. FAO. 1978. A Provisional Methodology for Soil Degradation, FAO, Rome, 84 pp. FAO. 1981. F A 0 Fertilizer Yearbook, FAO, Rome, Italy. FAOIUNEP. 1983. Guidelines for the Control of Soil Degradation, FAO, Rome, 38 pp. Floret, C. and Le Floch, E. 1973. ‘Production, sensibilite et evolution de la vegetation et du milien en Tunisie presaharienne’,

Floret, C., Le Floch, E. , Pontainicr, R. and Romane, F. 1977. ‘Case study on dcscrtification in Oglat Merteba Region, Tunisia’,

Fournier, F. 1967. ‘Research on soil erosion and soil conservation in Africa’, African Soils, 12, 53-96. Fukuda, H. 1976. Irrigation in the World, University of Tokyo Press, Tokyo, 329 pp. Gupta, R. K . and Abrol, I . P. in press. ‘Salt-affected soils: their reclamation and management for crop production’, in R. Lal. and

Hall, G. F., Daniels, R. B. and Foss, J . E. 1982. ‘Rates of soil formation and renewal in the USA’, pp, 23-29in B. L. Schmidt (ed.)

Ghuman, B. S., and Lal, R. 1987. ‘Effects of partial clearing on microclimate in a humid tropical forest’, Agricultural and Foresf

Grosse, B. 1967. ‘The productivity of scvcrely eroded Parabraunerde formed from loess under moderate climatic conditions’,

Grossman, R. B. and Carislc, F. J . 1969. ‘Fragipan soils of the Eaatcrn United States‘, pp. 237-279 in N. C. Brady, (ed.) Advances

Gupta, R. K. and Abrol, I. P. in press. ‘Saft-affected soils: thcir reclamation and management for crop production’, Advances in

Humphreys, G . S. 1984. ‘Thc environment and soils of Chimhu Province, Papua New Guinea with particular reference to soil

Jenkins, F. D. and Ayanaba, A. 1979. ‘Decomposition of carbon-14 labelled plant material under tropical conditions’, Soil Science

Kayombo, B. and Lal, R. 1986. ‘Effects of soil compaction by rolling on soil structure and development of maize in no-till and

Kovda, V. A. 1980. Land Aridizarion and Drought Control, Westview Prcas, Boulder, Colorado, p. 277. Lal, R. 1976a. ‘Soil erosion problems on an Alfisol in western Nigeria and their control’, ZITA Monograph, 1, Ibadan, Nigeria, 208

Lal, R. 1976b. ‘Soil erosion on Alfisols in Western Nigeria’, Geoderma, 16, 363-431. Lal, R. 1985. ‘Mechanized tillage systems effects on physical properties of an Alfisol in watersheds cropped to maize’, Soil Tillage

Lal, R. 1986a. ‘Conversion of tropical rainforest: agronomic potential and ecological consequences’, Advances irr Agrunomy, 39,

Lal, K. 1986b. ‘Soil surface management in the tropics for intensive land use and high and sustained production’, Advances in Soil

Lal, R. 1987a. Tropical Ecology and Physical Edaphology,Wiley. Chichester, 744 pp. Lal, R. 1987b. ‘Effects of soil erosion on crop productivity’, CRC Critical Reviews in Plant Sciences, 5, 303-368. Lal, R. 1988. ‘Soil erosion by wind and water: problems and prospects’, pp. 1-8 in R. Lal. (ed.) Soil Erosion Research Methods, Soil

Lal, R. in press. ‘Land degradation and impact on food and other resources’, in D. Pimentel and I. Hall. (eds), Food and Naturul

Lal, R. and Cummings, D. J . 1979. ‘Clearing a tropical forest, I. Effects on soil and microclimate’, Field Crops Research, 2,91-197. Larson, W. E., Pierce, F. J. and R. H. Dowdy. 1983. ‘The threat of soil erosion to long-term crop production’, Science, 219, 456. LeHouerou, H. N. 1977a. ‘The scapegoat’, Ceres, 10, 14-18. LeHouerou, H. N. 1977b. ‘The nature and causes of desertization’, pp. 17-38 in M. H. Glantz. (ed.) Desertification, Westview

Mabbutt, J . A. 1978. ‘The impact of desertification as revealed by mapping’, Environmental Conservation, 5, 45-56. Mbagwu, J. S., Lal, R. and Scott, T. W. 1984. ‘Effects of artificial desurfacing on Alfisols and Ultisols in Southern Nigeria Parts I

National Research Council. 1984. Environmental Change in the West African Sahel, Board of Science and Technology for

and management, ISRIC (International Soil Reference and Information Centre), Wageningen, the Netherlands 72 pp.

the Netherlands.

CEPE doc. No. 71, Montpellier.

Proceedings of the UN Conference on Desertification, Nairobi, Kenya, (mimeo), 147 pp.

B. A. Stewart. (eds) Soil Degradalion Advmice.~ in Soil Science 10, Springer-Verlag, New York.

Determinanfs of Soil Loss Tolerance, ASA Publication No. 45, American Society of Agronomy, Madison.

Meteorology, 40, 17-29.

Trans. Eighrh International Congre.s.s on Soil Science, 2, 729-736.

in Agronomy, Academic Press, New York.

Agronomy, 8.

erosion’, Research Rulletiri No. 35, Depr. of Primary Industries. Port Moresby, 109 pp.

Society of America Journal, 41, 912-916.

ploughing systems in a tropical Alfisol’, Soil and Tilkuge Reseurch, 7, 117-134.

PP,

Research, 6, 149-161.

173-263.

Science, 5, 1-109.

and Water Conservation Society, Ankeny, Iowa.

Resources, Academic Press, New York.

Press, Boulder, Colorado.

and II’, Soil Science Society of America Journal, 48, 823-838.

International Development, National Research Council, National Academy Press, Washington, DC, 96 pp.

SOIL DEGRADATION: I BASIC PKOCESSES 69

Ngatunga, E. L., Lal, R. and Uriyo, A. P. 1984. ‘Effects of surface management on runoff and soil erosion from some plots at

Norton, L. D., Hall, G. F., Smeck, N. E., and Bigham, J. M. 1984. ‘Fragipan bonding in a late-Wisonsinan loess-derived soil in

Olson, K. and Nizlyimana. 1988. ‘Effect of soil erosion on corn yields of seven Illinois soils’, Journal of Production Agriculture, I ,

Pimentel, D., Terhaune, E. C., Dyson-Hudson, R., Rochereau, S., Samis, R., Smith, E. A., Denman, D., Reifschmeider, D. and

Population Research Bureau, 1986. Quoted by: ‘International Union for Conservation of Nature and Natural Resources’, Gland,

Prospero, J. and Carlson, T. 1972. ‘Vertical and areal distribution of Saharan dust over the western equatorial North Atlantic

Roose, E. J. 1977. ‘Application of the use of Wischmeier and Smith in West Africa’, in Greenland, D. J . and Lal. R. (Eds) Soif

Singh, G. 1977. ‘Climate changes in the Indian Desert’, Desertification and its Control, ICAR, New Delhi, India, 25-30. Soane. B. D. 1985. ‘Traction and transport systems as related to cropping systems’, International Conference on Soil Dynamics

Soane, B. D., and Boone, F. R. 1986. ‘The effects of tillage and traffic on soil structure’, Soil and Tillage Research, 8, 303-306. Soil Survey Staff. 1975. ‘Soil taxonomy: A basic system of soil classification for making and interpreting soil surveys’, USD-SCS

Szabolcs, I. 1986. ‘Agronomic and ecological impact of irrigation on soil and water quality’, Advances in Soil Science, 4, 18s218. Thompson, C. H., Moore, A. W. and Horthcote, K. H. 1982. ‘Soils and land use’, Soils: an Australian Viewpoint, Division of Soils,

UNEP. 1977. Desertification: an Overview, United Nations Conference on Desertification, August 29-September 9, 1977, Nairobi,

UNEP. 1982. World’s Soil Policy, United Nations Environment Program, Nairobi, Kenya. UNEP. 1984. ‘General assessment of progress in the implementation of the Plan of Action to Combat Desertification 1978-84’,

Governing Council, 12th Session, 16 February 1984, (Mimeo). Nairobi, Kenya, 23 pp. UNEP. 1986. ‘Farming systems principles for improved food production and the control of soil degradation in the arid, semiarid and

humid tropics’, Expert Meeting Sponsored by UNEP, 20-30 June, 1973, ICRISAT, Hyderabad, India, 36 pp. Walling, D. E. 1988. ‘Measuring sediment yield from river basins’, pp. 39-74 in R. Lal. (ed.) Soif Erosion Research Methods, Soil

and Water Conservation Society, Ankeny, Iowa. World Bank. 1985. Desertification in the Sahelian and Sudanian Zones of West Africa, World Bank, Washington, DC, 60 pp. World Resources Institute. 1986. Annual Report, Washington, DC, 270-271.

Mlingano, Tanzania’, Geoderma, 33, 1-12.

East-Central Ohio’, Soil Science Society of America Journal, 48, 136G1366.

13-19.

Shepard, M. 1976. Land degradation: effects on food and energy resources, Science, 194, 149-155.

Switzerland.

Ocean’, Journal of Geophysical Research, 77, 5255-5265.

Conservation and Management in the Humid Tropics, Wiley, Chichester, 177-188.

Processes, 5 , 863-935.

Agricultural Handbook 436, US Government Printing Office, Washington, DC.

CSIRO, Melbourne/Academic Press, London, 757-775.

Kenya.

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