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GGR205 Soil Science Lecture 1

The Soils Around Us

July 3 2014

If not dirt, what is soil?

The relationship between soils and

disciplines (from Ellis & Mellor 1995)

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The pedological approach to soils

• Regards soils as complex biological, chemical and physical system

• Is both affected by, and influences, the environment

– sub-ecosystem

• Close relationship to organisms (e.g. plants and micro-organisms),

involving much co-evolution

• Acts as buffering agent of energy, chemicals and water in many

environmental systems

Think of pedology as the study of soils on the landscape in their natural state; and the history that has led to their present form

Radiation

budget

Short -wave

Long -wave

Albedo moisture content

thermal conductivity

Physical and chemical weathering of parent material

Water budget

Precipitation

Evaporation & transpiration

Soil moisture storage & hydraulic conductivity

To groundwater

Litterfall

Decomposition

Nutrient cycling

Leaching losses from soil

Nutrient retention & supply capacity

Gas exchange

Microbial community

O2

CH4

CO2

CH4

N2O

Soil properties

The “Pedosphere” as an Environmental Interface

Brady and Weil Figure 1.8

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The Soil Profile and Horizons

(Brady and Weil Figures 1.9 and 1.10)

- O = L,F,H horizons in Canadian Forest Soils

- Organic = >17 % organic carbon by dry weight

- E horizon – eluvial - not Canadian

- Lower case letter suffixes provide more detail- e.g. Ae in CSSC = a leached, ‘eluvial’ horizon

Pedon: the smallest

3D unit of soil that

permits study of the

soil profile (~1 m in

diameter)

The Soil Profile and Horizons

Soil Profile is all of the horizons together

Main Soil Horizons

O (organic LFH) Mainly plant remains litter (L) fibric (F) humic (H)

A (mineral) can accumulate organic matter and leach mineral matter

(eluviation)

B (mineral) can accumulate leached

mineral and organic matter

(illuviation)

C (mineral) Weathered bedrock or

parent material (little biological activity)

Except for C horizons, every main profile must have a lower case letter following when communicating about or classifying soils

The Soil Profile and Horizons

(Brady and Weil Figures 1.9)

Solum:

Regolith:

Bedrock = R horizon

in CSSC

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SOIL PROFILE AND HORIZONS

By classifying soils by horizons -> Information on soil

structure, formation, and properties

e.g.

Podzol Brunisol Fibrisol

SOIL STRUCTURE (PEDON)

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What Are Soils Made Of?

Soil Minerals

- Inorganic molecular constituents

- Extremely variable sizes: mixture of different particle sizes generates “soil texture” and are arranged together to form “soil structure” including clumps, pore spaces, etc

2mm > sand > 0.05mm

0.05mm > silt > 0.002mm

0.002mm > clay

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Soil Organic Matter

- Derived from plant material and

microbial decomposition products

- Oxidized back to CO2 over time while releasing nutrients (e.g. N, P)

- Humus is dark, heavily transformed organic matter.

Soil Organic Matter

- Mineral horizon: Less than 17% organic carbon

- Organic horizon: More than 17% organic C

- Soils themselves are ‘organic’ if more than one half of the profile is composed of organic horizon(s) (where might you find an organic soil?)

- Often need lab analysis to know % organic C

Soil Water

- Essential for photosynthesis and respiration and therefore

water availability controls plant and soil microbial activity

- Soil solution (soil water and its solutes) is a vital transfer

medium for nutrients to and carbon and CO2 from plant roots or soil microorganisms.

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Soil “Air” – Gas Exchanges

• Most plant roots need O2 from air, and must be well ‘ventilated’

• In poorly aerated soils, CO2 builds up from microbial and plant

respiration as O2 is consumed- common in flooded soils (e.g. wetlands)

• Also produce CH4 and N20 in flooded soils

Soil Biota

• Plant roots – the rhizosphere

• Most active area of soil

– fresh organic matter

– O2

– pore spaces

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Soil Biota

• Micro-organisms:

• Bacteria, archaea, and fungi

• dominant heterotrophic decomposers that form and consume

soil organic matter, cycle nutrients (e.g. nitrogen)

• Some pathogens (e.g. anthrax)

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Soil Biota

• Micro-organisms:

• Symbiotic mycorrhizal fungi

• Pathogenic microorganisms in soil

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Soil Biota

• Soil fauna: structuring the soil and grazing on

microorganisms and on plant roots

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So, can we define soil?

From Brady and Weil:

“A dynamic body composed of mineral and organic solids, gasses,

liquids, and living organisms that can serve as a medium for plant growth”

Or in a larger context: The collection of natural bodies (that is, soils)

occupying parts of the earth’s surface that is capable of supporting plant growth and has properties resulting from the effects of climate

and living organisms acting upon parent material, as conditioned by

topography, over time. (B&W)

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What Do Soils Do?

Soil: Many Functions

Figure 1.2 The many functions of soil can be grouped into six crucial ecological roles.

Soil: A medium for plant growth

- Physical support: roots anchor plants

– Soil structure is tightly linked to:

- Air and pore space: ventilation - O2 in, CO2 out

- Water: plants use water continuously- but it only rains periodically- soil water-holding capacity is critical

- Temperature control

- Protection from toxins - (adsorption- e.g. of Al+)

- Nutrients!

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Soil: A medium for plant growth

Anthropogenic Effects On Soil

• Agriculture – Deforestation – Urbanization - Greenhouse gases

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Soils: the Final Frontier (Science 2004)

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Historical Importance of Soils

Jared Diamond: “Collapse – how societies choose to fail or succeed”

Identifies soil resources and their conservation or loss/degradation as critical factor in decline of several societies:

- Norse in Greenland, 1000-1400 AD: soil erosion upon removal of turf for buildings and fuel

- Iceland: erosion of fragile volcanic soils upon removal of forests

Historical Importance of Soils • Easter Island: forest removal for fuel leading to erosion

• Haiti v Dominican Republic: absence of conservation v. preservation of forest cover and reduced erosion rates

• Australia: nutrient depletion of old, weathered soils and salinization of soils

Major Contemporary Issues

- Erosion of topsoil (e.g. U.S., China, Himalayas, Amazon)

- Compaction and sealing (e.g. Europe)

- Nutrient depletion of limited-fertility soils (e.g. Africa)

- Desertification (e.g. Central Asia, China)

- Salinization (e.g. Australia, semi-arid regions)

- Pollution – most of Developed World, central Asia and much of Developing World (e.g. Nutrient management in Canada).

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Erosion

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-Erosion of topsoil and forest soils

Soil Compaction

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Nutrient depletion

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Desertification

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Salinization

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Pollution

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Pollution

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Flooding

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The soil – food –

malnutrition – insecurity cycle

Lal, Science 304: 1623-

1627

Cycle of soil degradation and

food insecurity

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For Next Week

• Read:

– July 8 (Tuesday): Soil formation (Chapter 2), soil

classification (B&W Chapter 3)

– July 10 (Thursday): Soil physical properties (B&W Chapter 4), soil and water (B&W Chapter 5)

• Make sure you can access Blackboard

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Arctic brown soil landscape – southern Baffin Island

Arctic brown soil – Baffin Island

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A boreal forest landscape

Podzol beneath coniferous forest

LFH

Ae

Bh (humus accumulation)

Bf (Fe/Al accumulation)

Bm

C

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TEMPERATE FORESTS

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Brunisol beneath deciduous forest

Thin LFH A

Bm

C

Mer Bleue Bog, near Ottawa

S. Ontario (or Quebec)

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LFH

A

Ae: eluviated horizon

Bt: horizon with

clay accumulation

Bm

Cca: calcareous, clay-rich

parent material (commonly on limestone

-derived till)

pH

7.5

6.0

5.5

6.5

7.0

Luvisols (Niagara Peninsula, central BC)

Tall grass prairie

Black Chernozem soil,

tall grass prairie

Thick, organic matter

rich A horizon

B horizon with

CaCO3 removed

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A northern problem…. with contemporary climate change links

A northern peatland: post fire

Soil degradation and climate