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Soil Management: the basis of sustainable agriculture

FAB-465

Lecture 11

Lecture Outline

• Soil Formation

• Soil Structure/Texture

• Indicators of Soil Health or Fertility

• Soil Conservation Practices

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Soil Formation

• Dynamic Process carried out over millions of years

• Mainly brought about by weathering of “parent material” (both chemical and physical action)

– Continental shift

– Erosion

– Transport

– Weather

– Plants and animals in soil

Soil Formation • Complex process involving formation and

decay of organic matter

• Pedogenesis – processes involved in the formation of soil

• A number of different factors are involved – Parent material

– Climate

– Soil organisms and vegetation

– Topography

– Human activities

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• Soil is usually a

combination of sand,

silt and clay

• Assembly of particles

gives rise to aggregates

• Patterns of aggregation

vary, resulting in different

soil structure

Layers of Soil

Soil Profiles

• Plough Layer: 20-30 cm thick, organic matter and live roots, dark colour

• Deep Plough Layer: Less organic matter and live roots, lighter colour, often greyish

• Subsoil layer: hardly any organic matter, not very important for plant growth

• Parent rock layer: Bedrock or parent material

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Soil Structure

• Variation is mainly based on the permeability of water

ftp.fao.org

Soil Structure

• Soil structure mostly described in terms of the following properties of soil aggregates

– Grade (degree of aggregation)

– Class (average size)

– Types of aggregates (form)

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Soil Texture • Size of particles that make up soil

The Size of Sand, Silt and Clay

Name Particle Diameter

Clay below 0.002 millimeters

Silt 0.002 to 0.05 millimeters

Very fine sand Fine sand

Medium sand Coarse sand

Very coarse sand

0.05 to 0.10 millimeters 0.10 to 0.25 millimeters 0.25 to 0.5 millimeters

0.5 to 1.0 millimeters 1.0 to 2.0 millimeters

Gravel 2.0 to 75.0 millimeters

Rock greater than 75.0 millimeters (~2")

Soil Texture Triangle

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Indicators of Soil Health/Fertility

•

Biological Chemical

Physical

Some common examples of soil stressors

USDA Natural Resources Conservation Services

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Physical Indicators

• Aggregate Stability

• Available Water Capacity

• Bulk Density

• Infiltration

• Soil Crusts

• Soil Structure and Macropores

Physical Indicators

Aggregate Stability

• Ability of soil aggregates to resist disintegration

• Creation of pore space for water infiltration

• Inherent factors – Predominant type & amount of clay

– Adsorbed cations (Ca, N, Iron oxide content)

– Stability enhanced with Ca, Mg, Fe, Al -> formation of organic matter

• Dynamic – Organic Matter and Biological Activity

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Physical Indicators Available Water Capacity

• Maximum amount of plant available water a soil can provide

• Soil’s ability to retain water and make it available for plants – Saturation

– Field capacity

– Permanent Wilting Point

Physical Indicators

• Bulk Density

– Measure of soil compaction

– Structural support, water and solute movement, soil aeration

• Infiltration

– Downward entry of water into the soil (Inches per hour)

– Dependant on soil texture

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Physical Indicators

• Soil Crusts

– Thin, dense, somewhat continuous layers of non-aggregated soil particles on surface of tilled and exposed soils

– Influenced by soil texture, organic matter, Na content

– Indicates poor infiltration, reduced air exchange

– Increased Na content increased soil dispersion

– Restriction of seedling emergence

Soil Crusts

www.soilquality.org

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Chemical Indicators

• Soil Electrical Conductivity

• Soil pH

• Soil Nitrate

Chemical Indicators

• Soil pH – Ideal pH for most crops 6.5-7

– Sources of H+ ions include • artificial fertilizers

• Carbon dioxide released from organic matter decomposition

• Rain water, acid rain

– Liming, application of limestone to the field to increase pH

– Direct and indirect impact on plant growth

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Chemical Indicators

• Soil Electrical Conductivity

– The ability of soil water to carry electrical current

– Cations and anions from salts dissolved in water

– Saline soils have high electrical conductivity

• Soil Nitrate

– Soil texture and nitrate retention

– Potential to decrease soil pH (acidification)

– Dependent upon the N fertilizers applied

Biological Indicators

• Earthworms

• Particulate Organic Matter

• Potentially Mineralizable Nitrogen (PMN)

• Soil Enzymes

• Soil Respiration

• Total Organic Carbon

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Biological Indicators Earthworms

– Burrow narrow channels

– Feed on a mixture of soil and plant residues

– The depth of burrowing depends on the type of earthworms

– Excreted material released back into the soil (N, P, K and Ca) and microbes

– Fresh cast is the site of excess microbial and nutrient cycling activity

– Porosity, root development, aggregate stability, soil structure

Biological Indicators

Soil Enzymes

– Decomposition of plant residues, increase the rate of release of plant-available nutrients

• Particulate Organic Matter (POM

– All soil organic matter particles between 2mm and 0.053mm in size

– Biologically and chemically active

– Part of the easily decomposable pool of SOM

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Phosphate Solubilization by fungus on Pikovskaya medium

Biological Indicators

Soil Respiration

– The amount of carbon dioxide produced by the soil as a result of different soil activities

– Decomposition of organic matter,

– Affected by temperature, moisture, aeration etc

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Recommendations for Effective Soil Management

(i) liberal use of crop residues, animal dung and other biosolids, (ii) minimal disturbance of soil surface to provide a continuous cover of a plant canopy or residue mulch, (iii) Judicious use of sub-soil fertigation techniques to maintain adequate level of nutrient and water supply required for optimal growth, (iv) an adequate level of microbial activity in the rhizosphere for organic matter turnover and elemental cycling, and (v) use of complex cropping/farming systems which strengthen nutrient cycling and enhance use efficiency of input.