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By

Y. P. Singh&

Solomon Das

Forest Pathology Division

Forest Research Institute

Dehradun-248 006singh_yp@rediffmail.com

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Soil is a thin layer of material on the Earth's surface inwhich plants have their roots. It is made up of manythings, such as weathered rock and decayed plant and

animal matter. Soil is formed over a long period of time.Soil Formation takes place when many things interact,such as air, water, plant life, animal life, rocks, andchemicals.A dynamic natural body on the surface of the earth in

which plants grow, composed of mineral and organicmaterials and living forms.The collection of natural bodies occupying parts of theearths surface that support plants and that haveproperties due to the integrated effect of climate and

living matter acting upon parent material, as conditionedby relief, over periods of time. Source: Buckman, H.

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- top soil; high organic content

- litter layer; forested areas

- eluviation of clay, iron,aluminium

sand & silt left behind- illuviation: accumulation ofeluviated

materials- unconsolidated parent material

- solid rock

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Organic matter is the partially decomposed remains of soil organisms, animals and

plant life

It makes up only a small fraction of the soil (normally 2 to 10%)

Organic matter retains moisture (humus holds up to 90% of its weight in water), and is

able to absorb and store nutrients.

Importantly, organic matter is the primary food source for microorganisms and other

forms of soil life in dryland agriculture

Incorporating large amounts of high-carbon material (i.e. wheat stubble) can deprive

plants of soil derived nitrogen in the short term

Organic matter contributes to the development of the darker friable topsoil that retains

moisture and cycles nutrients for plant growth

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Organic matter is divided intotwo main categories:organic residues, i.e., plantmaterial, manures, etc. in somestage of partial decay, andstable soil humus

Stable soil humus, a small percentageof total soil organic matter, is the endproduct of organic matterdecomposition when performed underanaerobic, or oxygen free, conditions,

beneath the soil surface. Theresulting organic structures can behundreds of years old and areconsidered a slow renewableresource. Organic residues formed atshallow depths add little, if any, to

the reserves of stable soil humus

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All groups ofmicroorganisms are represented in most soils, and

because of their presence soil is transformed from an inert mass ofmineral and organic residues to a dynamic living system

Protozoa

Fungi

Algae

Bacteria

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Factors which effects thepopulation of soil

microorganisms:AerationPHMoistureTemperatureSoluble mineralsamount of organic materialChanges inorganic compoundsAppearance of the soilFertilityDegree of aggregation.

Moisture holding capacity

Aeration:Microbes consumeoxygen from soil air and give

out carbon dioxide. In theabsence of such gaseousexchange, carbon dioxideaccumulates in soil air andbecomes toxic to the microbes

Moisture:In the presence ofexcess water, waterlogging,anaerobic condition occur theaerobes become suppressedand inactive

Temperature: Temperature isthe most importantenvironmental factorinfluencing the biologicalprocesses and the microbialactivity. Most of the soil

organisms are mesophiles and0

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pH:Bacteria prefer near neutral toslightly alkaline reaction between

pH 6.5 and 8.0; fungi grow inacidic reaction between pH 4.5and 6.5; actinomycetes preferslightly alkaline conditions

Degree of aggregation: Some

organisms may play a beneficialrole indirectly be creating bettersoil physical condition, e.g. byimproving soil aggregation. Soilmicroorganisms cause soilaggregation probably by the gum

or polysaccharides produced bythem. Azotobacter, Beijerinckiaand Rhizobium are examples ofgum producing bacteria

Amount of organic material: It

enhances the population ofmicroor anism, since the de end

Fertility: Numbers and diversity ofsoil organisms maintains soil fertility

and productive quality of soil

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Soil health refers to the condition of the soil and its potential to sustain

biological functioning, maintain environmental quality, and promote plant

and animal health. The resistance of a soil refers to its capacity to retain

function during stress or disturbance, whilst its resilience refers to how

quickly it recovers after stress or disturbance.

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1. Soil organic matter equilibrium maintained

The soil is composed of approximately 90-98% minerals and only 2-10% organic matter. Soil organic matter

(SOM) consists of all materials found in, or on soil that originate from organic material. It comprises bothliving and dead organisms in various stages of decomposition and ranges in age from recent inputs to

thousands of years old. Of the SOM present in soil, approximately 15% of this is living (made up of roots,

fauna and micro-organisms). The microbial component of this living pool cycles rapidly and is considered

essential for organic matter decomposition and nutrient cycling, degradation of chemicals and soil

stabilisation.

2. Soil fertility is balanced

A healthy soil is not only fertile, but productive. Factors which limit plant growth such as weeds, subsoil

constraints, disease and climate can limit productivity even where soil fertility is adequate. In understanding

the interactions between soil fertility and plant growth, it is necessary to recognise that "the yield of a plant is

limited by a deficiency of any one essential element, even though all others are present in adequate amount"

3. Water entry, storage & supply optimisedIn most agricultural pursuits, production is limited by the capacity of soil to store and supply water to the

plant. Soil texture and structure influence the amount fo water able to be extracted from soil by influencing

the size of the 'bucket' and the depth to which root growth can extract water. Optimising plant nutrition results

in vigorous growth and increases the uptake of available water. Better water use efficiency slows leaching of

nutrients and deep drainage which may contribute to groundwater recharge, rising water tables and off-site

environmental impacts.

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4. Enhanced soil biological function

A myriad of organisms live in soil, some of which perform beneficial functions such as organic

matter decomposition and nutrient cycling, whilst others are associated with plant disease. Someof these organisms are visible to the naked eye (earthworms, mites, insects, etc.), but most are

microscopic (fungi, bacteria).

5. Supports productive land uses

Inherent soil properties and position in the landscape are factors that determine the capacity of a

soil to sustainably support different land uses. Limited opportunities exist to change soil depth,

parent material and soil texture which influence soil processes and management. We must

consider whether the land is capable of supporting long term ecological and profitable production

for example, without degrading soil condition

6. Enhances environmental & community health and well-being

Soil supports plant and animal health, as well as provides materials for industry and

infrastructure - producing the food, fuel and fibre to nourish, clothe and provide energy to the

world. Yet historically, agriculture has accelerated the decline in the soil resource.

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Soil fertility is the combined effects of three major interacting components. These are the chemical, physical and biological characteristics of thesoil. The physical and chemical characteristics of soil are far betterunderstood than that of the biological component, therefore we know quite

a lot about the desired chemical and physical status of soils. It is stilldifficult to define the desired biological status of soil because they are sodynamic and changes occur in much shorter time periods than physical andchemical changes.

Biological fertility, while difficult to define, provides us with great

opportunities for land management and monitoring because of its dynamicnature. It is thought that, the biological state of soils may be able toprovide early warning of land degradation, therefore enable us to employmore sustainable land management practices

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The biological components of soil perform a number of important processes, soil biological

fertility :

1.Soil organisms are most abundant in the surface layers of soil,

2.Soil organic matter is necessary for nutrient cycling and soil aggregation,

3.Maximum soil biological diversity depends on the diversity of organic matter and habitats,

4.Nitrogen fixing bacteria form specific associations with legumes under specific conditions ,

5.Nitrogen is released during organic matter breakdown, either into soil or into the soil microbial

biomass,

6.Arbuscular mycorrhizal fungi can increase phosphate uptake into plants in P-deficient soils,

7.Soil amendments can alter the physical and chemical environment of soil organisms,

8.Some crop rotations and tillage practices decrease the suitability of soil for plant pathogens,

9.Production systems based on soil biological fertility can be profitable,

10.Soil biological processes develop slowly, and the time required will differ for different soils,

environments and land management practices.

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1. Soil erosion should be controlled to minimise loss of soil organisms,

2. Plant organic matter should be retained to maximise nutrient cycling and soilaggregation processes,

3. Some disturbance of soil is necessary to maximise soil biological diversity,

4. Nitrogen fixing bacteria should be selected that match the host, soil characteristics

(such as pH) and environmental conditions,

5. Inputs of nitrogen fertiliser should be calculated to complement nitrogen cycling

from organic matter

6. Inputs of phosphorus fertiliser should be calculated to complement and enhance the

activities of arbuscular mycorrhizal fungi,

7. Any substance added to soil should be assessed in terms of its effects on soil

biological processes and soil biological diversity,

8. Crop rotations and tillage practices should be selected to avoid development of soilconditions that enhance the growth and survival of plant pathogens,

9. The capacity of a management practice to produce a commercial product should be

considered in parallel with its capacity to maintain and/or increase soil biological

fertility,

10.Sufficient time should be allowed for establishment or restoration of a level of soilbiological fertility appropriate for particular soils and land management.

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