Soil Environment and its Modification Point Wise Notes for Competitive exam

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Soil Environment

Soil fertility is the status or the inherent capacity of the soil to supply nutrients to plants in adequate amounts and in suitable proportions.(Soil Environment)

• According to modern usage, soil fertility is the capacity of the soil to produce crops of economic value and to maintain health of the soil without deterioration.

Soil productivity is the capacity of the soil to produce crops with specific systems of management and is expressed in terms of yields.

• All productive soils are fertile, but all fertile soils need not be productive due to some problems like waterlogging, saline or alkaline condition, adverse climate etc.

• Under above conditions, crop growth is restricted through the soil has sufficient amount of nutrients.(Soil Environment)

Soil physical environment is controlled by soil characters like texture, structure, aeration, water, mechanical resistance and depth of soil.

• Soil texture refers to the relative proportions of sand, silt and clay.

• Rock fragments larger than 2 cm in diameter are called stones.

• Materials between 2 cm and 2 mm diameter are called gravel.

• Soil mineral matter smaller than 2 mm in diameter is called fine earth.

• Sand, silt and clay together constitute fine earth.(Soil Environment)

• Sand particles of 0.2 to 2 mm in diameter are classified as coarse sand.

• Sand particles of 0.02 to 0.2 mm in diameter called fine sand.

• Sand particles are small pieces of un-weathered rock fragments.

• Unless sand particles are coated with clay or silt, they do not exhibit properties such as plasticity, cohesion, stickiness, moisture and nutrient retention etc.(Soil Environment)

• Because of large size of sand particles macropores exist between them which facilitate free movement of air and water.

• Size of silt particles are in the range of 0.02 to 0.002 mm.

• Because of an adhering film of clay, silt particles exhibit some plasticity, cohesion, adhesion and adsorption.

• Silt particles can hold more amount of water than sand but less than clay.

• Sand and silt particles are approximately spherical and cubical in shape.(Soil Environment)

• Clay fraction controls most of the soil physical and chemical properties.

• Clay particles are less than 0.002 mm in diameter.

• Clay particles have highest surface area since surface area is inversely related to size.

• Clay particles can adsorb and retain water and nutrients.

• Some clays swell on wetting and shrink on drying.

• Clay exhibit properties like flocculation, deflocculation and plasticity.

• Clay behaves like a weak acid which is neutralized by bases such as calcium and magnesium ions, thus serving as storehouse for several nutrients.(Soil Environment)

Soil texture is classified based on the proportion of predominant size fractions of sand, silt and clay.

• If soil contains more than 80% silt fraction, it is called as silty soil.

• If soil contains more than 85% sand fraction, it is called sandy soil.

• If soil contains 40% clay, it is called as clay soil.(Soil Environment)

• If sand, silt and clay are in sizeable proportion, then the soil is called as loamy soil.

Silty clay soil means a soil in which clay characteristics are outstanding and which also contains sufficient silt.

• Textural class of a soil can be known from textural triangle.

• If the soil contains 60% sand, 30% clay and 10% clay then texture is sandy loam.

Soil texture is a permanent feature of soil and its change over years is negligible.

• Soil texture can be changed by adding sand or silt or clay as an amendment to improve physical condition.(Soil Environment)

Tank silt is added to sandy soil to improve the water holding and nutrient retention capacities.

Sand is added to heavy clay soil to improve internal drainage.

• Application of rice husks at 4.5 t/ha decreased soil bulk density, increased total porosity, decreased penetration resistance, improved saturated hydraulic conductivity.

• Bulk density of mineral soils varies from 1.4-1.8.

• Particle density of mineral soils varies from 2.5 – 2.7.(Soil Environment)

White grub damage is less in crops grown in clay soil compared to loamy and sandy soils.

Soil texture influences soil physical and chemical properties like water holding capacity, nutrient retention, nutrient fixation, nutrient availability, drainage, strength, compressibility and thermal regime.

• Clay soils have a high capacity to adsorb and retain nutrients and moisture.

• Clay soils are difficult to handle in tillage operations.

• Loamy soils exhibit properties intermediate between sand and clay soils.

Loamy soils are considered best for agricultural production, because they retain more water and nutrients than sandy soils and have better drainage, aeration and tillage properties than clay soils.(Soil Environment)

• Suitability of a soil to a particular crop depends on texture in addition to soil depth, depth of water table, salinity and alkalinity.

• Rice, cotton, sorghum, coriander are grown on heavy textured soils which include clay loam, silty clay loam, silty clay and clay.

Medium textured soils like loams, silt loams, silts and sandy loams are suitable for most of the crops.

Sandy, loamy sand, sandy loam and sandy clay are light-textured soils and these are suitable for groundnut, potato, tobacco, pearl millet and leguminous fodder crops.(Soil Environment)

Primary soil particles viz., sand, silt and clay are usually grouped together in the form of aggregates.

• The arrangement of primary particles and their aggregates into certain defined patterns is called soil structure.

• Natural aggregates are called fragments.

• Artificially formed soil mass is called clod.

Stable aggregates are those that resist break down by disruptive forces such as water and wind.

Fine clay particles i.e., colloids, flock or group together due to cohesion and form a cluster.(Soil Environment)

Sand and silt particles stick to the clay cluster, thus forming an aggregate amount and nature of colloidal clay influences aggregate formation.

Calcium and hydrogen ions bring about better aggregation than magnesium and potassium ions.

Wetting of clay particles with a liquid like water is required to form aggregates because water molecules show dipole movement.

Sesquioxides i.e., iron and aluminium oxides, act as cementing agents for binding sand and silt particles to form aggregates. A part of the iron in solution acts as a flocculating agent and remaining acts as a cementing agent

Aggregates formed with sesquioxides are more stable than those formed by silicate clays.(Soil Environment)

Humic and fulvic acids produced during decomposition of organic matter are also sticky in nature and help in aggregate formation.

Colloidal organic matter is more effective in forming aggregates than clay.

Clay is adsorbed on humus forming clay-humus complex and it helps in forming stable aggregates.

Fungi and actinomycetes produce sticky materials which are helpful in forming aggregates.

Microbial products capable of bounding soil aggregates are polysaccharides, hemicelluloses and several other natural polymers.

• Above materials are attached to clay surfaces by means of cation bridges, hydrogen bonding, vander waal forces and anion adsorption mechanism.

Soil structure types are single grained, massive and aggregated.(Soil Environment)

• When particles are unattached to each other as in sandy soils, it is called single grained structure.

• When soil is tightly packed in large cohesive blocks, as in the case of clay, the structure is called massive.

• Between the above two extremes, an intermediate condition in which the soil particles are associated in quasi-stable small clods are known as aggregates or peds.(Soil Environment)

Aggregate structure is generally the most desirable condition for plant growth especially in the critical early stages of germination and seedling establishment.

• Diameter of macro-aggregates is more than 250 µm.

• Diameter of micro-aggregates is less than 250 µm.

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