Soil Environment and its Modification : Agrilearner

• 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.