The development of structure in arable soil depends on the following factors:
1.Climate: Climate has considerable influence on the degree of aggregation as well as on the type of structure. In arid regions, there is very little aggregation of primary particles. In semi-arid regions, the degree of aggregation is greater.
2.Organic matter: Organic matter improves the structure of a sandy soil as well as of a clayey soil. In case of a sandy soil, the sticky and slimy material produced by the decomposing organic matter and the associated microorganisms cement the sand particles together to form aggregates. In case of clayey soil, it modifies the properties of clay by reducing its cohesiveness. This helps making clay more crumby.
3.Tillage: Cultivation implements break down the large clods into smaller fragments and aggregates. For obtaining good granular and crumby structure, optimum moisture content in the soil is necessary. If the moisture content is high it will form large clods on drying. If it is low, some of the existing aggregates will be broken down.
4.Plants, roots and residues:
- Excretion of gelatinous organic compounds and exudate s from roots serve as a link
- Root hairs make soil particles to cling together. Grass and cereal roots Vs other roots
- Pressure exerted by the roots also hold the particles together
- Dehydration of soil leads to shrinkage forming cracks leading to aggregation
- Plant tops and residues, shade the soil and prevent it from extreme and sudden temperature and moisture changes and also from rain drop impedance.
- Plant residues also serve as a food to microbes which are the prime aggregate builders.
5.Animals : Among the soil fauna, small animals like earthworms, moles and insects that burrow in the soil are the chief agents that take part in the aggregation of finer particles.
6.Microbes: Algae, fungi, actinomycetes and bacteria keep the soil particles together. Fungi and actinomycetes exert mechanical binding by mycelia. Cementation by the products of decomposition and materials synthesized by bacteria encourages aggregate formation.
7.Fertilizers: Fertilizer like Sodium nitrate destroys granulation by reducing the stability of aggregates. A few fertilizers like CAN help in the development of good structures.
8.Wetting and drying: When a dry soil is wetted, the soil colloids swell on absorbing water. On drying, shrinkage produces strains in the soil mass gives rise to cracks, which break it up into clods and granules of various sizes.
9.Exchangeable cations: Ca, Mg –> Flocculating leading to good structure H, Na –> Deflocculating leading to poor structure
10.Inorganic cements: CaCO3 and sesquioxides
- Addition of lime to acidic soils improves their structure because lime stimulates growth of microbes (responsible for the improvement of soil structure). Addition of calcium nitrate also improves soil structure.
- Addition of large doses of partially decomposed organic manures like FYM, compost and green manure crops greatly improves soil structure.
- Surface of land covered with waste organic material like grasses, leaves, straw, etc. (mulches) keep the soil surface moist and cool so that microbes can multiply rapidly and improve soil structure.
- Mulches also protect the soil surface from the beating action of the raindrop that may destroy the soil structure to a great extent.
- There are some synthetic soil conditioners like polyacrylic acid (PAA), polyacrylonitrile (PAN) and vinyl acetate maleic acid copolymer (VAMA). But they are very costly.
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