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 clay soil. In case of a sandy soil, the sticky and slimy material produced by the decomposing organic matter and the associated microorganism 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 too high it will form large clods on drying. If it is too low some of the existing aggregates will be broken down.
4. Plants, Roots and Residues: Excretion of gelatinous organic compounds and exudates 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 held the particles together.
Dehydration of soil strains the soil due to shrinkage result in cracks lead to aggregation
Plant tops and residues shade the soil prevent it from extreme and sudden temperature and moisture changes and also from rain drop impedance.
Plant residues 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 etc., that burrow in the soil are the chief agents that take part in the aggregation of finer particles.
6. Microbes: Algae, fungi, actinomycetes and fungi keep the soil particles together. Fungi and actinomycetes exert mechanical binding by mycelia, Cementation by the products of decomposition and materials synthesized by bacteria.
7. Fertilizers: Fertilizer like Sodium Nitrate destroys granulation by reducing the stability of aggregates. Few fertilizers for example, CAN help in 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 ———-à H, Na
Flocculating Deflocculating
Good structure Poor structure
10. Inorganic cements: CaCO3 and Sesquioxides
11. Clay
12. Water
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