SANF- SOIL BIOLOGICAL ACTIVITY
Agriculture has been one of the primary occupations in India, since it generates around 50% employment in the country. It also contributes 17-18% to the total GDP of India. One of the most important aspects for farming is soil. To cultivate crops that are rich in nutrients, farmers must keep in mind the physical and chemical factors of the soil they plan to cultivate on. A good knowledge about soil can help the farmer in choosing right crops for them, thereby gaining profits from a healthy harvest.
Apart from chemical and physical properties, soil also comprises biological activities, which play a huge role in the development of the nutrients in the soil. Organic matter, microscopic and macroscopic organisms (e.g., fungal hyphae and invertebrates), detritus from fungi and animals, and bacteria, and biological exudates, all assist in stabilizing soil structure. The role of each part of the biomass differs according to its size.
The populations of soil organisms of all sizes are linked functionally through their roles in the degradation of various forms of organic material. The latter includes live and dead plant material and other live or dead organisms. Earthworms and other large invertebrates create, and inhabit, burrows and pores, and are very mobile. Small arthropods, micro fauna and fungi live mostly in larger voids and in association with roots.
Some Important Biological Aspects Are:
Both plants and animals provide inputs of organic matter to soils. Once within the soil organic residues can be distinguished on the basis of their chemical structure (e.g., old lignified humic substances that degrade slowly), by their source (plant or animal) or by location. The importance of litter (crop residue) and manure as inputs of organic matter, varies between cropping systems and spatially within a system.
Effects of organic matter on soil fertility:
Within a cropping system, manuring practice varies with location. On traditional farms, the area near the household or village is highly fertilized with human and animal manure while more distant fields receive little or no organic matter.
Cropping in dryland regions needs nitrogen to be economically successful. Two sources of nitrogen are from organic matter and from nitrogen-fixing bacteria associated with plant roots. Bradyrhizobium and Rhizobium species infect plant roots forming galls or nodules, and fix nitrogen from the soil atmosphere directly to the plants. Locally-adapted, heat-tolerant strains survive from crop to crop in wet-and-dry climates and, whether established by natural colonization or by inoculation of the crop seed at sowing, they subsequently fix variable quantities of nitrogen.
Various fungi facilitate uptake of nutrients by plants, particularly phosphorus. Numerous fungi live in close association with plant roots. One group, vesicular arbuscular mycorrhizal fungi (VAM), form both vesicles and arbuscules (knot-like structure) on the surface and within the root. They also colonize soil animals including earthworms and woodlice. They are most prolific in the topsoil, to about 10 cm depth. They facilitate uptake of nutrients, particularly phosphorus from soils low in that element. They provide some protection to the host plant; their presence being associated with decreased colonization by pathogens.
Weeds, pests and diseases all compete with or directly reduce the vigour of crops. Many pests and diseases are soil-borne. Weed life-cycles depend on replenishment of the soil seed bank and survival of the seeds against natural decay, predation by soil animals and depletion by human management, particularly cultivations. Ecological weed control thus aims to minimize recruitment of new seed into the soil as a long-term strategy as well as trying to artificially reduce the size of the weed seed bank in the soil.
The incidence of insect pests and crop diseases depends on the complexity of the cropping system and the types of crops grown. It depends too on management factors, such as the timing and type of tillage and the treatment of the crop residues. These aspects are so complex they warrant a Bulletin to themselves, so they are only touched upon here.
Inorganic nutrients occur in soil as ions and minerals, e.g., as oxides, silicates and phosphates, both adsorbed onto the surface of clay particles and organic matter, and in solution. A large proportion of some nutrients, most notably nitrogen, is found in organic matter in all but the most highly degraded soils, so organic matter and the organisms associated with it are given prominence in this chapter. Clay particles, because of their crystalline structures, carry an inherent electrical charge. This results in attractive forces and repulsive forces (electrostatic forces) which give clay species their particular characteristics. The inherent surface charge also causes a layer of associated ions to align next to the solid particles forming a so-called diffuse double layer because it consists of a relatively unexchangeable layer (the Stem layer) closest to the surface of the particle and an outer, readily exchangeable layer, of varying thickness, called the diffuse layer.
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