Soil is a complex system and consists of three phases namelysolid, liquid and gaseous substances. Soil is important for sustenance of lifeas it provides the foothold and majority of nutrients needed for growth ofplants and animals. Capacity of soil to supply the essential plant nutrientsin available form and in a suitable balance for luxuriant and healthy plantgrowth is called as soil fertility. Soil fertility depends not only on thepresence of inorganic substances (mineral matter), organic substances (organicmatter), water and air, but also on the presence of microorganisms (Fig. 1).
Yes, soil is not a dead mass. There is life in soil. Thereare microorganisms present in soil, which greatly influences the qualitativecomposition of the soil and thus, the soil fertility. Because of the presenceof enzymes, carbon dioxide and organic matter, the life in soil is responsiblefor making up numerous transformations, which changes plant nutrients toreadily available forms and make and stabilize desirable soil structure forluxuriant plant growth.
The soil organisms are classified into two groups (Fig. 2)viz. Soil flora and Soil fauna. These are again sub divided into soilmacro flora (roots of higher plants) and soil micro flora (bacteria, fungi,actinomycetes and algae); soil macro fauna (moles, ants and earthworms) andsoil micro fauna (protozoa and nematodes). The total population of all soillife is numbered in billions per gram of soil and the live weight may be asmuch as five MT/acre. Of these groups bacteria are most abundant in soil. Nextin order are actinomycetes followed by fungi. Algae are found under specificconditions. As per scientific calculation, the biomass of these four importantmicroorganisms is about 960 kg/acre.
The extent, to which the microbes are present in soil,depends on nature and chemical composition of soil. The greatest number ofmicrobes (100, 00, 00 per cu.cm) is found in top layer of the soil at a depthof 5-15 cm. In deeper layers (1.5 - 5 m) individual microorganisms are found.The number of microorganisms in the soil depends on the extent and nature oforganic matter soil receives and also on the nature of treating and fertilizingthe soil. For example, ploughed soil contains 2.5 times more microbes than the forestsoil.
Microorganisms play a very important role in supplyingnutrients to the crop plants by improving soil fertility through a number ofprocesses, which are explained here under:
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