Soil is a complex system and consists of three phases namely solid, liquid and gaseous substances. Soil is important for sustenance of life as it provides the foothold and majority of nutrients needed for growth of plants and animals. Capacity of soil to supply the essential plant nutrients in available form and in a suitable balance for luxuriant and healthy plant growth is called as soil fertility. Soil fertility depends not only on the presence of inorganic substances (mineral matter), organic substances (organic matter), water and air, but also on the presence of microorganisms (Fig. 1).

Yes, soil is not a dead mass. There is life in soil. There are microorganisms present in soil, which greatly influences the qualitative composition of the soil and thus, the soil fertility. Because of the presence of enzymes, carbon dioxide and organic matter, the life in soil is responsible for making up numerous transformations, which changes plant nutrients to readily available forms and make and stabilize desirable soil structure for luxuriant plant growth.

The soil organisms are classified into two groups (Fig. 2) viz. Soil flora and Soil fauna. These are again sub divided into soil macro flora (roots of higher plants) and soil micro flora (bacteria, fungi, actinomycetes and algae); soil macro fauna (moles, ants and earthworms) and soil micro fauna (protozoa and nematodes). The total population of all soil life is numbered in billions per gram of soil and the live weight may be as much as five MT/acre. Of these groups bacteria are most abundant in soil. Next in order are actinomycetes followed by fungi. Algae are found under specific conditions. As per scientific calculation, the biomass of these four important microorganisms 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 of microbes (100, 00, 00 per is found in top layer of the soil at a depth of 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 of organic matter soil receives and also on the nature of treating and fertilizing the soil. For example, ploughed soil contains 2.5 times more microbes than the forest soil.

Microorganisms play a very important role in supplying nutrients to the crop plants by improving soil fertility through a number of processes, which are explained here under:

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About the Authors

The authors are associated with Central Sericultural Research and Training Institute, Mysore.