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 cu.cm) 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
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:
About the Authors
The authors are associated with Central Sericultural Research and
Training Institute, Mysore.