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3.1.1 Decomposition
Decomposition is the physical breakdown and biochemical transformation of com-
plex organic molecules of dead material into simpler organic and inorganic mole-
cules. Bacteria and fungi, which comprise >90 % of the soil microbial biomass, are
responsible for most of the decomposition. Warm and moist conditions hasten the
process. Humic acids, produced by many microorganisms during decomposition,
form large molecules of stabilized organic matter in the soil. The stable humus is a
key component for soil aggregate formation and provides increased surface area for
nutrient adsorption thus increasing nutrient availability for plant uptake.
3.1.2 Mineralization
During mineralization, soil microbial activity converts organic matter into inorganic
compounds which can be used by plants.
3.1.3 Immobilization
Immobilization is the reverse of mineralization, i.e. the conversion of inorganic
compounds into organic compounds. Soil microorganisms (mainly bacteria and
fungi) consume inorganic molecules and incorporate them into their biomass, which
then become unavailable to plants. However, the soil organisms contribute signifi-
cantly to the formation and maintenance of soil structure by producing exopolysac-
charides, beneficial organic acids and enzymes. After the death and decay of
organisms, the immobilized nutrients are mineralized and become available to
plants.
3.1.4 Mineral Transformation
Mineral transformation converts one form of inorganic matter into another form.
Nitrogen cycling involves mineral transformation by many soil microorganisms.
Nitrogen-fixing bacteria (symbiotic and non-symbiotic) present in the nodules/soil
convert large amounts of inert atmospheric nitrogen (N 2 ) to ammonia (NH 3 ) with
the help of the enzyme nitrogenase. Ammonia is converted into nitrate (NO 3 −) by
nitrifying bacteria. Both ammonia and nitrate are plant usable forms. However,
nitrates can be quickly leached from the soil. Further, denitrifying bacteria convert
nitrate into gases (N 2 O) that are lost into the atmosphere. Several other soil micro-
organisms are involved in the mineral transformation of phosphorus, potassium,
sulfur, iron, manganese and zinc. Photosynthetic bacteria and algae present in the
top layer of soil can use the energy of sunlight and soil heat to convert organic mat-
ter and harmful gases into useful substances such as amino acids, nucleic acids,
sugars and other metabolites.
M. Grover et al.