Aeration : The air is essential for the growth of the aerobic organisms.
The water logging condition brings about a decrease in the abundance
of aerobic organisms. The change from an aerobic to a largely anaerobic
flora is effected by the disappearance of free oxygen as a result of
its utilization by oxygen-requiring microorganisms, so that only microorganisms
tolerant of low oxygen levels complete anaerobiosis are capable
of proliferation.
Temperature : Temperature governs all biological processes and it is
thus prime factor of concern to the microorganisms. Each microorganism
has an optimum temperature for growth. Most microorganisms
are mesophilic that can able to grow between 25-35oC. Certain species
develop best at temperature below 20oC and they are termed as
psychrophiles. Thermophilic microorganisms that grow readily at temperatures
of 45oC to 65oC.
pH : The neutral pH is favourable for many types of microorganisms.
Highly acidic or alkaline conditions tend to inhibit many common microbes.
The greater hydrogen ion concentration, the smaller is the size
of the microbial community. Soil-borne fungi are sensitive to high pH.
Organic and inorganic nutrients : These organic and inorganic
nutrients are very important for microorganisms a s these provide
nutrition for growth, activity and survival of microorganisms in soil. The
chemical factors are gases, acids, micro and macro elements and clay
minerals etc. In the soilsolution, gases and microorganisms are
dissolved. However, the dissolved components are in constantly
shifting equilibrium with the solid phase. The dead organic materials of
plant and animal origin serve as total organic matter, which later is
subjected to microbial colonization and decomposition. However, due
to incorporation of green manures , crop residues etc., in soil, the
community size of microorganisms gets increased. At the same time
application of these organic matter alters the composition of soil
microflora, microfauna and relative dominance of antagonistic microorganisms.
The types of vegetation and its growth stages of plant dominate
one or more groups of soil microorganisms. Increased population
of microorganisms can be found in the rhizosphere region according to
season, growth stages and abundant availability of nutrients.