Soil contains different kinds of micro-organisms. These living organisms feed on plant residues. The soil micro-organisms are of two types:
a. Microflora: Bacteria, actinomycetes, fungi and algae.
b. Microfauna: Protozoa and nematodes.
a. Microflora: Bacteria, actinomycetes, fungi and algae which come under this category vary greatly regarding their numbers and sizes and also from place to place and season to season. The density of population of these microflora varies due to food supply, moisture, temperature, aeration and reaction of the soil.
Bacteria:There are of two types viz., autotrophic and heterotrophic. The later decompose the organic matter and release plant food nutrients like nitrogen, phosphorus, etc. The autotrophic bacteria comprises nitrosomonas and nitrobacteria which oxidise ammonia to nitrate and nitrate to nitrate compounds respectively.
ii. Fungi: These organisms produce microscopic threads called mycelia and are found in the organic matter of plant roots. Some of them are saprophytic which obtain their energy from decomposing organic matter, while others are parasitic thriving on living plant roots. Fungi, which do not cause diseases, are useful to soil because they help in breaking down the organic matter.
iii.Actinomycetes: They are similar in size to bacteria but different in so far as they can grow in deeper layers even under dry conditions of soil and require less nitrogen than bacteria. They are in plenty in soils rich in organic matter and with neutral to slightly acidic reaction. They decompose the resistant parts of organic matter like cellulose.
iv.Algae: They are microscopic or very minute-sized plants having chlorophyll. They are usually found on the surface of wet soils of paddy fields. They help in adding organic matter to soil, improving the soil aeration.
b.Microfauna: There are two groups viz. Protozoa and nematodes.
i. Protozoa:These are unicellular animals, which feed on the organic matter and on the bacterial population in the soil.
ii. Nematodes:These are microscopic worms. Parasitic to plant roots are agriculturally important. A large number of them also feed on soil microflora and also on protozoa.