Soil supports a wide array of organisms of different body-sizes and taxonomic groups. Generally, soil organisms are classified into three major groups namely micro-fauna and micro-flora, meso-fauna or meio-fauna, and macro-fauna. Mesoflora and macroflora because occur above the surface of soil (land- surface):
It includes animals with body size within the range of 20p to 200p. It includes all Protozoa and small-sized mites, nematodes, rotifers, tradigrades and copepode Crustacea. Soil inhabiting protozoans like amoeba, ciliates, zoomastigine flagellates occur near the surface soils, while the testate forms like Thecamoelni, Euglyplia and Diffugia, have a wider vertical distribution. The common terrestrial polyclad is Bipaliuin. The nematodes such as Rhabditis, Diplogaster, Tylenclius, Heterodera, Aphelenchoides, Mononchus, Pratylenchus, Xipliineina and Criconemoides abound by as much as 1-3 million in raw humus soils to 20 million/m- in grassland soils.
The micro flora of soil includes bacteria, soil fungi, soil actionomycetes, blue green algae and algae. In soil, micro flora bacteria from about 90 percent of the total population fungi and algae together represent one percent and actionomycetes cover only 9 percent.
Soil bacteria grow fairly well in the neutral soils richly supplied with organic nutrients. Soil inhabitant bacteria fall into two categories namely-autotrophic bacteria and heterotrophic bacteria. The autotrophic bacteria derive their energy from the oxidation of simple carbon compounds or from inorganic substances and their carbon from the atmospheric.
The common autotrophic bacteria of soil are nitrifying bacteria, hydrogen bacteria, sulphur bacteria, iron bacteria, manganese bacteria, carbon monoxide bacteria and methane bacteria,. Most of soil bacteria are heterotrophic bacteria depending upon the organic matter of soil for their energy source and are primarily concerned with the decomposition of cellulose, and other carbohydrates, proteins, fats and waxes.
They bring about mineralization of organic matter of soil and release considerable amount of nitrogen, phosphorus and other nutrients for plants. The common nitrogen-fixing bacteria of soil are Rhizobium (occurs in root nodules of leguminous plants); Azobacter and Clostridium pasteurianum (the latter two are free occurring in soil).
Majority of soil fungi are found in acidic soils. They may be parasitic, saprophytic and symbiotic. Parasitic fungi of soil infect roots of plants and cause plant diseases such as cotton root rot and many kinds of wilts, rusts, blights and smuts. Certain wilt-forming fungi produce toxins which are harmful, for example, Fusarium lini, which causes wilt of flax (Alsi) and secrets HCN and Fusarium udum, a fungus causing wilt of pigeon pea (Arhar) secretes fusaric acid in the roots of host plants.