Distribution of different types of soil microorganisms
Soils contain five major groups of microorganisms. They are bacteria, actinomycetes, fungi, algae and protozoa. Among the soil microorganisms, bacteria are most dominant group of organisms. All kinds of bacteria are found in the soil. This is because all kinds of organic refuse are disposed off on the soil. Many of the soil bacteria perform useful functions like decomposition of organic matter, conver-sion of soil constituents into useful materials, production of antibiotics in the soil, and biogeochemical cycling of elements like carbon, nitro-gen, phosphorus, iron, sulfur and manganese.
The bacterial population of the soil exceeds the population of all other groups of microorganisms in both number and variety. Direct microscopic counts as high as several billions bacteria per a gram of soil have been reported.
The actinomycetes population as many as millions per gram of soil is present. The most predominant genera present in the soil are Nocardia, Streptomyces and Micromonospora. These organisms areresponsible for the characteristic musty or earthy odour soon after the rainfall. This is due to sporulation of actinomycetes. Actinomycetes are capable of degrading many complex organic substances and con-sequently play an important role in building soil fertility. The actino-mycetes have ability to synthesize and excrete antibiotics. Most of the antibiotics are produced by actinomycetes. The presence of antibiotic substances in soil can be detected with great difficulty.
The fungal population ranging from thousands to hundred thou-sands per gram of soil has been reported. They are aerobic in nature and found more numbers near the earth surface. They exist in the atmosphere as mycelial and spore stage. Fungi are active in decom-posing the major constituents of plant tissues, namely, cellulose, hemi-cellulose, lignin and pectin.
The population of algae in soil is very smaller than that of either bacteria or fungi. The major types present in the soil are the greenalgae and diatoms. Their photosynthetic nature accounts for their pre-dominance on the surface or just below the surface layer of soil. In a fertile soil biochemical activities of algae are masked by bacteria and fungi. In certain conditions, algae perform prominent and beneficial changes. For example, on barren and eroded lands they may initiate the accumulation of organic matter because of their ability to carry out photosynthesis and other metabolic activities.
Many soil protozoa are flagellates or amoebas; the population per gram soil ranges from a few hundred to several thousand in moist soils rich in organic matter. Protozoa are of significance since their dominant mode of nutrition involves ingestion of bacteria.
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