Free living bacteria and cyanobacteria prefer a variety of habitats with varying degree of nutrients, pH, oxygen, etc. Photosynthetic nitrogen fixing bacteria are divided into : (a) non sulfur purple, (b) purple sulfur and (c) green bacteria. Since water is a major component of cytoplasm, adequate amount of water must be required for their vegetative growth. The amount of water governs the concentration of oxygen ; therefore, oversupply of water limits gas exchange, lowers the available supply, and finally, creates anaerobic condition. Due to water-logging conditions, number of anaerobic bacteria increases and that of aerobic ones decreases in soil (Alexander, 1977).

Environmental factors which influence number, community size, vegetative growth and activity of microorganisms are temperature, organic matter, pH, inorganic fertilizers, light, oxygen, season, soil and depth. In water-logging fields (anaerobic condition) such as flooded soils, lakes, ponds, rice fields, etc. non-sulfur purple bacteria grow luxuriantly. Some times their numbers increase from 10⁴ to 10⁵ cells/g soil (Kabayashi et al, 1967).

The azotobacters are the most intensively investigated heterotrophic group. They are the aerobic bacteria possessing highest respiratory rates. Members of these genera are mesophilic, which require optimum temperature of about 30°C for their growth. Density of Azotobacters ranges from 10³ to 10⁶ per gram soil. Other dominant N₂ fixing aerobic bacteria present in soil are Beijerinckia and Derxia. Beijerinckia grows luxuriantly in acid soil in tropical region.

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