Many free-living and symbiotic bacteria are discussed in Biological Nitrogen Fixation which fixes atmospheric N2. Therefore, certain measures are adopted to increases number of such bacteria in soil which may increase the gross yield of nitrogen. The two methods, bacterization and green-manuring are the most widely used techniques.
Bacterization is a technique of seed-dressing with bacteria (as water suspension) for example, Azotobacter, Bacillus, Rhizobium etc. It has been proved that bacteria can successfully be established in root region of plants which in turn improve the growth of hosts. Bacterial fertilizers named 'azotobakterin' (containing cells of Azotobacter chroococcum) and ‘phosphobacterin' (containing cells of Baccilus megatehum var. phosphaticum) have been used in erstwhile U.S.S.R. and East European countries, respectively. These increased the crop yield about 10-20 per cent (Cooper, 1959). Subsequently bacterization of seeds in Russia, Czechoslovakia, Rumania, Poland, Bulgaria, Hungary, England and India has clearly demonstrated the increase in crop .yield such as wheat, barley, maize, sugarbeet, carrot, cabbage and potato. In rhizosphere, bacteria secrete growth substances and antibiotic secondary metabolites which contribute to seed germination and plant growth (Subba Rao, 1982; Dwivedi et al, 1989).
Moreover, informations gathered on associate symbiosis (i.e.
symbiosis between roots of grasses and Azospirillum)
has increased the interest on this bacterium to be used as seed inoculant for cereals (Dewan and Subba Rao, 1979). In recent years, free-living bacteria (Azotobacter),
and symbiotic (Rhizobium)
bacteria, and phosphate solubilizing ones (e.g. Bacillus megaterium, B. polymyxa,
and Pseudomonas striata)
are gaining much popularity. Such practices are being encouraged to save the chemical fertilizers, national economy and the environment.