Biofertilizers are micro-organisms which enhance the availability of nutrients like nitrogens and phosphorus to crop plants. Important biofertilizers are:
(i) Free-living N2-fixing Bacteria: They are bacteria of diverse nutritional status which live freely in soil. e.g. Azotobacter.
(ii) Symbiotic N2-fixing Bacteria: e.g. Rhizobium.
(iii) Free-living N2-fixing Cyanobacteria: A number of cyanobacteria (BGA) are capable to N2-fixation, e.g. Nostoc and Anabaena.
(iv) Symbiotic N2-fixing Cyanobacteria: e.g. Azolla oinnata
