Biofertilisersare organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. Rhizobium bacteria is found in symbiotic assosiation the root nodules of leguminousplants. These bacteria fix atmospheric nitrogen into organic forms, which is used bythe plant as nutrient.Azotobacter and Azospirillum also fix atmospheric nitrogen, but these are free-livingbacteria in the soil.
Mycorrhiza is a fungus that forms symbiotic association with plants. It absorbsphosphorus from soil and passes it to the plant. Cyanobacteria are autotrophic microbes that are widely distributed in aquatic andterrestrial environments and have the ability to fix atmospheric nitrogen.Blue green algae, not only add nutrients but also the organic matter and thusincrease fertility of the soil.