Biofertilizers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilizers are bacteria, fungi and cyanobacteria. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as nutrients. The bacteria involved in fixing atmospheric nitrogen are Rhizobium, Frankia, Beijerinkia, Rhodospirullum, Rhodopseudomonas, Azospirillum, Clostridium, Azotobacter, Anabaena, Nostoc etc. They enrich the nitrogen content of the soil. Fungi are also known to form symbiotic associations with plants (mycorrhiza). Many members of the genus Glomus form mycorrhiza. The fungal symbiont in these associations absorbs phosphorus from soil and passes it to the plant. Plants having such associations show other benefits also, such as resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development.