How do biofertilisers enrich the fertility of the soil?

Explanation

(i) Rhizobium leguminoserum (found as endosymbiont in the root nodules of leguminous plants) and Azospirillum (found in loose association with the roots of grasses, rice, sorghum, maize etc.) fix atmospheric nitrogen as nitrites and nitrates.
(ii) Anabaena azollae (a cyanobacterium) and many other species of blue-green algae (e.g., Nostoc, Aulosira, etc.) are important `N_2` - fixers of rice fields. Some of these are found in symbiotic association with certain plants while some are free living in water bodies. Such N -fixers decrease the nitrogen requirement so reduce the use of chemical fertilizers.
(iii) Certain fungi (e.g. Glomus) are found as symbionts with the roots of the higher plants e.g., oaks, Fugus, Eucalyptus, etc. and form mycorrhizae. The fungal hyphae increase the surface area for absorption of water and minerals (Nitrogen, Phosphorus and
Potassium) by the roots of higher plants. These also solubilise some insoluble organic compounds to make them available to the higher plants. These help in conversion of marginal soil into productive soil. These increase the drought tolerance and root pathogen resistance capacity of the plants.