How do biofertilizers enrich fertility of soil?

Biofertilizers are micro-organims which bring about nutrient enrichment of soil by enhancing their availability
(i) Manures- They are semi-decayed organic remains of various types -  farmyard manure, green manure, compost and vermicompost. As the manure decay further takes place they release minerals locked in them.
(ii) Nitrogen Fixing Organisms- They are certain bacteria and cyanobacteria which are capable of converting gaseous nitrogen into salts of nitrogen.
(iii) Mycorrhiza- It is an association between a fungus and roots of plants. Mycorrhiza is able to pick up inorganic nutrients from organic matter.