Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. They play a vital role in organic farming by reducing dependence on chemical fertilisers.

Bacteria act as biofertilisers in two ways. First, Rhizobium forms a symbiotic association with the roots of leguminous plants, forming nodules. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as a nutrient. Second, some bacteria like Azospirillum and Azotobacter can fix atmospheric nitrogen while living freely in the soil, thereby enriching the nitrogen content of the soil.

Cyanobacteria (also called blue green algae) are autotrophic microbes found in aquatic and terrestrial environments. Examples include Anabaena, Nostoc and Oscillatoria. Many of these can fix atmospheric nitrogen. In paddy fields, cyanobacteria serve as an important biofertiliser.

Apart from nitrogen fixation, blue green algae also add organic matter to the soil, which increases its fertility. This dual role makes them particularly valuable for soil enrichment.

Currently, biofertilisers are available commercially and farmers use them regularly to replenish soil nutrients and reduce the overuse of chemical fertilisers, which cause environmental pollution.