Which of the following biofertilisers are free living in the soil?

(A) Anabaena

(B) Azospirillum

(C) Rhizobium

(D) Azotobacter

Choose the correct answer from the options given below:

(B) and (D) only

The correct answer is Option (4) → (B) and (D) only

Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. The nodules on the roots of leguminous plants formed by the symbiotic association of Rhizobium. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as nutrient. Other bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.

Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc. For example, Azolla, a small aquatic fern, hosts the cyanobacterium Anabaena in its leaf cavities. This association enables Azolla to fix nitrogen, benefiting the growth of rice plants when grown together in paddy fields.