Which of the following is an example of a biofertiliser?

The correct answer is Option (3) → Cyanobacteria

Biofertilisers are living organisms that enrich the nutrient quality of the soil. The main groups are bacteria, fungi and cyanobacteria. They are used in place of, or along with, chemical fertilisers in organic farming.

Option (1) Common salt — Incorrect. Common salt is a chemical, not a living organism. Too much salt in the soil harms plant growth.

Option (2) Synthetic fertiliser — Incorrect. A synthetic fertiliser is a chemical fertiliser made in factories. Biofertilisers are used to reduce our dependence on such chemicals.

Option (3) Cyanobacteria — Correct. Many cyanobacteria (blue-green algae), such as Anabaena and Nostoc, fix atmospheric nitrogen. In paddy fields they serve as an important biofertiliser and also add organic matter to the soil.

Option (4) Insecticide — Incorrect. An insecticide is a chemical that kills insects. It does not add nutrients to the soil.

NCERT Extract:
Microbes as Biofertilisers – “… Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteria serve as an important biofertiliser. Blue green algae also add organic matter to the soil and increase its fertility.”