Consider the following organisms: 1. Agaricus 2. Nostoc 3. spirogyra Which of the above is/are used as biofertilizer/biofertilizers?

The correct answer is B (2 only). Nostoc is the only organism here used as a biofertilizer.

• Nostoc (2): correct. It is a filamentous, heterocystous cyanobacterium. Its heterocysts carry out biological nitrogen fixation, converting atmospheric N2 into ammonia with the enzyme nitrogenase. Nostoc is grown in rice paddies to add nitrogen to the soil, which is why it is a classic example of a biofertilizer.
• Agaricus (1): wrong. It is a basidiomycete mushroom, a heterotrophic fungus. The button mushroom (Agaricus bisporus) is an edible crop, not a soil inoculant or fertilizer.
• Spirogyra (3): wrong. It is a filamentous green alga found in freshwater ponds. It reproduces by conjugation and has no established role as a nitrogen-fixing inoculant in biofertilizer programmes.

The core concept is biological nitrogen fixation by free-living and symbiotic microbes. Azolla-Anabaena and Rhizobium are other standard examples. For Mains GS-III, this links to sustainable agriculture, reducing chemical fertilizer subsidy burden and soil health.

Trap: Aspirants often pick A because they assume any alga or mushroom is useful for soil nitrogen. Keep nitrogen fixers (cyanobacteria, Rhizobium, Azotobacter) separate from algae and fungi in your notes.