Identify the atmospheric nitrogen fixing, free living bacterium in the soil.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Azospirillum
A biofertiliser is a living organism that adds nutrients to the soil. Some bacteria fix atmospheric nitrogen while living free in the soil. Others fix it only in a partnership with a plant.
(1) Rhizobium — Incorrect. It fixes nitrogen only symbiotically, inside the root nodules of leguminous plants.
(2) Anabaena — Incorrect. It is a nitrogen-fixing cyanobacterium (a photosynthetic microbe). It belongs to a different group from the free-living soil bacteria the question asks for.
(3) Azospirillum — Correct. It is a free-living soil bacterium that fixes atmospheric nitrogen and so enriches the nitrogen of the soil. Azotobacter is another example.
(4) Glomus — Incorrect. It is a fungus that forms mycorrhiza. It absorbs phosphorus from the soil for the plant and does not fix nitrogen.
Note: A similar question was also asked in CUET (UG) 2025 on 29-05-2025 (Slot 1) and on 22-05-2025 (Slot 2), and in CUET (UG) 2023 on 30-05-2023 (Slot 1).
NCERT Extract:
Microbes as Biofertilisers – …
