Identify the free living nitrogen fixing soil bacteria.

The correct answer is Option (3) → Azospirillum

Option (1) Nostoc — Incorrect. Nostoc does fix nitrogen, but it is a cyanobacterium (blue-green alga), an autotrophic microbe. It is used as a biofertiliser mainly in paddy fields, and it is not one of the free-living soil bacteria named for this job.

Option (2) Rhizobium — Incorrect. Rhizobium fixes nitrogen, but it is symbiotic, not free-living. It lives in the root nodules of leguminous plants.

Option (3) Azospirillum — Correct. Azospirillum, like Azotobacter, is a bacterium that fixes atmospheric nitrogen while living freely in the soil. It needs no host plant to do so.

Option (4) Oscillatoria — Incorrect. Oscillatoria is another cyanobacterium, like Nostoc and Anabaena. It is a blue-green alga, not one of the free-living soil bacteria.

Note: A similar question was also asked in CUET (UG) 2026 on 30-05-2026 (Slot 2), in CUET (UG) 2025 on 26-05-2025 (Slot 2) and on 22-05-2025 (Slot 2), and in CUET (UG) 2023 on 30-05-2023 (Slot 1).

NCERT Extract:
Microbes as Biofertilisers – …