Bacteria which can fix atmospheric nitrogen while free-living in the soil are:
(A) Rhizobium
(B) Azospirillum
(C) Azotobacter
(D) Clostridium
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (C) only
Biofertilisers are organisms that make the soil richer in nutrients. Some bacteria fix nitrogen only when they live together with a plant (symbiotic). Others fix it while living on their own in the soil (free-living).
(A) Rhizobium — Incorrect. Rhizobium fixes nitrogen in the root nodules of leguminous plants. It is a symbiotic nitrogen fixer, not a free-living one.
(B) Azospirillum — Correct. It fixes nitrogen while free-living in the soil.
(C) Azotobacter — Correct. It is also a free-living nitrogen-fixing bacterium of the soil.
(D) Clostridium — Incorrect. Clostridium is not among the free-living nitrogen fixers named in NCERT. NCERT mentions Clostridium butylicum as the bacterium used to produce butyric acid.
Note: A similar question was also asked in CUET (UG) 2026 on 21-05-2026 (Slot 1) and on 20-05-2026 (Slot 1).
NCERT Extract:
Microbes as Biofertilisers – …
