Which of the following microorganisms are free-living in the soil and can fix the atmospheric nitrogen thereby enriching the nitrogen content of the soil?
(A) Azotobacter
(B) Rhizobium
(C) Azospirillum
(D) Cyanobacteria
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (C) only
Biofertilisers are organisms that enrich the nutrient quality of the soil. Some bacteria fix atmospheric nitrogen. Rhizobium does it in a symbiotic association, in the root nodules of leguminous plants. Other bacteria fix nitrogen while free-living in the soil. Azospirillum and Azotobacter are the examples.
(A) Azotobacter — Correct. It is a free-living nitrogen fixer in the soil.
(B) Rhizobium — Incorrect. It lives in a symbiotic association with the roots of leguminous plants. It is not free-living.
(C) Azospirillum — Correct. It is also a free-living nitrogen fixer in the soil.
(D) Cyanobacteria — Incorrect. They are a separate group, autotrophic microbes of aquatic and terrestrial environments, such as Anabaena and Nostoc. They are an important biofertiliser in paddy fields. They are not named among the free-living bacteria of the soil.
Note: A similar question was also asked in CUET (UG) 2026 on 07-06-2026 (Slot 2) and on 20-05-2026 (Slot 1).
NCERT Extract:
Microbes as Biofertilisers – …
