- A Plant roots alone through active mineral uptake
- B Rhizobium and other nitrogen-fixing bacteria
- C Mycorrhizal fungi that colonize root cortex
- D Earthworms that aerate and till the soil
Correct answer: B. Rhizobium and other nitrogen-fixing bacteria
Explanation: Nitrogen fixation: N2 gas → NH3 (ammonia) by nitrogen-fixing bacteria like Rhizobium (in legume root nodules) and free-living Azotobacter.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.