UPSC Prelims 2022 Q48: Environment & Ecology – Sustainable Agriculture & Forestry
Which of the following are nitrogen - fixing plants?
1. Alfalfa
2. Amaranth
3. Chickpea
4. Clover
5. Purslane (Kulfa)
6. Spinach
Select the correct answer using the code given below:
- (a) 1, 3 and 4 only
- (b) 1, 3, 5 and 6 only
- (c) 2, 4, 5 and 6 only
- (d) 1, 2, 4, 5 and 6
Answer: (a) 1, 3 and 4 only
Explanation
Which plants fix nitrogen?
Nitrogen fixation happens mainly in legumes (family Fabaceae).
Their root nodules host Rhizobium bacteria that turn atmospheric N₂ into ammonia.
| Plant | Family | Nitrogen-fixing? |
|---|---|---|
| Alfalfa | Fabaceae (legume) | Yes |
| Amaranth | Amaranthaceae | No |
| Chickpea | Fabaceae (legume) | Yes |
| Clover | Fabaceae (legume) | Yes |
| Purslane (Kulfa) | Portulacaceae | No |
| Spinach | Amaranthaceae | No |
Only the three legumes fix nitrogen.
Answer: (a) 1, 3 and 4 only.
Why each option is right or wrong
- (a) CORRECT. Alfalfa, chickpea and clover are legumes with Rhizobium root nodules.
- (b) Incorrect. Purslane and spinach are leafy plants without nitrogen-fixing nodules.
- (c) Incorrect. Amaranth, purslane and spinach do not fix nitrogen; only clover here does.
- (d) Incorrect. It leaves out chickpea, a legume, and adds three non-legumes.
Key point
Legumes (alfalfa, chickpea, clover) fix nitrogen through Rhizobium root nodules; leafy greens do not.
Remember
Nitrogen fixers = legumes: alfalfa, chickpea, clover.
Common mistakes
- Assuming leafy greens like spinach fix nitrogen because they are nitrogen-rich.
- Forgetting that alfalfa (lucerne) is a legume fodder crop.
- Treating amaranth as a pulse because its seeds are eaten like grain.
Exam tip
Ask: is it a legume? Pulses, beans, peas, clover and alfalfa fix nitrogen; most others do not.
Sources
- NCERT Class XI — Biology, Ch 12 (Mineral Nutrition: Nitrogen Fixation)
- ICAR — Pulses and Legume Fodder Crops