UPSC Prelims 2022 Q48: Nitrogen-Fixing Plants and Legumes — Explained
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:
- 1, 3 and 4 only
- 1, 3, 5 and 6 only
- 2, 4, 5 and 6 only
- 1, 2, 4, 5 and 6
Answer: (a) 1, 3 and 4 only
Statement-wise explanation
- 1: correct — Alfalfa (lucerne, Medicago sativa) belongs to the legume family, Fabaceae. Its roots carry nodules that house Rhizobium-group bacteria (Sinorhizobium meliloti). These bacteria turn nitrogen gas from the air into ammonia that the plant can use. This is why alfalfa is grown as a high-protein fodder and as a crop that adds nitrogen to soil.
- 2: incorrect — Amaranth (chaulai, rajgira) belongs to the family Amaranthaceae, not the legumes. It forms no root nodules and has no partnership with nitrogen-fixing bacteria. It is a leafy vegetable and a pseudo-cereal that takes its nitrogen from the soil.
- 3: correct — Chickpea (gram, chana, Cicer arietinum) is a pulse crop. Pulses are the dry seeds of legumes. Chickpea roots form nodules with Mesorhizobium bacteria, which fix nitrogen from the air. This is the main reason gram is a classic rotation crop on rabi fields.
- 4: correct — Clover (Trifolium, and berseem, Trifolium alexandrinum, in India) is a legume with root nodules that fix nitrogen through Rhizobium bacteria. Berseem is a major winter fodder in north India. It is valued because it adds nitrogen to the soil before the next crop.
- 5: incorrect — Purslane (kulfa, Portulaca oleracea) belongs to the family Portulacaceae. It is a succulent leafy plant, best known for using the C4 and CAM pathways to survive drought. It is not a legume and does not fix nitrogen.
- 6: incorrect — Spinach (palak, Spinacia oleracea) belongs to the family Amaranthaceae, the same family as amaranth and beet. It has no root nodules. As a fast-growing leafy crop, it actually needs a lot of soil nitrogen.
How to eliminate
Use one test: is the plant a legume, meaning a member of the pea and bean family that bears pods? Chickpea is gram, a pulse, so statement 3 must be in the answer. That rules out the options that leave out chickpea. Spinach is an ordinary leafy vegetable that people know needs heavy manuring, so you can safely drop it. That removes every option containing 6 and leaves only the one with 1, 3 and 4. If you know that alfalfa (lucerne) and clover (berseem) are fodder legumes, you can confirm that choice directly. Note the trap: amaranth, purslane and spinach are all leafy greens. UPSC grouped them together so that a candidate who guesses 'leafy and nutritious' falls for them. Fact to remember: in crops, nitrogen fixation through root nodules is almost entirely a legume (Fabaceae) trait.
Know more
Biological nitrogen fixation (BNF) converts nitrogen gas (N₂), which makes up about 78% of air, into ammonia (NH₃). Plants cannot use N₂ directly. The conversion is done by an enzyme called nitrogenase, which only some bacteria and archaea have. No plant can fix nitrogen on its own. A 'nitrogen-fixing plant' is a plant that houses such microbes.
How legumes do it
- Rhizobia enter the root hairs, and the plant forms root nodules around them.
- Inside the nodules, leghaemoglobin (which makes active nodules look pink) keeps oxygen low, because oxygen damages nitrogenase.
- The plant supplies sugars. The bacteria supply fixed nitrogen.
- After harvest, the roots and leftover plant material add nitrogen to the soil. This is why pulse–cereal rotation and intercropping cut the need for urea.
| Partnership | Microbe | Example hosts |
|---|---|---|
| Legume root nodules | Rhizobium, Bradyrhizobium, Mesorhizobium | Gram, tur, soybean, groundnut, alfalfa, clover, berseem |
| Actinorhizal nodules | Frankia (actinomycete) | Alnus (alder), Casuarina |
| Leaf cavities of a water fern | Anabaena (cyanobacterium) | Azolla, used as a biofertiliser in paddy |
| Free-living in soil or water | Azotobacter, Clostridium, blue-green algae | No host |
| Loose association with roots | Azospirillum | Cereals, grasses |
Common traps
- Groundnut and soybean are legumes too, even though they are grown as oilseeds.
- Many leafy greens, such as spinach, amaranth, purslane and fenugreek leaves, look similar. Only fenugreek (methi) is a legume.
- Industrial fixation (the Haber–Bosch process, used to make urea) is a chemical process, not a biological one.
NCERT Concepts to Revise
- Nitrogen fixation: The whole question turns on which of the six plants can host the process that turns nitrogen from the air into usable compounds.
- Pulses: Knowing that pulses are legume seeds helps you sort the listed plants into legumes and non-legumes.