Which of the following are nitrogen-fixing plants? 1. Alfalfa 2. Amarnath 3. Chickpea 4. Clover 5. Purslane (Kulfa) 6. Spinach Select the correct answer using the code given below:
Contents19
- A1, 3 and 4 only
- B1, 3, 5 and 6 only
- C2, 4, 5 and 6 only
- D1, 2, 4, 5 and 6
Show answer
Answer: (A) 1, 3 and 4 only
The answer is (A) Alfalfa, Chickpea, and Clover only (1, 3, and 4).
Nitrogen-fixing plants are those that can convert atmospheric nitrogen into usable form through bacteria (Rhizobium) in their roots.
Most nitrogen-fixing plants belong to the LEGUME family (Fabaceae).
CORRECT ones (nitrogen-fixers):
Alfalfa (lucerne): A legume. Has Rhizobium bacteria in root nodules that fix nitrogen.
Chickpea: A legume (dal/chana). Fixes nitrogen through root nodules.
Clover: A legume. Fixes nitrogen through symbiotic Rhizobium bacteria.
WRONG ones (NOT nitrogen-fixers):
Amaranth: NOT a legume. It's a grain/leafy vegetable family.
Purslane (Kulfa): NOT a legume. It's a succulent leafy vegetable.
Spinach: NOT a legume. It belongs to the Amaranthaceae family.
Simple rule: Legumes (plants that produce pods/dals) generally fix nitrogen. Non-legumes generally don't.
Nitrogen-fixing plants can convert atmospheric nitrogen into usable compounds through symbiotic bacteria in their root nodules, making them crucial for soil fertility and sustainable agriculture.
Nearly all nitrogen-fixing plants belong to the legume family (Fabaceae) - if it produces pods or is a dal/pulse crop, it likely fixes nitrogen.
Nitrogen-Fixing Plants
Environment nitrogen-fixing plants Alfalfa Chickpea Clover
Nitrogen-Fixing Plants: Legumes vs Non-Legumes
Legumes (Fabaceae family) are the main nitrogen-fixing plants
Alfalfa, Chickpea, Clover are nitrogen-fixers; Amaranth, Purslane, Spinach are not
Nitrogen fixation occurs through Rhizobium bacteria in root nodules
Simple rule: Plants that produce pods/dals generally fix nitrogen
What Are Nitrogen-Fixing Plants
Nitrogen-fixing plants convert atmospheric nitrogen (N₂) into ammonia that plants can use. This happens through symbiotic bacteria in specialized root structures called nodules. Most nitrogen-fixing plants belong to the Fabaceae (legume) family — the same family that gives us dals, beans, and peas.
Plants from the Question
Plant | Family | Nitrogen-Fixing? | Common Use |
|---|---|---|---|
Alfalfa (Lucerne) | Fabaceae (Legume) | Yes | Fodder crop |
Amaranth | Amaranthaceae | No | Grain/leafy vegetable |
Chickpea (Chana) | Fabaceae (Legume) | Yes | Pulse/dal |
Clover | Fabaceae (Legume) | Yes | Fodder/ground cover |
Purslane (Kulfa) | Portulacaceae | No | Leafy vegetable |
Spinach | Amaranthaceae | No | Leafy vegetable |
How Nitrogen Fixation Works
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Atmospheric N₂**
Nitrogen gas from air enters soil`"]
s2["`**Rhizobium Bacteria**
Bacteria in root nodules capture N₂`"]
s3["`**Enzyme Action**
Nitrogenase enzyme converts N₂ to NH₃`"]
s4["`**Plant Uptake**
Plant uses ammonia for protein synthesis`"]
s5["`**Soil Enrichment**
Dead plant parts add nitrogen to soil`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5Question Connection
This UPSC question tests your ability to identify legumes among mixed plant lists. The trap is including common vegetables (amaranth, spinach, purslane) that students might confuse as nitrogen-fixers. The correct answer (A) includes only the three legumes: Alfalfa, Chickpea, and Clover.
Amaranth looks like it could be a legume but belongs to Amaranthaceae family
Spinach and Purslane are leafy vegetables, not legumes — they don't fix nitrogen
Don't assume all green/healthy plants fix nitrogen — only legumes do it systematically
Chickpea = Chana — remember your common dal names are legumes
Options B, C, D include non-legumes — immediate elimination strategy
Legume Family (Fabaceae)
Environment legume Fabaceae
Legume Family: Key Features & Economic Importance
Fabaceae is the legume family with 18,000+ species worldwide
Key feature: Pod fruits that split along two sides when mature
Major source of plant protein and nitrogen fixation in agriculture
Includes all dals, beans, peas, groundnut, and fodder crops
Family Characteristics
Fabaceae (formerly called Leguminosae) is the third-largest plant family. The defining feature is legume fruits — pods that split open to release seeds. Most species form root nodules with Rhizobium bacteria for nitrogen fixation, making them valuable for soil fertility and sustainable agriculture.
Major Legume Categories
Category | Examples | Primary Use | Nitrogen Fixation |
|---|---|---|---|
Pulses (Dals) | Chickpea, Lentil, Black gram, Green gram | Human protein | Strong |
Beans | Kidney bean, Soybean, Lima bean | Human food | Strong |
Fodder Crops | Alfalfa, Clover, Berseem | Animal feed | Strong |
Oilseeds | Groundnut, Soybean | Oil extraction | Moderate |
Trees | Acacia, Cassia, Tamarind | Timber, gum | Variable |
Economic Importance
# Legume Benefits
## Nutrition
- High protein content
- Essential amino acids
- Low fat
- Rich in fiber
## Agriculture
- Nitrogen fixation
- Soil improvement
- Crop rotation
- Reduced fertilizer need
## Economy
- Food security
- Export crops
- Animal feed industry
- Rural employment
## Environment
- Carbon sequestration
- Biodiversity support
- Sustainable farming
- Reduced chemical inputsGroundnut is a legume (not a true nut) — fixes nitrogen unlike tree nuts
Soybean is both oilseed and legume — don't classify as cereal
Not all pod-bearing plants are legumes (e.g., vanilla, drumstick have pods but different families)
Tamarind tree is a legume but doesn't fix nitrogen significantly
Green gram = Moong, Black gram = Urad — know your dal names
Rhizobium-Legume Symbiosis
Environment Rhizobium
Rhizobium-Legume Symbiosis: Mechanism & Significance
Rhizobium bacteria live in legume root nodules and fix atmospheric nitrogen
Nitrogenase enzyme converts N₂ gas into ammonia (NH₃)
Mutualistic relationship: bacteria get carbohydrates, plants get nitrogen
Fixes approximately 200-300 kg nitrogen per hectare annually
The Symbiotic Process
Rhizobium bacteria form a mutualistic partnership with legume plants. The bacteria colonize plant roots, forming visible nodules where they convert atmospheric nitrogen into ammonia. In return, the plant provides carbohydrates from photosynthesis to feed the bacteria.
Nodule Formation Steps
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Root Hair Infection**
Rhizobium bacteria enter through root hairs`"]
s2["`**Infection Thread**
Bacteria travel through infection thread into root`"]
s3["`**Cell Division**
Root cells divide rapidly forming nodule tissue`"]
s4["`**Bacteroid Formation**
Bacteria transform into nitrogen-fixing bacteroids`"]
s5["`**Nitrogenase Activity**
Enzyme converts N₂ to NH₃ under oxygen-free conditions`"]
s6["`**Nitrogen Transfer**
Plant receives fixed nitrogen as amino acids`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5
s5 --> s6Types of Nitrogen Fixation
Type | Organisms | Location | Efficiency | Examples |
|---|---|---|---|---|
Symbiotic | Rhizobium + Legumes | Root nodules | Very High | Chickpea, Alfalfa, Clover |
Free-living | Azotobacter, Clostridium | Soil | Low | Independent soil bacteria |
Associative | Azospirillum | Root surface | Moderate | Grasses, cereals |
Industrial | Chemical process | Factories | High cost | Haber-Bosch process |
Root Nodule Structure
Source: ScienceDirect.com — Stem Nodules - an overview | ScienceDirect Topics · www.sciencedirect.com
Nitrogenase only works in oxygen-free conditions — nodules create this environment
Pink/red nodules indicate active fixation; white/green nodules are inactive
Rhizobium species are specific to certain legumes — not universal
Free-living nitrogen fixers exist but are much less efficient than symbiotic ones
Industrial nitrogen fixation consumes massive energy unlike biological process
Non-Nitrogen-Fixing Plants
Environment Amaranth Purslane Spinach
Non-Nitrogen-Fixing Plants: Families & Fertilizer Needs
Most plant families cannot fix atmospheric nitrogen
Amaranth, Spinach, Purslane belong to non-legume families
These plants depend on soil nitrogen or external fertilizers
Include major food crops like cereals, vegetables, fruits
Why Most Plants Don't Fix Nitrogen
Nitrogen fixation is energetically expensive and requires specialized bacteria and enzymes. Only certain plant families (mainly legumes) evolved this capability. Most plants — including cereals, vegetables, and fruits — obtain nitrogen from soil or fertilizers.
Non-Fixing Plants from Question
Plant | Family | Type | Nitrogen Source | UPSC Relevance |
|---|---|---|---|---|
Amaranth | Amaranthaceae | Grain/leafy | Soil + fertilizers | Drought-resistant crop |
Spinach | Amaranthaceae | Leafy vegetable | Soil + fertilizers | Iron-rich nutrition |
Purslane (Kulfa) | Portulacaceae | Succulent vegetable | Soil + fertilizers | Omega-3 rich wild edible |
Major Non-Nitrogen-Fixing Families
Plant Family | Common Examples | Economic Importance | Nitrogen Requirement |
|---|---|---|---|
Poaceae (Grasses) | Rice, Wheat, Maize, Sugarcane | Staple cereals | Very High |
Solanaceae (Nightshades) | Potato, Tomato, Brinjal, Chili | Vegetables | High |
Brassicaceae (Mustards) | Cabbage, Cauliflower, Mustard | Vegetables, oilseeds | Moderate |
Amaranthaceae | Spinach, Amaranth, Beetroot | Leafy vegetables | Moderate |
Rosaceae (Rose family) | Apple, Peach, Strawberry | Fruits | Moderate |
Agricultural Implications
Non-nitrogen-fixing crops require external nitrogen sources through:
Chemical fertilizers (urea, ammonium sulfate)
Organic manures (compost, farmyard manure)
Crop rotation with legumes
Green manuring using nitrogen-fixing cover crops
This makes them more input-intensive and environmentally costly compared to legumes.
Amaranth sounds exotic but it's just a common leafy vegetable/grain — not a legume
Spinach is iron-rich but cannot fix nitrogen despite being 'healthy'
Purslane (Kulfa) is omega-3 rich wild edible but belongs to Portulacaceae, not legumes
Don't confuse nutritional value with nitrogen-fixing ability
Cereals (rice, wheat, maize) are major crops but all require external nitrogen