How to prepare for Environment & Ecology for UPSC?

[THE VERDICT]: Sitter. Direct hit from NCERT Class X Geography (Ch 4) and Shankar IAS (Ch 2). If you missed this, your basic reading of 'Leguminous Crops' is weak. [THE CONCEPTUAL TRIGGER]: Bio-geochemical Cycles (Nitrogen Cycle) and Sustainable Agriculture (Biofertilizers). [THE HORIZONTAL EXPANSION]: Memorize the 'Fixer Squad': Legumes (Peas, Beans, Soy, Alfalfa, Clover, Gliricidia), Non-Legumes (Casuarina, Alder), and Aquatic (Azolla-Anabaena). Contrast with 'Nitrogen Hogs' (Spinach, Maize, Cotton). [THE STRATEGIC METACOGNITION]: When reading about the Nitrogen Cycle, stop treating 'Legumes' as a generic word. Unpack the category: What are the specific examples mentioned in NCERT? (Gram, Tur, Urad, Moong, Peas). Add fodder crops (Alfalfa, Clover) from Ecology texts.

Current set IAS · 2022 Q58

Pick exam & year, then Go.

Exam

Year

Not attempted Correct Incorrect ★ Bookmarked

Loading…

Q58 (IAS/2022) Environment & Ecology › Ecology & Ecosystem Basics › Biogeochemical cycles Official Key

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 :

Result

Your answer: — · Correct: A

Explanation

The correct answer is Option 1 (1, 3 and 4 only). Nitrogen fixation is a biological process where certain plants, primarily belonging to the Leguminosae (Fabaceae) family, host Rhizobium bacteria in their root nodules to convert atmospheric nitrogen into ammonia.

Alfalfa (1), Chickpea (3), and Clover (4) are all leguminous plants. They are well-known for their symbiotic relationship with nitrogen-fixing bacteria, making them essential for soil fertility and crop rotation.
Amaranth (2) belongs to the Amaranthaceae family and does not fix nitrogen.
Purslane (5) (Portulaca oleracea) and Spinach (6) (Spinacia oleracea) are non-leguminous leafy vegetables that lack the biological mechanism to fix atmospheric nitrogen; instead, they are heavy consumers of soil nitrogen.

Since only Alfalfa, Chickpea, and Clover possess nitrogen-fixing capabilities, Option 1 is the only scientifically accurate choice.

How others answered

Each bar shows the % of students who chose that option. Green bar = correct answer, blue outline = your choice.

Community Performance

Out of everyone who attempted this question.

47%

got it right

Detailed Concept Breakdown

7 concepts, approximately 14 minutes to master.

1. The Nitrogen Cycle: From Atmosphere to Soil (basic)

Nitrogen is one of the most paradoxical elements in nature. It makes up roughly 78% of the atmosphere Environment and Ecology, Majid Hussain, p.20, meaning it is present in every breath we take. However, despite being an essential building block for proteins, nucleic acids, and all living tissues — constituting nearly 16% of protein weight — most organisms cannot use it directly from the air Environment, Shankar IAS Academy, p.19. This is because atmospheric nitrogen (N₂) consists of two atoms held together by a very strong triple bond that most life forms cannot break.

To enter the food chain, nitrogen must first be "fixed" or converted into chemically reactive forms like ammonia (NH₃), nitrites, or nitrates. This "Nitrogen Fixation" occurs primarily through three pathways:

Biological Fixation: The most critical natural link. Specialized microorganisms, such as Rhizobium, live in a symbiotic relationship within the root nodules of leguminous plants (e.g., alfalfa, clover, beans, and peas). There are also free-living soil bacteria like Azotobacter and blue-green algae like Anabaena that perform this task Environment, Shankar IAS Academy, p.20.
Atmospheric Fixation: High-energy events like thunder and lightning provide enough energy to break N₂ bonds, allowing nitrogen to react with oxygen and reach the soil through rain as nitrates.
Industrial Fixation: Through human-led chemical processes (like the Haber-Bosch process), atmospheric nitrogen is converted into fertilizers. Today, the amount of nitrogen fixed by humans exceeds that of the natural cycle, sometimes leading to environmental imbalances Environment, Shankar IAS Academy, p.20.

Once nitrogen enters the soil as ammonia, it undergoes Nitrification. In this two-step process, Nitrosomonas bacteria convert ammonia into nitrite (NO₂⁻), which is then converted by Nitrobacter into nitrate (NO₃⁻) — the form most easily absorbed by plants Environment, Shankar IAS Academy, p.20.

Type of Bacteria	Function	Example
Symbiotic Fixers	Fix N₂ while living in plant roots	Rhizobium
Free-living Fixers	Fix N₂ independently in the soil	Azotobacter, Clostridium
Nitrifying Bacteria	Convert Ammonia to Nitrates	Nitrosomonas, Nitrobacter

Remember: Rhizobium helps the Roots of legumes. Nitrogen Fixation makes it Functional for plants.

Key Takeaway Nitrogen fixation is the essential "gatekeeper" process that converts inert atmospheric nitrogen into usable chemical forms, primarily through the help of specialized bacteria and leguminous plants.

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.20; Environment, Shankar IAS Academy (ed 10th), Functions of an Ecosystem, p.19-20

2. Biological Nitrogen Fixation: The Mechanism (intermediate)

To understand the nitrogen cycle, we must first address a biological paradox: while nitrogen (N₂) makes up nearly 78% of our atmosphere, it is chemically 'locked' by a powerful triple bond that most living organisms cannot break. Nitrogen is a critical building block of proteins and genetic material, constituting about 16% of all protein weight Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19. Biological Nitrogen Fixation (BNF) is the specialized process by which certain microorganisms 'fix' this inert gas, converting it into reactive forms like ammonia (NH₃) that plants can actually absorb Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Geomorphic Processes, p.45. This process occurs through two primary biological pathways. The most famous is the symbiotic relationship found in leguminous plants (like beans, peas, alfalfa, and chickpeas). These plants host Rhizobium bacteria in specialized root structures called nodules Science, Class VIII, NCERT (Revised ed 2025), The Invisible Living World, p.22. The plant provides the bacteria with sugar for energy, and in exchange, the bacteria provide the plant with fixed nitrogen. This is why farmers utilize crop rotation—planting legumes restores soil fertility naturally without the need for heavy chemical fertilizers. However, nitrogen fixation isn't exclusive to legumes. There are also non-symbiotic (free-living) organisms that fix nitrogen independently in the soil or water. These include aerobic bacteria like Azotobacter, anaerobic ones like Clostridium, and certain blue-green algae (Cyanobacteria) such as Anabaena and Nostoc Environment, Shankar IAS Academy, Functions of an Ecosystem, p.20. Understanding this distinction is vital for sustainable agriculture, as different bio-fertilizers are used for different crops.

Type of Fixation	Organism Examples	Typical Host/Habitat
Symbiotic	Rhizobium, Anabaena azollae	Legumes (Peas, Clover), Azolla (Water fern)
Free-living (Aerobic)	Azotobacter, Beijerinckia	Soil surface with oxygen access
Free-living (Anaerobic)	Clostridium, Rhodospirillum	Waterlogged soils/Oxygen-poor environments

Key Takeaway Nitrogen fixation is the essential 'biological gateway' that converts atmospheric N₂ into a usable form (NH₃), performed either by symbiotic bacteria in plant nodules or by free-living microbes in the soil.

Sources: Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19-20; Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Geomorphic Processes, p.45; Science, Class VIII, NCERT (Revised ed 2025), The Invisible Living World, p.22

3. Legumes and Root Nodules (intermediate)

To understand the nitrogen cycle, we must look at one of nature's most sophisticated partnerships: the relationship between leguminous plants and specialized bacteria. While 78% of our atmosphere is nitrogen gas (N₂), most plants are unable to use it in this gaseous form. They require nitrogen to be "fixed" into compounds like ammonia (NH₃) or nitrates (NO₃⁻) to build proteins and DNA. This is where Legumes (plants belonging to the Fabaceae or Leguminosae family) come in. These plants have evolved to host Rhizobium bacteria within specialized structures on their roots called nodules. Science, Class VIII NCERT, The Invisible Living World: Beyond Our Naked Eye, p.22

The process is a classic example of symbiosis: the plant provides the bacteria with carbohydrates (energy) produced through photosynthesis, and in exchange, the bacteria take inert nitrogen from the air and convert it into a chemically bound form that the plant can absorb. Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.20 This biological factory is so efficient that it often leaves the soil richer in nitrogen than before the crop was planted. Common examples of these nitrogen-fixing champions include Alfalfa, Chickpeas, Clover, Peas, Beans, and Sunhemp. Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.52

Because of this unique ability, legumes play a critical role in sustainable agriculture through crop rotation. Farmers often plant a leguminous crop before a "heavy feeder" (a non-leguminous crop like cotton or maize) to naturally replenish soil nutrients without relying solely on chemical fertilizers. Environment, Shankar IAS Academy, Agriculture, p.360 In contrast, common leafy vegetables like Spinach, Amaranth, and Purslane do not possess these nodules and act as consumers of soil nitrogen rather than providers.

Feature	Leguminous Crops (e.g., Alfalfa, Sunhemp)	Non-Leguminous Crops (e.g., Spinach, Maize)
Root Structure	Possess nodules (swollen regions)	No nodules; typically fibrous or standard tap roots
Nitrogen Source	Atmospheric N₂ (via bacteria)	Soil-based Nitrates/Ammonia
Soil Impact	Enriches soil nitrogen levels	Depletes soil nitrogen levels

Key Takeaway Root nodules are biological factories where Rhizobium bacteria convert atmospheric nitrogen into a usable form for legumes, making these plants essential for natural soil fertility and sustainable crop rotation.

Sources: Science, Class VIII NCERT, The Invisible Living World: Beyond Our Naked Eye, p.22; Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.20; Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.52; Environment, Shankar IAS Academy, Agriculture, p.360

4. Biofertilizers: Beyond Legumes (exam-level)

To understand biofertilizers, we must first look at the Nitrogen Paradox: nitrogen makes up nearly 78% of our atmosphere and is a fundamental building block of proteins and living tissue, yet most plants cannot 'breathe' it in to grow Shankar IAS Academy, Functions of an Ecosystem, p.19. For nitrogen to be useful, it must be 'fixed'—converted into chemical forms like ammonium (NH₄⁺) or nitrates (NO₃⁻). While modern agriculture often relies on industrial chemical fertilizers (the Haber-Bosch process), biofertilizers offer a sustainable biological alternative by using living microorganisms to enhance nutrient availability Shankar IAS Academy, Agriculture, p.364.

The most famous nitrogen-fixers belong to the Leguminosae (Fabaceae) family. These plants, such as Alfalfa, Chickpea, and Clover, form a symbiotic 'partnership' with Rhizobium bacteria. The bacteria live in specialized root nodules, taking sugars from the plant and, in exchange, converting atmospheric N₂ into ammonia that the plant can use. However, the world of biofertilizers extends far beyond just legumes. Nature employs several other 'specialists' to keep the nitrogen cycle moving:

Free-living Bacteria: Some microbes don't need a host plant at all. Azotobacter (aerobic) and Clostridium (anaerobic) fix nitrogen while living freely in the soil Shankar IAS Academy, Functions of an Ecosystem, p.20.
Blue-Green Algae (Cyanobacteria): Organisms like Anabaena and Nostoc are vital in wet ecosystems, such as paddy fields, where they fix nitrogen and increase soil fertility.
Non-Leguminous Symbiosis: Some non-legumes, like the Casuarina tree or Azolla (a water fern), also host nitrogen-fixing microbes.

In contrast, many common crops like Amaranth, Spinach, and Purslane are actually 'heavy feeders'—they consume large amounts of nitrogen from the soil but lack the biological machinery to fix it themselves. Understanding this distinction is the core of Integrated Nutrient Management (INM), which seeks to balance organic biofertilizers with inorganic ones to maintain high productivity without exhausting the soil Shankar IAS Academy, Agriculture, p.365.

Category	Examples	Mechanism
Symbiotic (Legumes)	Alfalfa, Clover, Chickpea, Soybeans	Rhizobium bacteria in root nodules.
Free-Living	Azotobacter, Clostridium	Fix nitrogen independently in the soil.
Non-Fixers (Consumers)	Spinach, Amaranth, Wheat, Corn	Absorb N from soil; do not fix atmospheric N₂.

Key Takeaway Biofertilizers facilitate nitrogen fixation through symbiotic relationships (like legumes with Rhizobium) or free-living organisms (like Azotobacter), reducing the need for chemical inputs in sustainable farming.

Sources: Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19-20; Environment, Shankar IAS Academy, Agriculture, p.364-365

5. Sustainable Agriculture: Green Manure & Crop Rotation (exam-level)

In the context of the Nitrogen Cycle, sustainable agriculture serves as a biological bridge to restore soil fertility without relying solely on synthetic inputs. At its core, Green Manure refers to the practice of growing specific crops—often legumes—and plowing them back into the soil while they are still green. This process does two critical things: it adds organic matter (humus) to improve soil structure and, more importantly, it returns atmospheric nitrogen to the earth in a plant-available form. Common green manuring crops in India include Sunhemp, Dhaincha, Clover, and Alfalfa Environment and Ecology, Majid Hussain, p.20. Sunhemp, for instance, belongs to the Fabaceae family; it is prized not just for its fibers but for its high cellulose content and ability to enrich the soil with nitrogen before the next cropping cycle Environment and Ecology, Majid Hussain, p.52.

Crop Rotation complements this by strategically alternating different types of crops on the same land. The scientific logic is simple: different plants have different nutritional needs. While cereal crops like wheat and rice—which were the focus of the Green Revolution Phase I (1966-72)—are heavy consumers of soil nitrogen Indian Economy, Vivek Singh, p.302, leguminous crops like Chickpeas and Barseem act as "bio-fertilizers." These legumes host Rhizobium bacteria in their root nodules, which perform biological nitrogen fixation. By rotating a nitrogen-depleting crop with a nitrogen-fixing one, farmers maintain a balanced biogeochemical flow, preventing the "soil exhaustion" that often results from monocultures and the excessive use of chemical fertilizers History (TN State Board), Envisioning a New Socio-Economic Order, p.119.

Feature	Leguminous Green Manure (e.g., Clover, Sunhemp)	Non-Leguminous Crops (e.g., Amaranth, Spinach)
Nitrogen Role	Nitrogen Fixers: Convert N₂ from air to soil via symbiosis.	Nitrogen Consumers: Rely on existing soil nitrogen or fertilizers.
Soil Impact	Enriches soil nutrients and adds organic biomass.	Depletes soil nutrients if not rotated or supplemented.

Key Takeaway Sustainable practices like Green Manuring and Crop Rotation utilize the biological nitrogen-fixing capabilities of the Fabaceae family to naturally replenish the soil's nutrient cycle, reducing dependence on chemical fertilizers.

Sources: Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.20; Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.52; Indian Economy, Vivek Singh, Agriculture - Part I, p.302; History (Tamilnadu State Board), Envisioning a New Socio-Economic Order, p.119

6. Distinguishing Nitrogen-Fixers from Other Crops (exam-level)

To understand soil fertility, we must distinguish between plants that deplete nitrogen and those that replenish it. While nitrogen (N₂) makes up 78% of our atmosphere, it is chemically inert and unusable by most plants. Biological Nitrogen Fixation (BNF) is the specialized process that converts this atmospheric nitrogen into ammonia (NH₃), a form plants can actually absorb.

This capability is not universal; it is primarily a hallmark of the Leguminosae (Fabaceae) family. These plants form a symbiotic relationship with Rhizobium bacteria. The bacteria reside in specialized "root nodules," where they trade fixed nitrogen for carbohydrates from the plant. Key examples of these "soil builders" include:

Pulses and Beans: Chickpea (Gram), Lentils, Peas, and Soya bean.
Fodder and Green Manure: Alfalfa (Lucerne), various Clovers (such as Berseem or Senji), Dhaincha, and Sunnhemp Environment, Shankar IAS Academy, Agriculture, p.354.
Soil Management: These crops are often used in green manuring to reduce soil erosion and naturally boost nutrient levels Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.20.

In contrast, many other common crops are heavy nitrogen consumers. They lack the biological machinery to fix nitrogen and instead rely entirely on soil reserves or chemical fertilizers. This group includes Cereals (Rice, Wheat, Maize) INDIA PEOPLE AND ECONOMY, NCERT, Land Resources and Agriculture, p.26 and most leafy vegetables like Spinach, Amaranth, and Purslane. Because of this difference, scientific farming utilizes crop rotation, planting deep-rooted legumes to fix nitrogen before sowing nutrient-demanding non-leguminous crops Environment, Shankar IAS Academy, Agriculture, p.360.

Feature	Nitrogen Fixers (Legumes)	Nitrogen Consumers (Non-Legumes)
Mechanism	Symbiosis with Rhizobium bacteria in root nodules.	Absorption of nitrates/ammonium from soil only.
Impact on Soil	Increases soil nitrogen content.	Depletes soil nitrogen content.
Examples	Alfalfa, Clover, Chickpea, Dhaincha, Pea.	Wheat, Rice, Spinach, Amaranth, Maize.

Key Takeaway Nitrogen fixation is largely restricted to leguminous plants (like clovers and pulses) that host symbiotic bacteria; most other crops, like cereals and leafy greens, are consumers that deplete soil nitrogen.

Sources: Environment, Shankar IAS Academy, Agriculture, p.354, 360; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.20; INDIA PEOPLE AND ECONOMY, NCERT, Land Resources and Agriculture, p.26

7. Solving the Original PYQ (exam-level)

Now that you have mastered the biological mechanics of the Nitrogen Cycle, this question tests your ability to apply that knowledge to specific botanical families. The core concept here is the symbiotic relationship between plants of the Leguminosae (Fabaceae) family and Rhizobium bacteria. As you learned, these bacteria reside in root nodules and convert atmospheric nitrogen into ammonia, a process essential for soil fertility. To solve this, you simply need to categorize the list into legumes (fixers) and non-legumes (consumers).

Walking through the list, Alfalfa (1), Chickpea (3), and Clover (4) are quintessential legumes often discussed in the context of crop rotation and sustainable agriculture. In contrast, Amaranth (2), Purslane (5), and Spinach (6) are leafy greens or herbs that actually deplete soil nitrogen rather than replenish it. By identifying that Spinach is a heavy feeder and not a fixer, you can use the elimination technique to discard any option containing '6'. This logical deduction lead us directly to the correct answer: (A) 1, 3 and 4 only.

UPSC frequently uses a 'commonality trap' by mixing well-known crops with slightly obscure ones like Purslane (Kulfa) to shake your confidence. A common mistake is assuming that all 'hardy' or 'green' plants contribute to soil health. However, as noted in NCERT Class 12 Biology, nitrogen fixation is a specialized biological niche. Always look for the distractors—in this case, the leafy vegetables—and remember that biological nitrogen fixation is almost exclusively the domain of the pulse and pea family in a typical agricultural context.

PROVENANCE & STUDY PATTERN

Guest preview

Don’t just practise – reverse-engineer the question. This panel shows where this PYQ came from (books / web), how the examiner broke it into hidden statements, and which nearby micro-concepts you were supposed to learn from it. Treat it like an autopsy of the question: what might have triggered it, which exact lines in the book matter, and what linked ideas you should carry forward to future questions.

Q. Which of the following are nitrogen-fixing plants ? 1. Alfalfa 2. Amaranth 3. Chickpea 4. Clover 5. Purslane (Kulfa) 6. Spinach Select th…

At a glance

Origin: Mixed / unclear origin Fairness: Moderate fairness Books / CA: 5/10 · 0/10

You're seeing a guest preview. The Verdict and first statement analysis are open. Login with Google to unlock all tabs.

This is a classic 'Identify the Legume' test disguised as a science question. Standard texts (NCERT Geography, Shankar Environment) explicitly list Alfalfa, Clover, and Chickpea as nitrogen fixers. The strategy is binary: Is it a legume (pulse/fodder)? Yes = Fixer. Is it a leafy vegetable (Spinach/Amaranth)? No = Consumer.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1: Is alfalfa (Medicago sativa) a nitrogen-fixing plant?
Statement 2: Is amaranth (Amaranthus spp.) a nitrogen-fixing plant?
Statement 3: Is chickpea (Cicer arietinum) a nitrogen-fixing plant?
Statement 4: Is clover (Trifolium spp.) a nitrogen-fixing plant?
Statement 5: Is purslane (Portulaca oleracea), also called kulfa, a nitrogen-fixing plant?
Statement 6: Is spinach (Spinacia oleracea) a nitrogen-fixing plant?

Statement 1

Is alfalfa (Medicago sativa) a nitrogen-fixing plant?

Origin: Direct from books Fairness: Straightforward Book-answerable

From standard books

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > The Nitrogen Cycle > p. 20

Presence: 5/5

“Te nitrogen cycle involves the major constituent of the atmosphere, (78.084 per cent of each breath we take). Nitrogen is also an important element in the formation of organic molecules, especially proteins, and therefore is essential to living processes. A simplifed view of the nitrogen cycle is portrayed in Fig. 1.9. Te vast atmospheric reservoir is however inaccessible directly to most organisms. Te key link to life is provided by nitrogen-fxing bacteria, which live principally in the soil and are associated with the roots of certain plants. For example, the legumes such as alfalfa, beans, clover, pulses, peas, soybeans, green-manuring crops and peanuts.”

Why this source?

Explicitly lists alfalfa among legumes associated with nitrogen-fixing bacteria.
Connects legumes (including alfalfa) to root-associated nitrogen fixation used in agriculture.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45

Presence: 4/5

“Further, bacteria and other soil organisms take gaseous nitrogen from the air and convert it into a chemical form that can be used by plants. This process is known as nitrogen fixation. Rhizobium, a type of bacteria, lives in the root nodules of leguminous plants and fixes nitrogen beneficial to the host plant. The influence of large animals like ants, termites, earthworms, rodents etc., is mechanical, but, it is nevertheless important in soil formation as they rework the soil up and down. In case of earthworms, as they feed on soil, the texture and chemistry of the soil that comes out of their body changes.”

Why this source?

Defines nitrogen fixation by Rhizobium in root nodules of leguminous plants.
Shows the mechanism by which leguminous hosts obtain fixed nitrogen from bacteria.

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > s r n r u l z N ,- / & f . -. : : u ' , \ S ACADEMY * d 6 # . , r '' t u f Y l ' ' J * w { d ) / u Y . / > p. 20

Presence: 4/5

“& srnrul<zrR ". N ,- / & f. -.:.:u' ,\SACADEMY *d6#.,r'' tufYl' 'J *w{d )/ uY./ include free living nitrifying bacteria (e.g. aerobic Azotobacter and anaerobic Clostridium) and symbiotic nitrifying bacteria living in association with leguminous plants and symbiotic bacteria living in non-leguminous root-nodule plants (e.g., Rhizobium) as well as blue-green algae (e.g., Anabaena, Spirulina). Ammonium ions can be directly taken up as a source of nitrogen by some plants, or are oxidized to nitrites or nitrates by two groups of specialized bacteria: Nitrosomonas bacteria promote transformation of ammonia into nitrite. Nitrite is then further transformed into nitrate by the bacteria Nitrobacter.”

Why this source?

Describes symbiotic nitrogen-fixing bacteria living in association with leguminous plants.
Names groups of free-living and symbiotic nitrogen fixers relevant to legumes.

Statement 2

Is amaranth (Amaranthus spp.) a nitrogen-fixing plant?

Origin: Weak / unclear Fairness: Borderline / guessy

Indirect textbook clues

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45

Strength: 5/5

Why relevant

Defines nitrogen fixation and states Rhizobium fixes N in root nodules of leguminous plants — establishing a common mechanism tied to legumes.

How to extend

A student can check whether Amaranthus is a legume or forms root nodules (presence would support possible symbiotic fixation).

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > c) The Nitrogen Cycle > p. 19

Strength: 4/5

“(c) The Nitrogen Cycle 0000 00 Nitrogen is an essential constituent of protein and is a basic building block of all living tissue. It constitutes nearly 16% by weight of all the proteins. There is an inexhaustible supply of nitrogen in the atmosphere but the elemental form cannot be used directly by most of the living organisms. Nitrogen needs to be 'fixed', that is, converted to ammonia, nitrites or nitrates, before it can be taken up by plants. Nitrogen fixation on Earth is accomplished in three different ways: • (i) By microorganisms (bacteria and blue-green algae) • (ii) By man using industrial processes (fertilizer factories) and • (iii) To a limited extent by atmospheric phenomenon such as thunder and lighting Certain microorganisms are capable of fixing atmospheric nitrogen into ammonium ions.”

Why relevant

Explains that most plants cannot use atmospheric N directly and lists microorganisms (bacteria, blue‑green algae) as the primary biological fixers.

How to extend

One can investigate whether Amaranthus hosts nitrogen‑fixing bacteria (e.g., nodulating bacteria or cyanobacteria) or relies solely on soil N.

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > The Nitrogen Cycle > p. 20

Strength: 4/5

“Bacteria colonies reside in nodules on the legume-roots and chemically combine the nitrogen from the air in the form of nitrates (NO3) and ammonia (NH3). Plants use these chemically bound organic matter. Anyone or anything feeding on the plants thus”

Why relevant

Notes bacteria in nodules on legume roots chemically combine atmospheric N into forms plants use — reinforces nodulation as a diagnostic feature.

How to extend

Check root morphology of Amaranthus for nodules or test roots for bacterial symbionts to infer possible fixation.

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > s r n r u l z N ,- / & f . -. : : u ' , \ S ACADEMY * d 6 # . , r '' t u f Y l ' ' J * w { d ) / u Y . / > p. 20

Strength: 4/5

Why relevant

Lists free‑living nitrogen fixers (e.g., Azotobacter, Clostridium) and symbiotic bacteria in both leguminous and some non‑leguminous root‑nodule plants and cyanobacteria — indicating multiple possible modes of fixation.

How to extend

A student could determine if Amaranthus associates with free‑living fixers (rhizosphere) or has non‑legume nodulation by checking species‑level literature or root microbiome studies.

Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 9: Agriculture > 11. Less Area under Leguminous and Fodder Crops > p. 9

Strength: 4/5

“The nitrogen fixing crops like pulses are getting less area under their cultivation. Consequently, the natural fertility of the soil is depleting and”

Why relevant

Points out 'nitrogen fixing crops like pulses' are legumes and their decline affects soil fertility — implies that crop taxonomy (legume vs non‑legume) matters for N fixation.

How to extend

Use basic botanical classification to see that Amaranthus belongs to Amaranthaceae (not legumes), which would make classical Rhizobium nodulation unlikely.

Statement 3

Is chickpea (Cicer arietinum) a nitrogen-fixing plant?

Origin: Direct from books Fairness: Straightforward Book-answerable

From standard books

NCERT. (2022). Contemporary India II: Textbook in Geography for Class X (Revised ed.). NCERT. > Chapter 4: The Age of Industrialisation > Major Crops > p. 85

Presence: 5/5

“Pulses: India is the largest producer as well as the consumer of pulses in the world. These are the major source of protein in a vegetarian diet. Major pulses that are grown in India are tur (arhar), urad, moong, masur, peas and gram. Can you distinguish which of these pulses are grown in the kharif season and which are grown in the rabi season? Pulses need less moisture and survive even in dry conditions. Being leguminous crops, all these crops except arhar help in restoring soil fertility by fixing nitrogen from the air. Therefore, these are mostly grown in rotation with other crops.”

Why this source?

Lists gram (chickpea) among pulses and labels these as leguminous crops.
Explicitly asserts that these leguminous crops (except arhar) restore soil fertility by fixing nitrogen from the air.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45

Presence: 5/5

Why this source?

Explains that Rhizobium bacteria live in root nodules of leguminous plants and fix atmospheric nitrogen.
Provides the biological mechanism by which leguminous plants obtain usable nitrogen.

Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 9: Agriculture > 11. Less Area under Leguminous and Fodder Crops > p. 9

Presence: 4/5

“The nitrogen fixing crops like pulses are getting less area under their cultivation. Consequently, the natural fertility of the soil is depleting and”

Why this source?

Refers to pulses as nitrogen-fixing crops, linking the crop category to nitrogen fixation.
Supports the general classification of pulses (which include chickpea) as contributors to soil nitrogen.

Statement 4

Is clover (Trifolium spp.) a nitrogen-fixing plant?

Origin: Direct from books Fairness: Straightforward Book-answerable

From standard books

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > The Nitrogen Cycle > p. 20

Presence: 5/5

Why this source?

Explicitly lists clover among legumes that are associated with nitrogen-fixing bacteria.
Identifies legumes (including clover) as key plants in the biological nitrogen cycle.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45

Presence: 5/5

Why this source?

Explains that Rhizobium bacteria live in root nodules of leguminous plants and fix nitrogen for the host.
Connects the mechanism (root-nodule symbiosis) to leguminous crops, supporting clover's role.

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > s r n r u l z N ,- / & f . -. : : u ' , \ S ACADEMY * d 6 # . , r '' t u f Y l ' ' J * w { d ) / u Y . / > p. 20

Presence: 4/5

Why this source?

Describes symbiotic nitrogen-fixing bacteria associated with leguminous plants and names Rhizobium.
Reinforces the general pattern of root-nodule symbiosis in legumes, applicable to clover.

Statement 5

Is purslane (Portulaca oleracea), also called kulfa, a nitrogen-fixing plant?

Origin: Weak / unclear Fairness: Borderline / guessy

Indirect textbook clues

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > c) The Nitrogen Cycle > p. 19

Strength: 5/5

Why relevant

Defines nitrogen fixation and lists the main agents: microorganisms (bacteria and blue‑green algae), industrial processes, and lightning.

How to extend

A student could use this to ask whether purslane is associated with known N‑fixing microorganisms or exposed to such agents in its habitat.

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45

Strength: 5/5

Why relevant

Explains that Rhizobium in root nodules of leguminous plants fixes nitrogen for the host.

How to extend

A student can check whether purslane is a legume or forms root nodules (if not, this reduces the likelihood of Rhizobium symbiosis).

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > s r n r u l z N ,- / & f . -. : : u ' , \ S ACADEMY * d 6 # . , r '' t u f Y l ' ' J * w { d ) / u Y . / > p. 20

Strength: 4/5

Why relevant

Mentions both free‑living nitrogen‑fixing bacteria (e.g., Azotobacter, Clostridium) and symbiotic bacteria including those in non‑leguminous root nodules and cyanobacteria (Anabaena).

How to extend

A student could investigate whether purslane hosts non‑legume root‑nodule symbionts or is commonly associated with free‑living N‑fixers in soil.

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > The Nitrogen Cycle > p. 20

Strength: 4/5

Why relevant

Describes the general mechanism where bacterial colonies in root nodules chemically bind atmospheric nitrogen to forms plants can use.

How to extend

Use this to test for the physical presence of root nodules or bacterial colonization on purslane roots as evidence for symbiotic fixation.

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > 13. Complementrary Crops > p. 12

Strength: 2/5

“Crops which are grown to beneft each other in inter-cropping e.g. Jowar (millets) with Lobiya. Jowar (millets) receives nitrogen from Lobiya and Lobiya requires support from Jowar (millets).”

Why relevant

Gives an example of intercropping where a legume supplies nitrogen to a companion crop, illustrating that nitrogen contribution is commonly from leguminous partners.

How to extend

A student could check whether purslane is ever described as a leguminous companion or as benefiting/adding N in crop rotations.

Statement 6

Is spinach (Spinacia oleracea) a nitrogen-fixing plant?

Origin: Weak / unclear Fairness: Borderline / guessy

Indirect textbook clues

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45

Strength: 5/5

Why relevant

Defines nitrogen fixation and states that Rhizobium bacteria live in root nodules of leguminous plants to fix nitrogen for the host.

How to extend

A student can check whether spinach is a legume or forms root nodules with Rhizobium — if not, it is unlikely to be a Rhizobium-based nitrogen-fixer.

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > c) The Nitrogen Cycle > p. 19

Strength: 4/5

Why relevant

Gives the general rule that nitrogen fixation is carried out by microorganisms (bacteria and blue‑green algae), industrial processes, or lightning, implying plants fix nitrogen only via microbial symbiosis or special associations.

How to extend

A student could ask whether spinach hosts nitrogen‑fixing microorganisms (symbiotic or endophytic) or relies on soil/industrial sources instead.

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > s r n r u l z N ,- / & f . -. : : u ' , \ S ACADEMY * d 6 # . , r '' t u f Y l ' ' J * w { d ) / u Y . / > p. 20

Strength: 4/5

Why relevant

Lists types of nitrogen‑fixing bacteria: free‑living (Azotobacter, Clostridium), symbiotic with legumes, and symbiotic bacteria in non‑legume root nodules, showing multiple biological modes but emphasizing specific associations.

How to extend

A student can investigate whether spinach is known to associate with any of these free‑living or symbiotic nitrogen‑fixers or to develop root nodules.

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > The Nitrogen Cycle > p. 20

Strength: 4/5

Why relevant

Explains bacteria in nodules on legume roots chemically combine atmospheric nitrogen into forms plants use, reinforcing the pattern that nodulation is a key indicator of plant nitrogen‑fixing partnerships.

How to extend

A student could examine spinach root morphology (presence/absence of nodules) or literature on nodulation to assess whether it participates in this typical pathway.

Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Life-giving gases > p. 280

Strength: 3/5

“• Plants require carbon dioxide to survive while animals and many other organisms need oxygen for their survival. Nitrogen is fixed by bacteria and lightning to produce ammonia used in the construction of nucleotides and amino acids.”

Why relevant

Notes that nitrogen is fixed by bacteria (and lightning), underscoring that fixation is usually an abiotic or microbial process rather than a direct plant trait.

How to extend

A student might therefore check whether spinach itself biochemically fixes N (unlikely from this rule) or instead depends on microbial/abiotic fixed nitrogen in soil.

Pattern takeaway: UPSC tests 'Applied Botany' through the lens of Agriculture. They don't ask for the bacterial strain name (Rhizobium vs Azotobacter) as often as they ask for the host plant. The pattern is to mix common kitchen ingredients (Spinach, Chickpea) with fodder crops (Alfalfa, Clover) to test observation skills.

How you should have studied

[THE VERDICT]: Sitter. Direct hit from NCERT Class X Geography (Ch 4) and Shankar IAS (Ch 2). If you missed this, your basic reading of 'Leguminous Crops' is weak.
[THE CONCEPTUAL TRIGGER]: Bio-geochemical Cycles (Nitrogen Cycle) and Sustainable Agriculture (Biofertilizers).
[THE HORIZONTAL EXPANSION]: Memorize the 'Fixer Squad': Legumes (Peas, Beans, Soy, Alfalfa, Clover, Gliricidia), Non-Legumes (Casuarina, Alder), and Aquatic (Azolla-Anabaena). Contrast with 'Nitrogen Hogs' (Spinach, Maize, Cotton).
[THE STRATEGIC METACOGNITION]: When reading about the Nitrogen Cycle, stop treating 'Legumes' as a generic word. Unpack the category: What are the specific examples mentioned in NCERT? (Gram, Tur, Urad, Moong, Peas). Add fodder crops (Alfalfa, Clover) from Ecology texts.

Concept hooks from this question

📌 Adjacent topic to master

👉 Legumes as nitrogen-fixing crops

💡 The insight

Leguminous plants (e.g., alfalfa) are commonly associated with root bacteria that fix atmospheric nitrogen.

High-yield for questions on soil fertility, crop choice and sustainable agriculture. Links agriculture to biogeochemical cycles and cropping systems; useful for MCQs and short-answer questions on crops that improve soil nitrogen.

📚 Reading List :

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > The Nitrogen Cycle > p. 20
FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45

🔗 Anchor: "Is alfalfa (Medicago sativa) a nitrogen-fixing plant?"

📌 Adjacent topic to master

👉 Rhizobium and root-nodule symbiosis

💡 The insight

Rhizobium bacteria form root nodules on legumes and convert atmospheric nitrogen into plant-usable forms.

Essential for explaining mechanisms behind biological nitrogen fixation; connects microbiology with agronomy and environmental science topics. Enables answers on biofertility, crop rotations and soil nutrient management.

📚 Reading List :

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45
Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > s r n r u l z N ,- / & f . -. : : u ' , \ S ACADEMY * d 6 # . , r '' t u f Y l ' ' J * w { d ) / u Y . / > p. 20

🔗 Anchor: "Is alfalfa (Medicago sativa) a nitrogen-fixing plant?"

📌 Adjacent topic to master

👉 Bio-fertilizers and nitrogen-fixing microorganisms

💡 The insight

Bio-fertilizers contain live nitrogen-fixing microbes used to augment plant-available nitrogen.

Relevant to policy and practice questions on sustainable farming and fertilizer alternatives; links to chapters on agriculture, soil management and eco-friendly inputs, and helps frame solutions-based answers.

📚 Reading List :

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 25: Agriculture > Bio-fertilizers > p. 364
Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > The Nitrogen Cycle > p. 20

🔗 Anchor: "Is alfalfa (Medicago sativa) a nitrogen-fixing plant?"

📌 Adjacent topic to master

👉 Modes of nitrogen fixation

💡 The insight

Nitrogen is fixed into biologically available forms by microorganisms, industrial processes, and atmospheric phenomena.

High-yield concept for questions on nutrient cycles, agriculture and environmental management; helps answer questions on sources of soil nitrogen, fertilizer policy, and ecosystem functioning. Mastering this enables candidates to link biological processes to agricultural inputs and policy debates.

📚 Reading List :

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > c) The Nitrogen Cycle > p. 19
Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 25: Agriculture > Bio-fertilizers > p. 364

🔗 Anchor: "Is amaranth (Amaranthus spp.) a nitrogen-fixing plant?"

📌 Adjacent topic to master

👉 Symbiotic nitrogen fixation in root nodules

💡 The insight

Rhizobium bacteria form root nodules on legumes and fix nitrogen; symbiotic root-nodule associations can occur in non-leguminous plants as well.

Essential for questions on crop selection, soil fertility restoration, and sustainable farming (e.g., crop rotation with legumes). It connects ecology, agriculture and rural economy topics and helps reason about which crops contribute to soil nitrogen.

📚 Reading List :

FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45
Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > s r n r u l z N ,- / & f . -. : : u ' , \ S ACADEMY * d 6 # . , r '' t u f Y l ' ' J * w { d ) / u Y . / > p. 20

🔗 Anchor: "Is amaranth (Amaranthus spp.) a nitrogen-fixing plant?"

📌 Adjacent topic to master

👉 Biofertilizers and microbial augmentation of soil nitrogen

💡 The insight

Biofertilizers contain live nitrogen-fixing microorganisms applied to seeds or soil to increase microbial nitrogen-fixation processes.

Directly relevant for policy and practice questions on sustainable agriculture, reducing chemical fertilizer use, and improving soil health. Understanding this concept allows candidates to discuss interventions (biofertilizers) for augmenting soil fertility.

📚 Reading List :

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 25: Agriculture > Bio-fertilizers > p. 364
Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > The Nitrogen Cycle > p. 20

🔗 Anchor: "Is amaranth (Amaranthus spp.) a nitrogen-fixing plant?"

📌 Adjacent topic to master

👉 Leguminous pulses fix atmospheric nitrogen

💡 The insight

Pulses such as gram (chickpea) are leguminous and fix atmospheric nitrogen, restoring soil fertility.

High-yield for agriculture and environment questions: explains why pulses are prioritized in crop planning and soil management. Links to topics on sustainable agriculture, food security, and cropping patterns; useful for questions on soil fertility and crop choices.

📚 Reading List :

NCERT. (2022). Contemporary India II: Textbook in Geography for Class X (Revised ed.). NCERT. > Chapter 4: The Age of Industrialisation > Major Crops > p. 85
Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 9: Agriculture > 11. Less Area under Leguminous and Fodder Crops > p. 9

🔗 Anchor: "Is chickpea (Cicer arietinum) a nitrogen-fixing plant?"

🌑 The Hidden Trap

Frankia & Casuarina. While everyone knows Rhizobium+Legumes, the Frankia bacteria fixes nitrogen in non-leguminous trees like Casuarina and Alnus. This is the next logical 'hard' question.

⚡ Elimination Cheat Code

The 'Leafy Green' Logic. Spinach and Amaranth are grown for their lush green leaves. Biologically, rapid leaf growth requires *consuming* massive amounts of soil nitrogen (nitrates). If they fixed their own, they wouldn't be such heavy feeders. Also, Alfalfa and Clover are famous 'Green Manure' crops—Spinach is never used as green manure.

🔗 Mains Connection

GS-3 Agriculture & Economy. Nitrogen fixing plants = Reduced Urea Import Bill. Linking natural N-fixation to the government's 'Soil Health Card' and 'Neem Coated Urea' schemes creates a holistic answer for Mains.

Change set

Question map

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 :

Explanation

Detailed Concept Breakdown

1. The Nitrogen Cycle: From Atmosphere to Soil (basic)

2. Biological Nitrogen Fixation: The Mechanism (intermediate)

3. Legumes and Root Nodules (intermediate)

4. Biofertilizers: Beyond Legumes (exam-level)

5. Sustainable Agriculture: Green Manure & Crop Rotation (exam-level)

6. Distinguishing Nitrogen-Fixers from Other Crops (exam-level)

7. Solving the Original PYQ (exam-level)

PROVENANCE & STUDY PATTERN

SIMILAR QUESTIONS

Which one among the following kinds of organisms resides in the roots of pulse plants to do nitrogen fixation ?

Which one among the following kinds of organisms resides in the roots of pulse plants to do nitrogen fixation ?

Which of the following is/are used as biofertilizers ? I. Azolla II. Blue-green algae III. Alfalfa. Select the correct answer using the codes given below.

Which one of the following crops enriches the nitrogen content in soil ?

5 Cross-Linked PYQs Behind This Question