Soil Fertility, Fertilisers and Manures
Essential plant nutrients, NPK and deficiency symptoms, soil pH, urea, DAP and nano urea, manures and biofertilisers, and balanced fertilisation for CGPSC Mains GS-II.
Prerequisites
- Soil Types of India: what each soil is made of and where it is found
- Climate and Soils of Chhattisgarh: the Kanhar, Dorsa, Matasi and Bhata soils of the state
- Acids, Bases and Salts: pH and neutralisation, needed for liming and gypsum
The Hook: Khaira in the Paddy Nursery
A farmer in Bemetara notices that, a few weeks after transplanting, patches of his paddy have turned rusty brown and stunted. His neighbour says: “Add more urea.” He does, and nothing improves. The agricultural extension officer looks once and says khaira (खैरा). The crop is short of zinc, not nitrogen. A small dose of zinc sulphate fixes it.
The lesson is older than the Green Revolution: a plant is held back by whichever nutrient is most lacking, and adding more of another does nothing. How many nutrients does a crop need? Why does India use so much urea? Can cow dung and bacteria replace chemical fertilisers? This page answers those questions.
Why it matters: Mains GS-II asks about soil health, NPK imbalance, nano fertilisers, biofertilisers and integrated nutrient management. GS-III (economy) asks about fertiliser subsidy; that policy side is in Agriculture and the Economy. Here the focus is the science.
Soil Fertility: the Basics
Soil fertility (मृदा उर्वरता) is the soil’s ability to supply nutrients in adequate amounts and balance. Soil productivity is its ability to produce crops, which also depends on water, climate and management. A soil can be fertile but unproductive (for example, fertile but waterlogged).
The 17 essential elements
A nutrient is essential if the plant cannot complete its life cycle without it, it cannot be replaced by another element, and it is directly involved in the plant’s metabolism. Seventeen elements meet these criteria:
| Group | Elements | Source |
|---|---|---|
| Non-mineral | Carbon (C), hydrogen (H), oxygen (O) | Air and water |
| Primary macronutrients | Nitrogen (N), phosphorus (P), potassium (K) | Soil and fertilisers; needed in the largest amounts |
| Secondary macronutrients | Calcium (Ca), magnesium (Mg), sulphur (S) | Soil, lime, gypsum |
| Micronutrients | Iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), nickel (Ni) | Needed in tiny amounts, but just as essential |
What each major nutrient does
| Nutrient | Main role in the plant | Deficiency symptom | Mobility |
|---|---|---|---|
| Nitrogen (N) | Part of proteins, nucleic acids and chlorophyll; drives leafy growth | Yellowing (chlorosis) of older leaves first; stunted growth | Mobile in plant, so old leaves show it first |
| Phosphorus (P) | Energy transfer (ATP), nucleic acids, root growth, flowering, seed formation | Purple or dark green leaves; poor roots; delayed maturity | Mobile |
| Potassium (K) | Opens and closes stomata; enzyme activation; water regulation; disease and lodging resistance; grain quality | Scorching of leaf margins in older leaves; weak stems | Mobile |
| Sulphur (S) | Part of some amino acids; oil content in oilseeds | Yellowing of young leaves | Less mobile |
| Zinc (Zn) | Enzyme activation; growth hormones | Khaira disease of rice (rusty brown patches); “little leaf” | Immobile |
| Iron (Fe) | Chlorophyll formation | Chlorosis of young leaves with green veins (interveinal) | Immobile |
| Boron (B) | Cell wall formation; pollen and seed set | Hollow stem, poor seed set; “brown heart” in some root crops | Immobile |
| Molybdenum (Mo) | Part of nitrogenase; nitrogen metabolism | Whiptail of cauliflower | Moderately mobile |
Rule of thumb: deficiency of a mobile nutrient shows first on older (lower) leaves, because the plant moves it to new growth. Deficiency of an immobile nutrient shows on younger (upper) leaves.
Soil reaction (pH) and how to correct it
| Problem soil | Cause | Correction |
|---|---|---|
| Acidic soil (pH below about 6) | High rainfall leaches bases; laterite and red soils | Liming (चूना) with calcium carbonate raises pH; makes P and Mo more available |
| Saline soil | Excess soluble salts | Leaching with good-quality water; drainage |
| Alkaline or sodic soil (high pH, excess sodium) | Poor drainage, canal over-irrigation in dry areas | Gypsum (calcium sulphate) replaces sodium; then leaching. Pyrite can also be used |
Chhattisgarh angle: Much of the state has red and yellow soils and lateritic Bhata soils, many of them acidic, especially in the Bastar plateau and the northern hills. These soils are also often low in phosphorus, and zinc deficiency is common in intensively cropped paddy. Liming, phosphorus and zinc management are therefore core advice from IGKV, Raipur.
Chemical Fertilisers
Common fertilisers and nutrient content
| Fertiliser | Nutrient content | Type | Notes |
|---|---|---|---|
| Urea | 46% N | Straight (nitrogen) | Most-used fertiliser in India; amide form, converted to ammonium and then nitrate in soil |
| Ammonium sulphate | About 20–21% N + about 24% S | Straight (N) | Also supplies sulphur; acidifies soil |
| DAP (diammonium phosphate) | 18% N, 46% P₂O₅ | Complex | Main phosphatic fertiliser; largely imported or made from imported raw material |
| SSP (single super phosphate) | 16% P₂O₅ (plus sulphur and calcium) | Straight (P) | Made in India from rock phosphate; useful for oilseeds and pulses because of its sulphur |
| MOP (muriate of potash, potassium chloride) | 60% K₂O | Straight (K) | India has no commercial potash reserves, so it is wholly imported |
| NPK complexes | e.g. 10:26:26, 12:32:16 | Complex | Supply all three primary nutrients |
| Zinc sulphate | Zinc (and sulphur) | Micronutrient | Corrects khaira in rice |
Straight fertilisers supply one primary nutrient. Complex fertilisers supply two or more through a chemical reaction. Mixed fertilisers are physical blends.
The nitrogen problem: why urea is overused and wasted
India’s crops recover only a modest share of the nitrogen applied; nitrogen-use efficiency in Indian field conditions is commonly estimated at about 30–50%. The rest is lost by:
| Loss pathway | What happens | Environmental cost |
|---|---|---|
| Volatilisation | Urea turns into ammonia gas, which escapes, especially from wet surface-applied urea in heat | Air pollution |
| Leaching | Nitrate (NO₃⁻), which is negatively charged and not held by soil, washes below the root zone | Groundwater nitrate pollution; “blue baby syndrome” (methaemoglobinaemia) in infants |
| Denitrification | In waterlogged soil, bacteria turn nitrate into nitrogen gases, including nitrous oxide (N₂O) | N₂O is a powerful greenhouse gas |
| Runoff | Fertiliser washes into water bodies | Eutrophication (algal blooms) |
Why urea is overused: urea is priced far below cost under the fertiliser subsidy, while phosphatic and potassic fertilisers moved to Nutrient Based Subsidy (NBS) in 2010 and became costlier. Farmers respond by using more nitrogen and less P and K. The result is a skewed NPK ratio.
Making fertiliser smarter
| Innovation | What it is | Science |
|---|---|---|
| Neem-coated urea (नीम लेपित यूरिया) | Mandatory for all domestically produced and imported urea since 2015 | Neem oil compounds slow nitrification, so nitrogen stays in the ammonium form longer and is lost less. Also stopped diversion of subsidised urea to industry |
| Nano urea (liquid) | Launched by IFFCO in June 2021; nano DAP followed in 2023; both notified under the Fertiliser Control Order, 1985 | Nanoscale nitrogen particles sprayed on leaves are claimed to be absorbed more efficiently. A 500 ml bottle is marketed as replacing one bag of urea. Independent field trials have questioned the yield claims, so present it as promising but still debated |
| Slow- and controlled-release fertilisers | Coated urea (e.g. sulphur- or polymer-coated) | Release nutrients over weeks to match crop demand |
| Customised and fortified fertilisers | Blends with micronutrients, zinc-coated urea | Match the soil’s actual deficiencies |
| Fertigation | Fertiliser dissolved in drip irrigation water | Places nutrients at the roots in small doses |
Policy tools linked to soil science
| Measure | Year | Scientific purpose |
|---|---|---|
| Nutrient Based Subsidy (NBS) | 2010 | Subsidy fixed per kg of N, P, K and S in P and K fertilisers. Urea is outside NBS |
| Soil Health Card Scheme (मृदा स्वास्थ्य कार्ड) | Launched 19 February 2015 at Suratgarh, Rajasthan | Soil tested for 12 parameters (N, P, K, S, Zn, Fe, Cu, Mn, B, pH, EC, organic carbon); crop-wise fertiliser advice |
| Neem-coated urea | 2015 | Slower nitrification, less diversion |
| One Nation One Fertiliser ("Bharat" brand) | 2022 | Single brand for subsidised fertilisers |
| PM-PRANAM | Approved 2023 | Rewards states that reduce chemical fertiliser use compared with their past average, with part of the subsidy saved |
| World Soil Day | 5 December (UN) | Awareness of soil health |
Manures: Organic Sources of Nutrients
Manures (खाद) are organic materials of plant or animal origin. They supply nutrients in small amounts but greatly improve soil structure, water-holding capacity, cation exchange capacity and soil biological activity.
| Type | Examples | Typical nutrient content | Notes |
|---|---|---|---|
| Bulky organic manures | Farmyard manure (FYM) (गोबर खाद), compost | FYM roughly 0.5% N, 0.2% P₂O₅, 0.5% K₂O | Large quantities needed; main value is organic matter |
| Vermicompost (केंचुआ खाद) | Organic waste digested by earthworms, mainly Eisenia fetida (red wiggler) | Richer than FYM; contains growth-promoting substances | Faster than ordinary composting |
| Green manure (हरी खाद) | Fast-growing legumes ploughed into the soil while green: sunhemp (सनई), dhaincha (ढैंचा, Sesbania) | Adds N and organic matter | Sesbania rostrata forms stem nodules and suits waterlogged paddy fields |
| Green leaf manure | Leaves of Glyricidia, karanj, neem collected and incorporated | Adds N and organic matter | Useful where land cannot be spared for a green manure crop |
| Concentrated organic manures | Oilcakes (neem, castor, groundnut cake), bone meal, fish meal | Higher nutrient content | Neem cake also has pest-repellent and nitrification-inhibiting effects |
Biofertilisers
Biofertilisers (जैव उर्वरक) are preparations of living microorganisms that increase the availability of nutrients to plants, by fixing nitrogen, dissolving phosphorus or helping roots absorb nutrients. They are regulated as fertilisers under the Fertiliser Control Order, 1985.
| Biofertiliser | Organism type | How it helps | Crops |
|---|---|---|---|
| Rhizobium | Symbiotic bacterium in root nodules | Fixes nitrogen | Legumes only (gram, soybean, arhar, groundnut); crop-specific strains |
| Azotobacter | Free-living, aerobic bacterium | Fixes nitrogen; produces growth hormones | Cereals, vegetables, cotton |
| Azospirillum | Associative bacterium living around and in roots | Fixes nitrogen | Cereals and millets: maize, sorghum, ragi, rice |
| Acetobacter (Gluconacetobacter diazotrophicus) | Endophyte inside the plant | Fixes nitrogen | Sugarcane |
| Blue-green algae (cyanobacteria) | Free-living: Anabaena, Nostoc, Tolypothrix | Fix nitrogen in standing water | Paddy |
| Azolla | Water fern with the cyanobacterium Anabaena azollae living in its leaves | Fixes nitrogen; also used as cattle feed | Paddy |
| Phosphate-solubilising bacteria (PSB) | Bacillus, Pseudomonas | Dissolve fixed (insoluble) phosphate | All crops |
| Mycorrhiza (VAM/AM fungi) | Fungus forming a symbiosis with roots | Fungal threads extend the root system; improve phosphorus and water uptake | Most crops and tree nurseries |
| Potash- and zinc-solubilising bacteria | Various bacteria | Make K and Zn available | All crops |
Limitations: they are living products, so they have a short shelf life, are sensitive to heat and storage, are crop- or strain-specific (Rhizobium especially), and their effect depends on soil conditions. They supplement chemical fertilisers; they rarely replace them fully.
Integrated Nutrient Management (INM)
INM (एकीकृत पोषक तत्व प्रबंधन) combines chemical fertilisers + organic manures + biofertilisers + crop residues + legumes in rotation, based on soil testing, to supply nutrients in balance while keeping soil healthy.
graph LR
A["Soil test
(Soil Health Card)"] --> B["Balanced chemical
fertilisers (NPK + Zn, S)"]
A --> C["Organic manures
(FYM, vermicompost, green manure)"]
A --> D["Biofertilisers
(Rhizobium, PSB, BGA, Azolla)"]
A --> E["Legumes in rotation
and residue recycling"]
B --> F["INM: higher yield,
healthier soil, less waste"]
C --> F
D --> F
E --> F- Imbalanced fertilisation: urea-heavy, P and K short, micronutrients ignored
- Falling soil organic carbon: residue burning, little manure, intensive tillage
- Multi-nutrient deficiencies: zinc, sulphur and boron deficiencies widespread
- Acidification and salinisation: acid soils in high-rainfall regions (including parts of Chhattisgarh), salinity in canal-irrigated dry areas
- Policy bias: cheap urea, weak extension, soil tests not followed by action
- Way forward: INM, soil-test-based dosing, neem-coated and slow-release products, biofertilisers, green manuring, rational pricing of nutrients
Memory Tricks
Don’t Confuse
| This | vs | That |
|---|---|---|
| Soil fertility: nutrient supply | vs | Soil productivity: actual crop output |
| N deficiency: chlorosis of older leaves | vs | Fe deficiency: chlorosis of younger leaves |
| Liming: for acidic soils | vs | Gypsum: for alkaline/sodic soils |
| Rhizobium: symbiotic, legumes, root nodules | vs | Azotobacter: free-living; Azospirillum: associative, cereals |
| Azolla: a water fern | vs | Anabaena: the cyanobacterium inside it that fixes N |
| Manure: organic, bulky, improves soil structure | vs | Fertiliser: concentrated nutrient source, usually chemical |
| NBS (2010): covers P and K fertilisers | vs | Urea: outside NBS, fixed low price |
Common Mistakes
- “Azolla fixes nitrogen by itself.” Imprecise. The cyanobacterium Anabaena azollae in its leaves fixes nitrogen.
- “Rhizobium can be used for any crop.” False. It is specific to legumes, and to particular legume groups.
- “India mines its own potash.” False. MOP is wholly imported.
- “Khaira disease of rice is caused by a fungus.” False. It is a zinc deficiency disorder.
- “Neem coating adds nitrogen.” False. Neem coating slows nitrification and so reduces losses; it adds no nutrient worth counting.
- “Nano urea’s yield benefit is settled science.” Avoid this. Its yield claims have been questioned by independent trials; write “debated”.
Practice
All questions below are original practice questions written in CGPSC style. They are not previous-year questions.
Level 1: Foundation
- The nitrogen content of urea is ______ per cent.
- Khaira disease of rice is caused by deficiency of ______.
- Acidic soils are corrected by applying ______, and sodic soils by applying ______.
Answers
- 46 per cent.
- Zinc.
- Lime; gypsum.
Level 2: Prelims-style MCQs
Q1. Consider the following statements about biofertilisers:
- Azotobacter is a free-living nitrogen-fixing bacterium.
- Azolla is a cyanobacterium widely used in paddy fields.
- Mycorrhizal fungi mainly improve the uptake of phosphorus.
Which of the statements given above is/are correct?
- (a) 1 only
- (b) 1 and 3 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer and explanation
Q2. Match List I (Deficiency) with List II (Symptom or disorder):
| List I | List II |
|---|---|
| A. Nitrogen | 1. Whiptail of cauliflower |
| B. Zinc | 2. Yellowing of older leaves first |
| C. Molybdenum | 3. Scorching of leaf margins |
| D. Potassium | 4. Khaira disease of rice |
- (a) A-2, B-4, C-1, D-3
- (b) A-3, B-4, C-1, D-2
- (c) A-2, B-1, C-4, D-3
- (d) A-2, B-4, C-3, D-1
Answer and explanation
Q3. Neem coating of urea improves nitrogen-use efficiency mainly by:
- (a) Adding extra nitrogen from neem oil
- (b) Slowing the conversion of ammonium to nitrate in the soil
- (c) Increasing the solubility of urea
- (d) Killing nitrogen-fixing bacteria
Answer and explanation
Q4. Which of the following fertilisers is entirely imported into India because the country has no commercially viable reserves of its raw material?
- (a) Urea
- (b) Single super phosphate
- (c) Muriate of potash
- (d) Ammonium sulphate
Answer and explanation
Q5. Under the Soil Health Card Scheme, which of the following is not among the parameters tested?
- (a) Organic carbon
- (b) Electrical conductivity
- (c) Zinc
- (d) Soil microbial DNA
Answer and explanation
Level 3: Mains Answer Writing
Q1. (60 words) What are biofertilisers? Name two and state their limitations.
Model answer outline
- Living microorganisms that make nutrients available by fixing nitrogen, solubilising phosphate or extending root uptake.
- Examples: Rhizobium (legume nodules), Azolla–Anabaena (paddy), PSB, mycorrhiza.
- Limitations: short shelf life, heat-sensitive, crop/strain-specific, variable field results; they supplement, not replace, fertilisers.
Q2. (150–200 words) “India’s soils are hungry for balance, not for more fertiliser.” Discuss with reference to the NPK imbalance and integrated nutrient management.
Model answer outline
- Intro: Fertiliser use rose with the Green Revolution, but the response per kg has fallen; the use pattern is skewed towards nitrogen.
- The imbalance: benchmark N:P:K of 4:2:1 versus seasonal ratios of roughly 10:4:1 in 2023–24; cheap urea outside NBS (2010) versus costlier P and K.
- Scientific costs: Liebig’s law, so excess N cannot offset P, K, S or Zn deficits; N losses through volatilisation, leaching (groundwater nitrate) and denitrification (N₂O); falling soil organic carbon; widespread zinc and sulphur deficiencies.
- INM as the answer: soil-test-based doses (Soil Health Card, 2015); neem-coated and slow-release urea; FYM, vermicompost and green manure; biofertilisers; legumes in rotation; micronutrient correction.
- Chhattisgarh angle: acidic red-yellow and Bhata soils need liming and phosphorus; zinc deficiency (khaira) in intensive paddy; gauthan vermicompost under Godhan Nyay (2020) as an organic input experiment.
- Conclusion: Balance and soil biology, supported by rational nutrient pricing (PM-PRANAM incentives), not more urea, will raise productivity.
Quick Revision Table
| Fact | Detail |
|---|---|
| Essential elements | 17: C, H, O + N, P, K + Ca, Mg, S + Fe, Mn, Zn, Cu, B, Mo, Cl, Ni |
| Law of the Minimum | Liebig: the scarcest nutrient limits growth |
| Mobile-nutrient deficiency | Older leaves first (N, P, K) |
| Urea / DAP / SSP / MOP | 46% N / 18-46 / 16% P₂O₅ / 60% K₂O |
| MOP | Wholly imported |
| N losses | Volatilisation, leaching, denitrification (N₂O), runoff |
| Benchmark NPK | 4:2:1 |
| NBS | 2010; urea excluded |
| Neem-coated urea | Mandatory 2015; nitrification inhibitor |
| Nano urea | IFFCO, June 2021; nano DAP 2023; yield claims debated |
| Soil Health Card | 19 Feb 2015, Suratgarh (Rajasthan); 12 parameters |
| PM-PRANAM | 2023; incentive to states to cut chemical fertiliser |
| Acid / sodic soil | Lime / gypsum |
| Khaira (rice) | Zinc deficiency |
| Vermicompost worm | Eisenia fetida |
| Green manures | Sunhemp, dhaincha; Sesbania rostrata stem nodules |
| Paddy biofertilisers | BGA (Anabaena, Nostoc), Azolla–Anabaena azollae |
| Sugarcane biofertiliser | Acetobacter (Gluconacetobacter) |
Key Takeaways
- Seventeen elements are essential; the scarcest one limits yield (Liebig).
- Nitrogen is cheap and overused; much of it is lost to air and water, causing pollution and greenhouse gases.
- Neem coating, slow-release forms, soil testing and balanced NPK raise efficiency; nano urea is promising but debated.
- Manures build soil structure and carbon; biofertilisers add nitrogen and phosphorus biologically but are living, specific and short-lived.
- Integrated Nutrient Management is the scientific synthesis Mains answers should end on.
What’s Next
- Organic and Natural Farming: farming without synthetic inputs
- Pest Management and IPM: protecting the crop you have fed
- Soil Erosion and Conservation: keeping the soil in place
- Pollution and Waste Management: nitrate and eutrophication in context