Fertilizers are chemical substances — natural or synthetic — that supply one or more essential nutrients to soil, enabling plants to grow. The three macronutrients every plant needs in bulk are nitrogen (N), phosphorus (P), and potassium (K). When all three are present in one formulation, you call it an NPK fertilizer.
Here is why this matters beyond agriculture: the industrial processes that manufacture fertilizers are among the most consequential chemical processes ever scaled up. The Haber process for ammonia synthesis, the Contact process for sulphuric acid, and the treatment of phosphate rock all belong to the same family of large-scale industrial chemistry that NDA GAT tests systematically.
Think of fertilizers this way — soil is a bank account, and every crop withdrawal depletes nutrients. Fertilizers are deposits. The chemistry question is always: what chemical form of a nutrient can a plant actually absorb? Plants cannot absorb molecular nitrogen N₂ from the air despite it making up 78% of the atmosphere. They absorb nitrogen in ionic form: NH₄⁺ (ammonium) or NO₃⁻ (nitrate). That is precisely why the industrial fixation of nitrogen — converting inert N₂ into reactive compounds — is the foundational chemistry of the entire fertilizer industry.
Phosphorus in soil exists largely as insoluble calcium phosphate Ca₃(PO₄)₂. Plants cannot absorb it in that form. Converting it to water-soluble forms like Ca(H₂PO₄)₂ (monocalcium phosphate) is the chemistry behind superphosphate manufacture.
Analogy: think of rock phosphate as a locked vault of phosphorus. Sulphuric acid is the key that unlocks it into plant-available form.
Fertilizers are classified by nutrient content and by origin:
By nutrient type:
By origin:
1. Ammonia via the Haber Process
The feedstock for almost all nitrogen fertilizers is ammonia NH₃. The Haber process synthesises it from nitrogen and hydrogen:
N₂ + 3H₂ ⇌ 2NH₃ (ΔH = −92 kJ/mol)
Conditions: temperature ~450°C, pressure ~200 atm, iron catalyst with promoters (K₂O, Al₂O₃).
Look — this reaction is exothermic, so lower temperature favours the product. But too low a temperature slows the rate. The 450°C is a compromise between yield and rate. High pressure also favours the product side (fewer moles of gas on the right: 2 vs. 4 on the left, Le Chatelier's principle).
2. Urea — CO(NH₂)₂
The highest-nitrogen solid fertilizer: 46% N by mass. Manufactured by reacting liquid ammonia with carbon dioxide under pressure:
2NH₃ + CO₂ → NH₂COONH₄ → CO(NH₂)₂ + H₂O
In soil, urea is hydrolysed by urease enzyme: CO(NH₂)₂ + H₂O → 2NH₃ + CO₂, releasing ammonium ions available to plants.
3. Ammonium Nitrate — NH₄NO₃
Prepared by neutralising ammonia with nitric acid: NH₃ + HNO₃ → NH₄NO₃
Contains 34% N. Both ammonium (NH₄⁺) and nitrate (NO₃⁻) forms of nitrogen are directly absorbed. Also used as an oxidiser in explosives — dual-use chemistry that NDA can test.
4. Ammonium Sulphate — (NH₄)₂SO₄
Contains 21% N. Also supplies sulphur. Produced as a by-product of coke ovens and caprolactam manufacture.
5. Calcium Cyanamide (Nitrolim) — CaCN₂
Manufactured by heating calcium carbide with nitrogen gas:
CaC₂ + N₂ → CaCN₂ + C (at ~1000°C)
In moist soil: CaCN₂ + 3H₂O → CaCO₃ + 2NH₃
The ammonia is then nitrified to nitrate. Key classification points: Nitrolim is a nitrogenous fertilizer only (no P or K, so not NPK). It is inorganic/synthetic (not organic). It also has herbicidal properties at high doses.
1. Superphosphate of Lime
Rock phosphate Ca₃(PO₄)₂ is insoluble. Treating it with concentrated sulphuric acid makes it soluble:
Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄
The product is a mixture of calcium dihydrogen phosphate (water-soluble, plant-available) and calcium sulphate (gypsum, largely inert). Contains about 16–20% P₂O₅ equivalent.
2. Triple Superphosphate (TSP)
If you use phosphoric acid H₃PO₄ instead of sulphuric acid:
Ca₃(PO₄)₂ + 4H₃PO₄ → 3Ca(H₂PO₄)₂
No gypsum by-product. Higher phosphorus content (~46% P₂O₅). The word "triple" refers to roughly three times the phosphorus content of ordinary superphosphate.
3. Nitrophosphate (Odda Process)
Using nitric acid on rock phosphate:
Ca₃(PO₄)₂ + 4HNO₃ → Ca(H₂PO₄)₂ + 2Ca(NO₃)₂
Products contain both phosphate and nitrate, making this a combined N–P fertilizer. This is why NDA options often include "nitric acid" as a distractor — it IS used in fertilizer chemistry, just for a different product than superphosphate.
Muriate of potash (KCl) and sulphate of potash (K₂SO₄) are the main ones. Mostly mined from mineral deposits (sylvite, carnallite) rather than synthesised. NDA rarely goes deep here, but you should know the names.
The fertilizer industry drives demand for:
S + O₂ → SO₂, then 2SO₂ + O₂ ⇌ 2SO₃, absorbed into oleum, diluted) — used for superphosphateThese industrial acid processes are connected to fertilizer questions. If a question gives you "nitric acid + rock phosphate", it's pointing to nitrophosphate, not superphosphate.
The word "superphosphate" contains no hint of the acid used — so memorise by alliteration: Super-SULPHATE uses Sulphuric acid. The 'S' sound runs through all three: Superphosphate, Sulphuric, Sulphate (the by-product CaSO₄). When you see the options sodium hydroxide / sulphuric acid / HCl / nitric acid, eliminate anything that is not an acid first (NaOH is out), then recall the S–S–S chain. This reduces a 4-option question to a 2-option question in under 5 seconds vs. trying to recall the equation from scratch (~30 seconds).
Nitrolim (CaCN₂) questions always arrive as multi-statement formats. Use the NI-NO grid before reading options: NI = Nitrogen only (not NPK); NO = Not Organic (it is synthetic/inorganic). Any statement calling it NPK → wrong. Any statement calling it organic → wrong. Statements about "nitrogen gas + calcium carbide" and "releases ammonia in soil" → correct. This eliminates two of four statements in under 10 seconds, leaving you to choose between options containing only statements I and II. Standard elimination: 45 seconds; this grid: 12 seconds.
Single superphosphate uses H₂SO₄ and gives ~16–20% P₂O₅. Triple superphosphate uses H₃PO₄ (phosphoric acid — three H's) and gives ~46% P₂O₅. The "three H's" in phosphoric acid → triple product. If an option says "phosphoric acid on rock phosphate", it is TSP not ordinary superphosphate. This pattern prevents swapping the two in option-matching questions — 0 extra steps vs. writing out both equations.
Memorise Haber process as 4–2–Fe: 400–500°C (use 450 as midpoint), 200 atm pressure, Fe catalyst. Four–two–iron: you can say it in one breath. Standard textbook recall requires scanning three separate facts; this compresses them into a 3-item code you can reproduce in under 3 seconds.
NDA frequently offers "nitric acid" as a distractor in superphosphate questions. Burn this in: nitric acid → nitrophosphate (because the product contains nitrate). Sulphuric acid → superphosphate (product contains sulphate by-product). The acid name predicts the by-product name. Eliminates the nitric acid option in under 3 seconds without writing any equation.
When you see a fertilizer question in the NDA GAT, run this decision tree:
Step 1 — Identify the fertilizer name. Is it superphosphate? Nitrolim? Urea? Ammonium nitrate?
Step 2 — If it asks about manufacture/preparation:
Step 3 — If it is a multi-statement question about Nitrolim: Apply NI-NO grid immediately (not NPK, not organic).
Step 4 — Eliminate acids that don't fit: NaOH is a base, not an acid — immediately out of any "acid treatment" question. HCl is not used industrially for phosphate fertilizers.
Step 5 — If % nitrogen content appears: Urea = 46% (highest), ammonium nitrate = 34%, ammonium sulphate = 21%. Order: U > AN > AS.
This framework handles 90% of fertilizer MCQs without writing a single equation on paper.
Why this question: This is the single most repeated fertilizer question in recent NDA papers. The distractor "nitric acid" catches candidates who confuse nitrophosphate with superphosphate.
Solving path: The question asks for the reaction that makes superphosphate of lime. Eliminate option (a) immediately — NaOH is a base, not an acid, and bases don't dissolve rock phosphate to release plant-available phosphate. Option (c) HCl is not used commercially for phosphate manufacture. Option (d) nitric acid does react with calcium phosphate, but produces nitrophosphate, a different fertilizer. Option (b) sulphuric acid is correct: Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄. The by-product calcium sulphate (gypsum) is the signature of superphosphate manufacture.
Why this question: Identical in content to the previous one — NDA repeated this question across sets. If you solved the first, this is a free mark.
Solving path: Exact same logic applies. Note that option (d) nitric acid is a recurring trap — nitric acid is used in the Odda/nitrophosphate process to make a combined N-P fertilizer, which is categorically different from superphosphate. The correct answer is sulphuric acid. Recognising repeated questions in NDA is itself a time-saving skill — once you have confirmed it is the same question, select option (b) without rereading.
Why this question: Multi-statement questions on Nitrolim test whether you have memorised all four properties: preparation route, soil behaviour, nutrient class, and organic vs. inorganic classification.
Solving path: Apply the NI-NO grid. Statement III says Nitrolim is an NPK fertilizer — wrong, it supplies only nitrogen (NI = nitrogen only). Statement IV says it is an organic fertilizer — wrong, it is a synthetic inorganic compound (NO = not organic). That eliminates options (b), (c), and (d) which contain III or IV. Statement I is correct: CaC₂ + N₂ → CaCN₂ + C. Statement II is correct: in soil, CaCN₂ hydrolyses and eventually releases NH₃. Answer is option (a), I and II only.
Confusing superphosphate with nitrophosphate. Sulphuric acid → superphosphate (by-product: CaSO₄). Nitric acid → nitrophosphate (by-product: Ca(NO₃)₂). The acid name predicts the product family. Do not swap these in option-matching.
Calling Nitrolim an NPK fertilizer. CaCN₂ contains only N (and Ca). There is no phosphorus, no potassium. NPK requires all three. This traps candidates who assume "complex compound = multiple nutrients".
Calling Nitrolim an organic fertilizer. Organic fertilizers are carbon-based, derived from biological material (compost, manure, bone meal). Nitrolim is an industrially synthesised inorganic compound, despite the fact that the cyanamide group CN₂²⁻ contains carbon. The carbon is structural, not the source of organic matter.
Reversing the Haber process Le Chatelier argument. Exothermic reaction → low temperature favours product. But rate also matters — the industrial compromise is ~450°C, not the thermodynamically optimal lower temperature. Questions that ask "why not use lower temperature?" expect you to cite rate/catalyst efficiency, not equilibrium position alone.
Mixing up % nitrogen content. Urea has the highest nitrogen content at 46%, not ammonium nitrate (34%) or ammonium sulphate (21%). A question asking "which fertilizer has the highest nitrogen percentage" tests this directly.
Assuming all acid–phosphate reactions give superphosphate. Only H₂SO₄ gives superphosphate. H₃PO₄ gives triple superphosphate. HNO₃ gives nitrophosphate. Three different acids, three different products — treat each as a distinct reaction, not variations on one theme.