NPK fertilisers are manufactured using INTEGRATED INDUSTRIAL PROCESSES — several reactions are linked.
KEY STARTING MATERIAL: AMMONIA (from Haber process)
NITROGEN COMPOUNDS:
Ammonium nitrate (NH₄NO₃): ammonia + nitric acid.
Ammonium sulfate: ammonia + sulfuric acid.
Ammonium phosphate: ammonia + phosphoric acid.
PHOSPHORUS COMPOUNDS:
Phosphoric acid made from phosphate rock + sulfuric acid.
Superphosphate: calcium phosphate (from rock) treated with H₂SO₄.
POTASSIUM COMPOUNDS:
Potassium chloride or potassium sulfate — mined from natural mineral deposits.
No synthesis needed — extracted and purified.
BLENDING:
Different salts blended in specific ratios to give the required NPK formulation.
Different crops and soil types need different N:P:K ratios.
Fertiliser bags labelled with N:P:K ratio (e.g. 20:10:10).
Environmental Issues with Fertilisers
EUTROPHICATION:
Excess fertiliser washed into waterways by rain (LEACHING).
Nitrogen and phosphorus stimulate algal growth (algal bloom).
Algae block light → aquatic plants die.
Bacteria decompose dead plants → use up oxygen.
Fish and other organisms die — DEAD ZONE.
NITRATE IN DRINKING WATER:
Excess nitrate in groundwater can contaminate drinking water supplies.
High nitrate in infants: 'blue baby syndrome' (methaemoglobinaemia) — nitrate reduces blood oxygen.
EU limit: 50 mg/L nitrate in drinking water.
ACID RAIN:
Ammonium fertilisers can be oxidised in soil to nitrate → releases H⁺ ions → soil acidification.
REDUCING ENVIRONMENTAL IMPACT:
Precision agriculture — apply only what is needed.
Slow-release fertilisers — coated to release gradually.
Organic farming — using manure instead of synthetic fertilisers.
Buffer zones — vegetated strips near waterways to absorb runoff.
⚠️ Common Mistake
NPK fertilisers contain nitrogen, phosphorus AND potassium — not just nitrogen. The Haber process makes AMMONIA, which is then used to make ammonium salts for fertilisers. Potassium comes from MINING, not synthesis. Eutrophication is caused by excess nutrients (N and P) entering waterways, not just nitrates.
N = proteins/leaves, P = roots/ATP, K = enzymes/photosynthesis. NPK fertilisers: ammonium salts (from NH₃ + acids), potassium from mining. Manufacturing: integrated processes using Haber ammonia + acids. Environmental issues: eutrophication (leaching → algal bloom → deoxygenation), nitrate in drinking water. Precision farming reduces impact.
🎯 Matching Activity — NPK Fertilisers
Match each nutrient to its role in plant growth and a deficiency symptom. — drag the symbols on the right to match the component names on the left.
Nitrogen (N)
Drop here
Phosphorus (P)
Drop here
Potassium (K)
Drop here
Eutrophication
Drop here
Enzyme function and photosynthesis — deficiency: poor fruit, brown leaf edges
Fertiliser leaching into waterways → algal bloom → deoxygenation → fish death
Protein synthesis → leaf and stem growth — deficiency: yellow leaves
Root development and ATP energy transfer — deficiency: poor roots, purple leaves
⭐ Higher Tier Only
Write equations for the Ostwald process (oxidation of NH₃ → NO → NO₂ → HNO₃). Write equations for production of ammonium salts. Calculate the percentage by mass of N, P or K in a fertiliser compound. Evaluate strategies to reduce eutrophication from fertiliser use with reference to chemical and agricultural approaches.
🔬 Triple Science Only
Production and uses of NPK fertilisers (4.10.4.2) is chemistry-only — not in Combined Science.
🎯 Test Yourself
Question 1 of 2
1. How is ammonium nitrate fertiliser manufactured?
2. What is eutrophication and how does it lead to fish death?
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