Introduction to Fertilisers
Cambridge IGCSE Chemistry 0620 · Agriculture & Industry
🏦 The Soil Bank Account
Every time a farmer harvests wheat, they make a withdrawal from the soil's nutrient account. The wheat took nitrogen from the ground to build its proteins — and walked away with it. Eventually, the soil goes bankrupt. Fertilisers are the cash deposits that keep the soil wealthy enough to grow the next crop.
📖 What Are Fertilisers — Exactly?
Definition (Cambridge Mark Scheme): Fertilisers are substances added to soil to replace elements removed by crops when they are harvested.
Plants need nitrogen (N) to make proteins, DNA, and chlorophyll. Without N, leaves turn yellow and growth stops. The most important fertiliser compounds supply this nitrogen in a form that roots can actually drink.
🔬 Root Zoom: Watch Ions Travel
Plants don't eat solid food — they drink ions dissolved in water. Follow the NH₄⁺ and NO₃⁻ ions from granule to root tip.
NH₄NO₃ sits as solid white granules on the soil surface.
⚡ The Formula Check: Double Nitrogen Power
Ammonium Nitrate is a chemist's masterpiece. Look carefully at the formula — nitrogen hides in two separate locations:
NH₄⁺ (Ammonium ion)
1 nitrogen atom in the cation. Roots absorb and use for amino acid synthesis.
NO₃⁻ (Nitrate ion)
1 nitrogen atom in the anion. Highly mobile in soil water, absorbed instantly.
💧 The Solubility Gatekeeper
Why don't we just grind up nitrogen-rich rocks and sprinkle them on fields? Select to find out:
❌ Why Insoluble = Useless
Plants absorb nutrients through root hair cells by osmosis and active transport. These processes only work with dissolved ions in the soil water. A rock sitting in the ground is as useful to a plant as a locked bank vault is to someone who needs cash. The ions are trapped and inaccessible.
↳ Topic 7.3 Rule: All nitrates are soluble. All ammonium salts are soluble. That's why we use them!
🎯 Compound Selector Challenge
A farmer needs to add nitrogen to depleted soil. Which compound is the best fertiliser?
📋 Key Fertiliser Compounds
| Compound | Formula | Type | N% |
|---|---|---|---|
| Ammonium Nitrate | NH₄NO₃ | Ammonium salt | 35% |
| Ammonium Sulfate | (NH₄)₂SO₄ | Ammonium salt | 21% |
| Ammonium Phosphate | (NH₄)₃PO₄ | Ammonium salt | 28% + P |
| Potassium Nitrate | KNO₃ | Nitrate salt | 14% + K |
📌 Core Summary (Exam Ready)
✅ Fertilisers replace elements lost when crops are harvested.
✅ Plants need nitrogen to make proteins and grow.
✅ Ammonium salts (e.g. NH₄NO₃, (NH₄)₂SO₄) and nitrates are used.
✅ These compounds are chosen because they are highly soluble in water.
✅ Solubility means ions can be absorbed by plant roots from soil water.
✅ NH₄NO₃ contains nitrogen in two forms: NH₄⁺ and NO₃⁻ — maximum efficiency.
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