Fertiliser Compounds
Identifying, explaining, and applying fertiliser chemistry
📋 Quick Reference — Key Fertiliser Facts
| Compound | Formula | Nutrients | Solubility |
|---|---|---|---|
| Ammonium nitrate | NH₄NO₃ | N (×2) | ✓ Soluble |
| Ammonium sulfate | (NH₄)₂SO₄ | N, S | ✓ Soluble |
| Potassium nitrate | KNO₃ | N, K | ✓ Soluble |
| Ammonium phosphate | (NH₄)₃PO₄ | N, P | ✓ Soluble |
| Calcium carbonate | CaCO₃ | None (NPK) | ✗ Insoluble |
| Sodium chloride | NaCl | None (NPK) | ✓ But toxic |
Section A: The Compounds
10 marksName two types of salts commonly used as agricultural fertilisers.
💡 Hint: Think about the nitrogen-containing anions plants need most.
Name the three essential elements supplied by NPK fertilisers and state the role of nitrogen (N) in plant growth.
💡 Hint: N, P, K — what does each letter stand for?
Give the chemical name and formula of ONE ammonium salt used as a fertiliser.
Ammonium Nitrate — NH₄NO₃
💡 Hint: The Haber process makes NH₃ — what salt does it react with to make?
Which of the following is NOT commonly used as a fertiliser?
💡 Hint:
Ammonium nitrate (NH₄NO₃) is a widely used fertiliser. State TWO reasons why it is particularly effective as a nitrogen source compared to other nitrogen compounds.
💡 Hint: Look at its formula — how many nitrogen atoms does it contain?
Section B: Properties
10 marksWhy is it essential that fertilisers are soluble in water?
💡 Hint: How do plant roots actually absorb nutrients from soil?
Explain why calcium carbonate would be a poor fertiliser even if it supplied the correct nutrients.
💡 Hint: Think about its solubility from the solubility rules.
A student claims that any nitrogen-containing compound could be used as a fertiliser. Evaluate this claim by giving ONE example that supports it and ONE that disproves it.
💡 Hint: Nitrogen gas (N₂) exists — could you add that to soil?
Excess fertiliser application can cause eutrophication. Explain the sequence of events from fertiliser run-off to a decrease in dissolved oxygen in the water.
💡 Hint: Follow the chain: fertiliser → algae → bacteria → oxygen.
Which property is MOST important when selecting a compound for use as a fertiliser?
💡 Hint:
Section C: Examples & Application
16 marksWhich of the following would make the BEST fertiliser?
💡 Hint:
A student tests three white powders (A, B, C) to identify which is a fertiliser. Powder A: insoluble in water. Powder B: soluble, turns damp red litmus blue when heated with NaOH. Powder C: soluble, no gas produced with NaOH. Which powder is most likely the fertiliser? Explain your reasoning.
💡 Hint: What gas turns damp red litmus blue? What does that tell you about the anion?
Potassium nitrate (KNO₃) is used as a fertiliser. State which TWO essential nutrients it provides for plant growth.
💡 Hint: Look at both ions in the formula.
Describe how you would prepare a pure, dry sample of ammonium sulfate starting from dilute sulfuric acid and aqueous ammonia. Include observations and relevant equation.
💡 Hint: This is a neutralisation reaction — acid + alkali → salt + water.
A farmer has a field with nitrogen-deficient soil. Evaluate the use of ammonium nitrate (NH₄NO₃) as a fertiliser for this field. In your answer, include: (i) why ammonium nitrate is effective, (ii) the environmental risk, and (iii) one alternative approach.
💡 Hint: Think: chemical properties, eutrophication, and sustainable alternatives.
🎓 Examiner's Golden Rule:
"A good fertiliser must be: (1) soluble — so it dissolves in soil water, (2) a source of N, P, or K — so it feeds the plant, (3) not toxic — so it doesn't kill the crop."
Cambridge IGCSE Chemistry 0620 • Topic 10.2.1 • Fertiliser Compounds
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