Production and uses of NPK fertilisers — AQA GCSE Chemistry
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Production and uses of NPK fertilisers explained
NPK fertilisers supply nitrogen, phosphorus and potassium.
Read the full explanation
Potassium chloride and potassium sulfate are mined as soluble potassium salts, so they dissolve in soil water and roots absorb the potassium ions. Phosphate rock is also mined, but it consists mainly of calcium phosphate, which has very low solubility. Insoluble compounds cannot release phosphate ions quickly enough into soil solution for crops to absorb, so spreading phosphate rock directly gives little available phosphorus. To make it useful, the rock must be treated with an acid to convert the insoluble calcium phosphate into soluble phosphate salts. For example, treating phosphate rock with sulfuric acid produces a mixture containing calcium sulfate and soluble calcium phosphate, which plants can take up.
Your focus
- Identify potassium chloride, potassium sulfate and phosphate rock as materials obtained by mining.
- Explain why phosphate rock cannot be used directly as a fertiliser in terms of solubility.
- Describe how treatment with acid converts phosphate rock into soluble salts suitable for fertilisers.
Production and uses of NPK fertilisers exam tips
Marking Points
- Potassium chloride and potassium sulfate are obtained by mining and are soluble potassium salts used to supply the K in NPK fertilisers.
- Phosphate rock is obtained by mining and is mainly insoluble calcium phosphate.
- A fertiliser must release its nutrient ions into soil solution so plant roots can absorb them.
- Phosphate rock is too insoluble to release phosphate ions at a useful rate, so it is not applied directly as a fertiliser.
- Treating phosphate rock with an acid converts insoluble calcium phosphate into soluble phosphate salts that plants can absorb.
Examiner Tips
- 💡Link each named substance to its nutrient: potassium chloride and potassium sulfate supply potassium, while phosphate rock supplies phosphorus after treatment.
- 💡Use the word soluble when explaining why phosphate rock must be treated before use.
- 💡If asked why a substance is not a fertiliser, refer to the availability of nutrient ions to roots rather than to the element being absent.
Common Mistakes
- Thinking phosphate rock contains no phosphorus: it does contain phosphorus, but in an insoluble form that plants cannot readily absorb.
- Assuming all mined minerals are directly usable as fertilisers: solubility in soil water is the key requirement, and phosphate rock fails this test.
- Confusing the roles of potassium and phosphorus: potassium salts supply K, while phosphate rock is a source of P after chemical treatment.