NPK fertilisers
NPK fertilisers
Plants need the elements nitrogen (N), phosphorus (P) and potassium (K) to grow well. are of compounds containing these elements in suitable amounts. They replace elements the crops take from the soil, which improves farm productivity.
The compounds must be so plants can absorb them through their roots.
Key terms
Where the elements come from
- Nitrogen: from the Haber process is used to make and . For example: NH₃ + HNO₃ → NH₄NO₃ (ammonium nitrate). The ions are NH₄⁺ and NO₃⁻.
- Potassium: potassium chloride and potassium sulfate are mined and can be used directly.
- Phosphorus: is mined, but it is insoluble, so it is treated with acids:
- with nitric acid → phosphoric acid and calcium nitrate
- with sulfuric acid → single superphosphate (a mixture of calcium phosphate and calcium sulfate)
- with phosphoric acid → triple superphosphate (calcium phosphate)
Key terms
Making ammonium sulfate
Ammonium sulfate is a nitrogen fertiliser:
2NH₃ + H₂SO₄ → (NH₄)₂SO₄
| In the lab | In industry | |
|---|---|---|
| Method | ammonia solution with dilute sulfuric acid, then | A continuous process of several stages |
| Starting materials | Bought from chemical suppliers | Made from raw materials (air, natural gas, sulfur, water) |
| Scale | Small batches, a few grams | Huge amounts, many tonnes |
| Equipment | Cheap glassware | Expensive plant, but automated |