Every harvest removes nitrogen, phosphorus and potassium from the soil. NPK fertilisers put them back, and the nitrogen usually comes as an ammonium salt or a nitrate made from ammonia and an acid.
Ammonium salts (such as ammonium nitrate and ammonium sulfate) and nitrates (such as potassium nitrate) are used as fertilisers to increase crop yields. NPK fertilisers provide the three elements plants need most: nitrogen (N), phosphorus (P) and potassium (K). Ammonium fertiliser salts are made by neutralising ammonia with an acid, for example NH3 + HNO3 → NH4NO3. Extended: plants need nitrogen to make proteins for growth (healthy leaves), phosphorus for root growth, and potassium for flowers and fruit and resistance to disease.
Add an alkali to an ammonium salt in the simulator and test the gas given off. This is why farmers never spread lime at the same time as an ammonium fertiliser: the nitrogen escapes as ammonia.
| Fertiliser salt | Formula | Elements supplied | % nitrogen by mass |
|---|---|---|---|
| ammonium nitrate | NH4NO3 | N | 35.0 |
| ammonium sulfate | (NH4)2SO4 | N | 21.2 |
| ammonium phosphate | (NH4)3PO4 | N and P | 28.2 |
| potassium nitrate | KNO3 | K and N | 13.9 |
Worked example: ammonium nitrate, Mr = (2 × 14) + (4 × 1) + (3 × 16) = 80. It contains two nitrogen atoms, so % N = (28 ÷ 80) × 100 = 35.0%. An NPK fertiliser is a mixture of salts such as these, blended so that it supplies all three elements.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which three elements are supplied by an NPK fertiliser?
Which compound is used as a nitrogen fertiliser?
Which acid reacts with ammonia to make ammonium nitrate?
Which fertiliser supplies both potassium and nitrogen?
Ammonia is an alkali, so it neutralises acids to give ammonium salts. Only one product forms: there is no water in these equations.
Extended. Each element has its own job in the plant. A deficiency shows up in a different part of the plant.
| Element | Needed for | Sign of deficiency |
|---|---|---|
| nitrogen (N) | making proteins; growth of stems and healthy green leaves | stunted growth, yellow leaves |
| phosphorus (P) | root growth | poor roots, purple-tinged leaves |
| potassium (K) | flowers and fruit; resistance to disease | poor fruit, yellow leaf edges |
Plants take up ions such as nitrate and ammonium dissolved in soil water, not nitrogen gas from the air, which is why the 78% nitrogen in air does not feed most crops. Fertilisers must be soluble. And too much fertiliser washes into rivers, where nitrates and phosphates cause deoxygenation of the water.
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(Extended) Why do plants need nitrogen?
What is the percentage by mass of nitrogen in ammonium sulfate, (NH4)2SO4? (Ar: H = 1, N = 14, O = 16, S = 32)
Which is the balanced equation for making ammonium sulfate?
A farmer spreads calcium hydroxide (lime) on a field at the same time as ammonium nitrate. Why is this a mistake?
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