Production & Use of Fertilisers (AQA GCSE Chemistry)

Role of NPK fertilisers in plant growth

• Compounds containing nitrogen, potassium and phosphorus are used as fertilisers to increase crop yields

• NPK fertilisers are formulations containing appropriate ratios of all three elements

• From these three essential elements:

  • Nitrogen promotes healthy leaves,

  • Potassium promotes growth, healthy fruit and flowers

  • Phosphorus promotes healthy roots

• A distinct advantage of artificial fertilisers is that they can be designed for specific needs whereas in natural fertilizers, such seaweed or manure, the proportions of elements cannot be controlled

• Fertiliser compounds contain the following water soluble ions:

  • Ammonium ions, NH₄⁺ and nitrate ions, NO₃^–, which are sources of soluble nitrogen

  • Phosphate ions, PO₄^3-, which are a source of soluble phosphorus

• Most common potassium compounds dissolve in water to produce potassium ions, K⁺

Use of ammonia in manufacturing fertilisers

• Ammonia is an alkaline substance and neutralises acids producing a salt and water

• The salt it produces contains the ammonium ion, NH₄⁺, which is a component of several fertilisers

• Ammonia also undergoes oxidation to produce nitric acid, HNO₃

• Nitric acid is used as the source of the nitrate ion, NO₃^–, which is another important ion found in fertilisers

• Ammonium nitrate, a fertiliser and one of the most important ammonium salts, is made by reacting ammonia with nitric acid:

NH₃ (aq) + HNO₃ (aq) → NH₄NO₃ (aq)

• It is prepared on large scale industrial proportions but can also be prepared in the laboratory using a different method

• In the laboratory it is prepared by titrating ammonia with sulfuric acid:

2NH₃ (aq) + H₂SO₄ (aq) → (NH₄)₂SO₄ (aq)

Comparing lab and industrial production of ammonium sulfate

Use of phosphate rock

• The Earth’s crust also contains useful minerals which are useful raw materials for making fertilisers

• Phosphate rocks are mined and then treated to produce various water soluble compounds that can be used in fertilisers

• The rock itself is insoluble in water so it is usually reacted with:

  • Nitric acid - phosphoric acid and calcium nitrate are produced

    • The phosphoric acid  is neutralised with ammonia forming ammonium phosphate

  • Sulfuric acid - a mixture of calcium phosphate and calcium sulfate is produced 

    • This mixture is known as single superphosphate

  • Phosphoric acid - calcium phosphate is produced

    • This is known as triple superphosphate

• Potassium chloride and potassium sulfate are also mined

  • They are easier to extract as potassium compounds are water soluble