Production & Use of Fertilisers (AQA GCSE Chemistry)

• 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⁺

• 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

• 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