AQA GCSE Chemistry

Revision Notes

10.4.4 Production & Use of Fertilisers

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, PO43-, which are a source of soluble phosphorus
  • Most common potassium compounds dissolve in water to produce potassium ions, K+

Role of NPK Fertilisers in Plant Growth Table, downloadable IGCSE & GCSE Chemistry revision notes

Exam Tip

Fertilisers must be water soluble so the nutrients they provide can be effectively absorbed and transported by the plant.

Use of Ammonia in Manufacturing Feritlisers

  • 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)

Use of Ammonia in Manufacturing Fertilisers Table, downloadable IGCSE & GCSE Chemistry revision notes

Exam Tip

Notice that when writing ammonia solution as NH3 (aq), water does not appear to be a product of the neutralisation reaction. However, ammonia solution may also be written as, NH4OH (aq), ammonium hydroxide, in which case water is produced:

NH4OH (aq) + HNO3 (aq) → NH4NO3 (aq)  +  H2O (l)

Either formula may be used to show the reactions.

Use of Phosphate Rock

  • The Earth’s crust also contains useful minerals which are useful raw materials for making fertilisers
  • Phosphate rocks contain potassium chloride and potassium sulfate as they provide potassium
  • The rock itself is insoluble in water to it is usually reacted with acid to produce useful compounds with water soluble compounds

Exam Tip

Phosphate rocks can supply potassium and phosphorus compounds only. Nitrogen containing compounds have to be added to produce a fertiliser.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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