Fertilisers
NPK fertilisers and the elements plants need.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 10.2.1State the elements supplied by NPK fertilisers and why plants need them.
4 minute read
Fertilisers
Plants make their own sugars by photosynthesis, but sugars alone cannot build every part of a plant. Proteins, DNA and cell membranes all need extra elements that a plant can only get from the soil through its roots — and intensive farming removes these elements faster than natural processes replace them, so farmers add them back as fertilisers.
The NPK elements
Nitrogen, Phosphorus and Kpotassium are the three elements added in bulk fertilisers, each for a different job:
- Nitrogen is needed to make proteins, which are essential for cell growth — a nitrogen-deficient plant grows slowly and has yellowing leaves.
- Phosphorus (as phosphates) is needed for healthy root growth and for respiration.
- Potassium helps enzymes work and improves resistance to disease, and helps flowering and fruiting.
Making NPK fertilisers
Ammonium salts and nitrates (e.g. ammonium nitrate, NH₄NO₃, which conveniently supplies nitrogen twice over — once from the ammonium ion and once from the nitrate ion) are manufactured by neutralising ammonia with the appropriate acid, e.g. ammonia + nitric acid → ammonium nitrate. Industrially, ammonia itself is made by the Haber process (N₂ + 3H₂ ⇌ 2NH₃).
Think of it like this
NPK fertiliser is like a multivitamin for soil: photosynthesis supplies the plant's "calories" (sugars), but N, P and K are the specific nutrients the plant cannot make for itself and must absorb through its roots.
Worked examples
Method, step by step
Name the acid and alkali needed to manufacture ammonium sulfate fertiliser, and write a word equation for the reaction.
- 1Ammonium salts are made by neutralising ammonia with the corresponding acid.
- 2For a sulfate salt, the acid is sulfuric acid.
- 3Word equation: ammonia + sulfuric acid → ammonium sulfate.
Sulfuric acid and ammonia solution; ammonia + sulfuric acid → ammonium sulfate.
Common misconceptions
- Thinking fertilisers are a plant's food in the way sugar is — plants make their own food by photosynthesis; fertilisers supply mineral elements the plant cannot photosynthesise.
- Mixing up which element does what — nitrogen is for leafy/protein growth, phosphorus is for roots, potassium is for flowering/fruiting and disease resistance.
In the exam
- If asked why crop yields fall without fertiliser, the answer is that repeated harvesting removes N, P and K from the soil faster than natural processes can replace them.
- Ammonium nitrate is a favourite exam salt because it supplies nitrogen from both parts of the formula — be ready to identify it as a nitrogen fertiliser made by neutralisation.