Production and Uses of NPK Fertilisers
Get to grips with Production and Uses of NPK Fertilisers for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) chemistry topic on how nitrogen, phosphorus and potassium compounds are made and combined into NPK fertilisers.
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Topic overview
NPK fertilisers contain compounds of nitrogen, phosphorus and potassium, the three elements plants need in the largest quantities. This is a Triple-only topic.
They are formulations, because they are mixtures of compounds blended in measured proportions to give the required percentages of each element. Ammonia from the Haber process is used to make ammonium salts and nitric acid, which are nitrogen sources.
Potassium chloride, potassium sulfate and phosphate rock are all mined, but phosphate rock cannot be used directly. It is treated with nitric acid to give phosphoric acid and calcium nitrate, with sulfuric acid to give single superphosphate, or with phosphoric acid to give triple superphosphate.
Revision notes
What NPK fertilisers are
They contain compounds of nitrogen, phosphorus and potassium — the three elements plants need in the largest quantities.
They are formulations: mixtures blended in measured proportions to give required percentages of each element.
Nitrogen sources
Ammonia from the Haber process can be reacted with acids to produce ammonium salts.
Ammonia can also be oxidised to make nitric acid, which is used to produce ammonium nitrate. In industry these are made on a much larger scale than in a laboratory.
Phosphorus and potassium sources
Potassium chloride and potassium sulfate are mined and used directly.
Phosphate rock is mined but cannot be used directly. Treated with nitric acid it gives phosphoric acid and calcium nitrate. With sulfuric acid it gives single superphosphate. With phosphoric acid it gives triple superphosphate.
Key points
- NPK stands for nitrogen, phosphorus and potassium.
- NPK fertilisers are formulations.
- Ammonia from the Haber process gives nitrogen compounds.
- Potassium chloride and sulfate are mined and used directly.
- Phosphate rock cannot be used directly.
- Phosphate rock is treated with acids to make it usable.
Worked examples
Example 1
State the three elements provided by an NPK fertiliser. [3 marks]
Model answer
Nitrogen (1 mark), phosphorus (1 mark) and potassium (1 mark).
Example 2
Explain why NPK fertilisers are described as formulations. [2 marks]
Model answer
They are mixtures of different compounds (1 mark) blended in carefully measured proportions to give the required percentage of each element (1 mark).
Example 3
Explain why phosphate rock cannot be used directly as a fertiliser. [2 marks]
Model answer
The phosphate in the rock is insoluble, so plants cannot absorb it (1 mark). It must be treated with an acid to produce a soluble phosphate compound that plants can take up (1 mark).
Common mistakes
Saying phosphate rock is used directly.
It must be treated with acid first because it is insoluble.
Forgetting that NPK fertilisers are formulations.
The measured proportions are what make them formulations.
Confusing the three superphosphate products.
Nitric acid, sulfuric acid and phosphoric acid each give different products.
Saying potassium compounds need treatment.
Potassium chloride and sulfate are mined and used directly.
Exam tips
- Learn what each letter of NPK stands for.
- Explain formulations through measured proportions.
- Give solubility as the reason phosphate rock is treated.
- Remember this is a Triple-only topic.
Key terms
- NPK fertiliser
- A fertiliser containing nitrogen, phosphorus and potassium.
- Formulation
- A mixture blended in measured proportions.
- Phosphate rock
- A mined material needing acid treatment before use.
- Superphosphate
- A soluble phosphate fertiliser made by treating phosphate rock.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.