NPK fertilisers supply nitrogen, phosphorus and potassium
- Plants need nitrogen, phosphorus and potassium to grow well.
- NPK fertilisers supply all three elements in the right proportions.
- An NPK fertiliser is a formulation, a carefully mixed blend of several salts that together contain N, P and K.
- Using them improves agricultural productivity by giving crops the nutrients they need.
Key Idea
- The letters N, P and K stand for the three key nutrient elements.
- An NPK fertiliser blends salts so the mixture contains the right percentage of each.
Ammonia is used to make ammonium salts
Definition
Ammonium salt
A salt containing positively charged ammonium ions and negatively charged ions from an acid.
- Ammonia from the Haber process is a base, so it is neutralised by acids to make ammonium salts.
- Ammonia is neutralised with nitric acid to make ammonium nitrate: NH3+HNO3→NH4NO3\text{NH}_3 + \text{HNO}_3 \rightarrow \text{NH}_4\text{NO}_3NH3+HNO3→NH4NO3.
- Ammonium nitrate is a good fertiliser because it contains nitrogen from two sources, the ammonia and the nitric acid.
Example
- Ammonia can also be used to make nitric acid, which is then used to make more ammonium nitrate.
- The result is a soluble salt that plant roots can take up easily.
Making ammonium nitrate in the lab and in industry
- In the lab, dilute ammonia and dilute nitric acid are combined by titration, then the solution is crystallised to give pure ammonium nitrate.
- Lab preparation is done in small batches by hand.
- In industry, hot concentrated ammonia and nitric acid react in a continuous process on a huge scale.
- The industrial neutralisation is strongly exothermic, and the heat released evaporates water to help concentrate the product.
Exam technique
- When you compare the lab and industrial methods, contrast the scale, the concentration of the reactants and batch versus continuous working.
- Note that industry uses the heat of the reaction to evaporate water, which the lab method does not.
Potassium and phosphorus come from mined rock
Definition
Raw material
A natural material used to make a product, such as crude oil for plastic or wood for paper.
- Potassium chloride and potassium sulfate are mined and can be used directly as fertilisers.
- Phosphate rock is also mined, but it cannot be used directly.
- Its phosphorus compounds are insoluble, and plants can only take up soluble compounds.
Note
- These salts are dug straight from the ground as raw materials.
- Phosphate rock must be treated with acid first to make its phosphorus available to plants.
Treating phosphate rock with acids
Definition
Phosphate rock
A mined rock containing insoluble phosphorus compounds that can be treated with acids to make fertilisers.
- Phosphate rock is reacted with acids to turn it into soluble compounds that plants can use.
- With nitric acid, it makes phosphoric acid and calcium nitrate.
- With sulfuric acid, it makes single superphosphate, a mixture of calcium phosphate and calcium sulfate.
- With phosphoric acid, it makes triple superphosphate, which is calcium phosphate.
Self review
- What three elements are supplied by an NPK fertiliser?
- Write the equation for making ammonium nitrate from ammonia and nitric acid.
- Give two differences between the lab and industrial preparation of ammonium nitrate.
- Why can phosphate rock not be used directly as a fertiliser?
- Name the product made when phosphate rock is treated with sulfuric acid.
