NPK Fertilisers

This lesson covers:

  1. What fertilisers are
  2. Why we use fertilisers 
  3. How we make fertilisers 

NPK fertilisers

Illustration showing fertiliser being applied to soil with green plant sprouts.

A fertiliser is a substance that is applied to soil, in order to supply plants with nutrients.

Diagram showing the composition and formulation of NPK fertilisers, including nitrogen, phosphorus, and potassium compounds.

How fertilisers are made

Illustration showing the transition from using cow manure to formulated fertilisers.

In the past, fertilisers were usually made from animal waste such as cow manure. Now though, we tend to use formulated fertilisers, which are made in factories. 

Formulated fertilisers

Illustration showing a formulated fertiliser mixture in a round-bottom flask.

A formulation is a mixture that has been designed as a useful product. So if we make fertilisers by combining certain chemicals in a specific ratio, then we would call it a formulated fertiliser.

What formulated fertilisers contain

Diagram showing nitrogen phosphorus and potassium elements in npk fertilisers.

The three main elements that plants needs from the soil are are nitrogen (N), phosphorus (P), and potassium (K). For this reason, fertilisers normally consist mainly of nitrogen, phosphorus, and potassium compounds.


This is why we sometimes call them NPK fertilisers. NPK fertilisers are formulations of various salts containing appropriate percentages of these three elements.

Fertilisers typically contain three essential elements. What are they?

Potassium

Magnesium

Chlorine 

Nitrogen

Phosphorus

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Which part of a plant absorbs minerals ions? 

Leaves 

Roots 

Stem

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Producing nitrogen (N) fertiliser


  1. Nitrogen is important to plants as it is required to make amino acids and hence proteins, which are essential for growth.
  2. The main source of nitrogen in fertilisers is ammonia, which remember is made in the Haber process. 
  3. You need to know how ammonia can be used to make nitric acid and ammonium salts:
  • Ammonia can react with oxygen and water in a series of reactions to make nitric acid.
  • Ammonia can also react with acids (including nitric acid) to form ammonium salts. 
  1.  How scientists carry out these reactions is different depending on whether they're done in industry or in a lab. You may be asked to compare these in an exam, but you will be given the information required. 

What is the main reason plants need nitrogen?

To make chlorophyll for photosynthesis

To make fatty acids that can be used to produce triglycerides

To make amino acids that can be used to produce proteins 

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Producing phosphorus (P) fertiliser


  1. Phosphate can be mined from the ground as phosphate rock.
  2. However, because the phosphate salts in the rock are insoluble, plants can’t use them as nutrients, and so they can't be used directly in fertilisers.
  3. Instead, we have to react the phosphate rocks with acids to produce soluble salts:
  • Reaction with nitric acid produces phosphoric acid and calcium nitrate.
  • Reaction with sulfuric acid produces calcium sulfate and calcium phosphate (this mixture is known as single superphosphate).
  • Reaction with phosphoric acid only produces calcium phosphate (also called triple superphosphate).


What useful compound is produced when phosphate rock is reacted with phosphoric acid?

Single superphosphate

Calcium nitrate

Triple superphosphate

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Producing potassium (K) fertiliser


Potassium fertiliser is the simplest to produce. Potassium chloride and potassium sulphate can be both be mined from the ground and then used directly in fertilisers.

The compound KNO3 is used in fertiliser as it provides two essential elements to plants. These are and nitrogen.

potassium

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NPK fertilisers are formulations of different salts. 


Explain what is meant by the term 'formulation'.

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What do plant fertilisers do?

They help insects pollinate plants

They provide food for plants to grow

They provide mineral ions needed by plants

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