4.10.4.2 - Production and uses of NPK fertilisers

What are NPK fertilisers?

NPK fertilisers are formulations containing salts of nitrogen, phosphorous and potassium in the right percentages.

NPK fertilisers are formulations of various salts containing appropriate percentages of the elements.

Why are compounds of nitrogen, phosphorus and potassium used as fertilisers to improve agricultural productivity?

As a plant’s growth and life processes are affected if these compounds go missing due to being used by a previous crop

Fertilisers therefore replace these missing elements or provide more of them , helping increasing the crop yield as crops now grow bigger and faster

Why are formulated fertilisers better than manure to fertilise fields?

• more widely available

• easier to use

• don’t smell

• have just enough of each nutrient to maximise productivity

What is ammonia used to produce which are useful in fertilisers?

Ammonia can be used to manufacture ammonium salts and nitric acid

Ammonia can be reacted with oxygen and water in a series of reactions to make nitric acid

You can also react ammonia with acids to get ammonium salts

What does ammonia react with to produce nitric acid?

Ammonia reacts with oxygen and water in a series of reactions

Describe how these reactions (involving ammonia) are carried out in industry.

• The reaction is carried out in giant vats, at high concentrations resulting in a very exothermic reaction.

• The heat released is used to evaporate water from the mixture to make a very concentrated ammonium nitrate product

Describe how these reactions (involving ammonia) are carried out in the lab.

• The reaction is carried out on much smaller scale by titration and crystallisation.

• The reactants are at a much lower concentration than in industry, so less heat is produced and it’s safer for a person to carry out

• After the titration, the mixture then needs to be crystallised to give pure ammonium nitrate crystals

• Crystallisation isn’t used in industry because it’s very slow

Name what compounds can be mined to be used as a source of potassium.

• Potassium chloride

• Potassium sulphate

How do we obtain phosphate rock?

Mining

Why can the phosphate rock not be used directly as fertiliser?

The phosphate salts in the rock are insoluble meaning plants can’t use them as nutrients

Meaning the rock cannot be used directly as fertiliser

List all the types of acid that phosphate rock is reacted with to produce soluble phosphates (that can be used as fertiliser).

• Reaction with nitric acid (forms phosphoric acid and calcium nitrate)

• reaction with sulphuric acid (from calcium sulphate and calcium phosphate - otherwise known as single superphosphate)

• Reaction with phosphoric acid (forms calcium phosphate otherwise can be called triple superphosphate)