Common fertilisers and their importance

In a nutshell

Fertilisers are used to facilitate plant growth for crops. NPK fertiliser is the most common and it contains nitrogen $$(N)$$, phosphorous $$(P)$$​ and potassium $$(K)$$​.

What is fertiliser?

Fertilisers provide mineral ions to plants which are needed for healthy growth. As plants grow they absorb these minerals ions from the water in soil. Fertilisers are typically sold as powders or granules and are dissolved in the soil by rain or mixed with water and sprayed into soils.

NPK fertiliser

Common fertilisers include $$N-P-K$$​ fertilisers which stands for nitrogen $$(N)$$​, phosphorus $$(P)$$​ and potassium $$(K)$$​. Fertiliser compounds must be soluble in water so they can dissolve in the water in the soil and be absorbed by the plants. Nitrogen is a key component of fertilisers as it helps the plants produce proteins such as enzymes.

Sources of soluble nitrogen are ammonium ions $$(NH_4^+)$$​ and nitrate ions $$(NO_3^-)$$.​

Diammonium phosphate is the most common source of soluble phosphorus, in the form of phosphate ions $$(PO_4^{2-})$$. All potassium compounds are able to dissolve in water to provide $$K^+$$​ ions.

Common fertilisers

Importance of ammonia for making fertiliser

Ammonia is a key component of fertilisers as it can be used to make both ammonium ions or nitrate ions. Both of these are useful soluble forms of nitrogen. Ammonium ions can be formed when reacting ammonia with acid to form $$NH_4^+$$​. Nitrate ions $$(NO_3^-)$$​ are formed by oxidising ammonia ​.

Ammonia and nitric acid can be reacted together to form ammonium nitrate. A common compound in fertilisers as it has two sources of soluble nitrogen.

​$$ammonia\ +\ nitric\ acid\ \rightarrow\ ammonium\ nitrate \\ \ \\NH_3(aq)\ +\ HNO_3(aq)\ \rightarrow\ NH_4NO_3(aq)$$​​

This can then be heated to remove the water to purify the ammonium nitrate.

Mining phosphates and potassium

Potassium chloride and potassium sulphate can be mined and used as source of potassium.

Phosphate rocks can also be mined. However phosphate rocks are not soluble and therefore they need to be treated with acids to produce soluble sulphate salts.

Reacting phosphate rock with phosphoric acid produces calcium phosphate, $$Ca_3(PO_4)_2$$, a very common form of soluble phosphates.