Properties and Uses of Fertilisers

Fertilisers are chemical compounds added to the soil to supply nutrients to crops. They replace elements that plants absorb. This improves plant growth and promotes the development of crops, so fertilisers are important in agricultural production.

NPK fertilisers are fertilisers that contain:

  • Nitrogen (N) – Promotes the development of stems and leaves
  • Phosphorus (P) – Supports strong root growth
  • Potassium compounds (K) – Improves water and nutrient uptake

These are the three essential elements for plant growth, which most fertilisers will contain. For fertilisers to be absorbed by plant root hair cells, they must be soluble in water:

  • Ammonium ions NH4⁺, and nitrate ions, NO3⁻, are sources of soluble nitrogen.
  • Phosphate ions, PO4³⁻, are a source of soluble phosphorus
  • Most common potassium compounds dissolve in water to produce potassium ions, K⁺

NPK fertilisers are formulations of different salts, containing the essential element in the right proportions. The table below lists some examples of fertilisers, their formulas and the important elements they provide:

FertiliserFormulaEssential element(s)
Ammonium nitrateNH4NO3Nitrogen
Ammonium sulfate(NH4)2SO4Nitrogen
Ammonium phosphate(NH4)3PO4Nitrogen and phosphorus
Potassium nitrateKNO3Nitrogen and phosphorus

Fertilisers can be produced in laboratories or industrial settings, depending on the required quantity.

One advantage of using artificial fertilisers over natural alternatives, such as manure, is the ability to control the quantity of each element. This allows farmers and gardeners to change the nutrient supplementation according to their soil’s specific needs.

Using Ammonia to Make Fertilisers

Ammonia (NH3) is an alkaline substance that plays an important role in the formation of nitric acid and fertilisers. Ammonia undergoes oxidation to form nitric acid, which can be reacted with more ammonia to produce ammonium nitrate.

Ammonia + Nitric acid  → Ammonium nitrate

NH3 (aq) + HNO3 (aq)NH4NO3 (aq)

Ammonium nitrate is the main nitrogen compound in NPK fertilisers and is widely used as a fertiliser. Also, ammonium sulfate can be produced in a laboratory by reacting ammonia with sulfuric acid.

Ammonia + Sulfuric acidAmmonium sulfate

2NH3(aq) + H2SO4(aq)(NH4)2SO4(aq)

Mining Phosphate Rock for Fertilisers

The potassium in NPK fertilisers comes from potassium chloride and potassium sulfate, which are minerals that can be mined from the Earth’s crust. These compounds can be used directly but need to be chemically processed before being used to make fertilisers.

Phosphate rocks can be obtained by mining. However, these rocks are insoluble, so they are typically treated with acids to form soluble potassium salts.

Treating phosphate rock with:Compound(s) produced
Nitric acidPhosphoric acid and calcium nitrate. Phosphoric acid contains phosphorous, but it cannot be applied directly to plants. Therefore, it is neutralised with ammonia to produce ammonium phosphate.
Sulfuric acidA mixture of calcium sulfate and calcium phosphate (also known as single superphosphate).
Phosphoric acidTriple superphosphate (calcium phosphate).