Fertilisers

Plants need mineral nutrients to grow. They take these nutrients up through their roots from the soil. If the soil does not contain enough of an essential nutrient, plant growth can be limited. Fertilisers are substances added to soil to replace or increase the supply of these nutrients.

NPK fertilisers

Three essential elements supplied by fertilisers are nitrogen ($$\ce{N}$$), phosphorus ($$\ce{P}$$) and potassium ($$\ce{K}$$). An NPK fertiliser contains compounds that provide all three elements. It can improve plant growth when the soil would otherwise lack one or more of them.

The letters NPK name the elements supplied; they do not mean that the fertiliser contains the pure elements. The elements are present in compounds that provide nutrients plants can take up.

Nitrogen fertilisers

Some fertilisers supply nitrogen in particular. Ammonium salts and nitrates are used because they are sources of nitrogen for plants. Adding them to soil can improve growth when plants do not have enough nitrogen.