Plants need certain elements to grow well. When a farmer harvests a crop, these nutrients are taken away from the soil, so they must be put back. Fertilisers are substances added to soil to replace nutrients and promote plant growth.
The three main elements are nitrogen (N), phosphorus (P) and potassium (K). Nitrogen supports leaf growth and the making of the green pigment chlorophyll. A plant short of nitrogen has pale, yellowing leaves and grows slowly. Phosphorus supports root growth. Potassium supports flower and fruit growth and helps the plant stay healthy.
A fertiliser contains these elements as compounds, not as pure elements. Ammonium nitrate supplies nitrogen, potassium chloride supplies potassium, and ammonium phosphate supplies nitrogen and phosphorus. A fertiliser that has compounds of all three is called an NPK fertiliser. The compounds must be soluble in water so that the roots can absorb them from the soil water.