Plants need many elements, but nitrogen (N), phosphorus (P) and potassium (K) are needed in the largest amounts. When a crop is harvested these elements are carried away instead of returning to the soil when plants rot, so farmers add fertilisers to replace them. A fertiliser that contains compounds of all three is called an NPK fertiliser.
Each element does a different job. Nitrogen is used to make proteins and helps leaves and stems grow. A plant short of nitrogen grows slowly and has yellow leaves. Phosphorus helps roots to grow and develop, so a plant short of phosphorus has weak roots and stays small. Potassium helps flowers and fruit to develop, and a plant short of it has poor flowers and fruit and leaves with brown, dead edges.
Plants absorb these elements through their roots as soluble ions, so the compounds in a fertiliser must be soluble in water. Ammonium nitrate supplies nitrogen. Ammonium phosphate supplies both nitrogen and phosphorus. Potassium chloride supplies potassium, and potassium nitrate supplies both potassium and nitrogen.