Crops take nitrogen, phosphorus and potassium out of the soil. Fertilisers containing compounds of these elements improve agricultural productivity by replacing what the crops remove. An NPK fertiliser contains compounds of all three elements. NPK fertilisers are formulations of various salts, mixed so that they contain the right percentages of nitrogen, phosphorus and potassium.
Industrial production uses several integrated processes and a variety of raw materials. Ammonia from the Haber process is used to make ammonium salts and nitric acid. For example, ammonia reacts with nitric acid to make ammonium nitrate, and with sulfuric acid to make ammonium sulfate. Potassium chloride, potassium sulfate and phosphate rock are obtained by mining.
Phosphate rock cannot be used directly as a fertiliser because it is insoluble in water, so plants cannot take it up. It is treated with acid to make soluble salts. Nitric acid gives phosphoric acid and calcium nitrate. Sulfuric acid gives calcium phosphate and calcium sulfate. Phosphoric acid gives calcium phosphate only.
In the laboratory, an ammonium salt can be prepared by adding ammonia solution to an acid, then evaporating some of the water and leaving the solution to crystallise. This is done in small batches with pure reagents. In industry the same compounds are made continuously on a huge scale, using raw materials such as ammonia from the Haber process and rock from mines.