The main compound of nitrogen in NPK fertilisers is ammonium nitrate, its formula is NH4NO3. To make this, we use ammonia which is produced by the Harber process. We can use the ammonia to produce nitric acid. We then react the nitric acid with more ammonia to make ammonium nitrate. The potassium in NPK fertilisers comes from the salt potassium chloride or potassium sulfate.