These are chemicals plants use to grow and are often divided into macronutrients and trace elements. These terms are in themselves are of limited use as if the plant needs the chemical to grow, in however small a quality, its absence is going to cause problems. In practice the main ones are nitrogen (N), phosphorus (P), potassium(K), Calcium (Ca), magnesium (Mg) and sulphur (S). Of these the most important are nitrogen, phosphorus and potassium and they are the NPK referred to on packets of fertilizer.