potassium.jpg A macronutrient is a nutrient that is used by the plant in large quantities as opposed to a micronutrient, which is needed in very minute quantities. Nitrogen (N), phosphorus (P) and potassium (K) are macronutrients. Fertilizers are labelled with three numbers indicating what percentage of each of these elements they contain. For example, 20-20-20 is 20 per cent nitrogen (N), 20 per cent phosphorus (P), and 20 per cent potassium (K). These percentages represent the quantity present of each element by weight, but the figures won’t total 100 per cent because the elements are held within larger molecules and not in a pure form.