There are three numbers listed on all fertilizer packaging.
The first number is nitrogen (N) which is responsible for promoting strong leaf and stem growth as well as plant colour. So, when foliage is normally a bright green and suddenly becomes pale, chances are nitrogen is required.
The second number is phosphorus (P) which promotes strong root development that is essential for delivering nitrogen and other nutrients to the leaves. It also encourages greater flower production, in the case of fruit, results in a larger crop.
The third number is potassium (K) which is needed for general vigor. It strengthens plants to resist insects and disease and to better withstand poor soil and drought.
Together, the three elements are referred to as primary or macronutrients. Micronutrients include copper, iron, manganese, zinc, boron, chlorine, and molybdenum. Some or all of these may be included in the formulation and are listed accordingly. The actual number represents the percentage present of that element. Fertilizer often contains in smaller quantities such secondary nutrients as calcium, magnesium, and sulphur.