Nitrogen, Phosphorus and PotassiumPlants need a lot of nutrients to grow, but the three major elements they require include nitrogen (N), phosphorus (P) and potassium (K). (Note: Fertilizer package labels usually have three prominent numbers listed: the first number is for nitrogen, the middle for phosphorus and the last for potassium.) Most soil test kits (figure A), which can be purchased from a garden center, measure these three elements. Some kits also measure the soil's pH, which is important because certain plants like the pH at a certain level in order to grow properly.