Yellowing of leaves can be caused by a number of nutrient deficiencies as well as other unrelated factors. Chlorophyll is the substance that carries out photosynthesis and is also the source of the green colour of most leaves. Yellowing of the foliage implies that chlorophyll is being broken down by the plant. The reason may be direct because nitrogen or magnesium are deficient which form part of the actual molecule or indirect because a micronutrient such as manganese, zinc or iron that are important for the correct functioning of photosynthesis are deficient therefore stopping the process and leading to chlrophyll breakdown. Other reasons are more complex which is why sulphur, potassium and calcium may also produce this symptom when deficient.