- a lack of iron in the growing medium, resulting in yellowing leaves. Iron deficiency occurs in chalky soils, or in soil that has been limed too much; it can be corrected by applying iron chelate solution. The role of iron in the physiology of plants appears to be associated with specific enzymatic reactions and the production of chlorophyll. Iron deficiency occurs always in soils with a pH of over 7.5. It affects young leaves, which appear scorched. It should be dealt with by reducing the pH level by adding peat. Aluminium sulphate or ferrous sulphate can be used, but they may have the effect of making the phosphate in the soil unavailable to plants, resulting in phosphorus deficiency.