Tomato plants were grown with limited or luxury water supply and given nitrogen either as the ammonium ion or the nitrate ion. The ‘active’ iron fractions of the leaves as extracted by etherized tenth molar hydrochloric acid showed no relationship with the total iron but a very significant linear relationship was found between the active iron fraction and the ratio of total phosphorus to total iron. Recalculation of results presented by other investigators for maize plants grown in soil with various levels of moisture, phosphorus and iron also showed a highly significant linear relationship between the active iron fraction and the ratio of total phosphorus to total iron.