FE, MN and ZN addition to nutrient solutions for tomato and cucumber grown in inert substrates

Abstract: In experiments in a 2×2×3 factorial layout the addition of Fe, Mn and Zn to nutrient solutions for hydroponics was studied with crops grown under greenhouse conditions. Standard as well high concentrations of up to 25 or 50 µmol/L were included as targeted levels in the root environment of the elements under investigation. One tomato crop on poly urethane slabs and two successive cucumber crops on glass wool slabs were grown in a recirculation system. The concentrations required in the nutrient solution supplied to maintain the targeted concentrations in the root environment during cultivation were for Fe and Zn respectively, 1/3 to 1/5 of the targeted concentrations. The concentrations required for Mn were more or less equal to the targeted concentrations. The Fe contents in the young leaves were not significantly affected by the different Fe concentrations in the root environment. The Zn contents in the young leaves of the tomato crop were scarcely affected by the concentration in the root environment, while those for cucumber proportionally increased with the external concentrations. The Mn concentrations in the root environment strongly affected the contents of this element in the plant material. The high concentration of a metal micro nutrient as realised in the experiments could strongly reduce or increase, by up to 30%, the contents of other metal micro nutrients of young leaves. In addition to ionic competition the use of chelates may strongly influence these processes. For example, depending on the pH in the root environment and the chelate type used, Fe in the chelate complex can easily be replaced by a different metal micro nutrient. In the pH range between 5 and 6, the recommended range for most crops in hydroponics, Fe in the chelate complex may be replaced by Zn and Cu. The effects of high metal micro nutrient concentrations on the yield of tomato and cucumber were small and mostly not consistent nor significant. However, a significant yield reduction of, on average, 8% was found with high Zn applications for both successive cucumber crops. The yield reduction was equal for both high Zn levels of 25 and 50 µmol/L. A high concentration of the one metal micro nutrient can reduce the uptake of other metal micro nutrients so much that deficiency can appear, and recommendations to prevent this are discussed. In this discussion special attention is given to the fact that with the replacement of Fe by other metal micro nutrients, a high solubility of a micro nutrient can be combined with a low availability to plants. Such situations can easily lead to misleading interpretations of analytical data of solutions from the root environmen