Zn availability in nutrient solutions for cucumber (Cucumis sativus L.) in hydroponics as affected by Fe-chelates and pH

In soil-less culture systems Fe is usually supplied as chelate to ensure an adequate availability of this element. As chelates have affinity for many metal ions these chelates will interact with other cation nutrients in nutrient solutions. This affects the availability of Fe and other nutrients. The effects of Fe-chelate types, pH, and Zn concentrations on the availability of Fe, Zn and other micro nutrients in the nutrient solution were tested with rock wool grown cucumber as test crop. In the first experiment the Fe-chelates DTPA, EDDHA and HEEDTA (25 μmol L-1) and Zn target values of 5, 25 and 50 μmol L-1 in the root environment were compared. In the second experiment DTPA and EDDHA and Zn target values of 5 and 50 μmol L-1 at two pH ranges: 5-5.5 and 6.0-6.5 were compared. No yield nor fruit quality were significant affected in both trials. Chlorosis occurred severely at the high pH levels but was not significantly different among the chelate- or Zn treatments Necrosis of leaf margin occurred with the DTPA treatments, specifically at high pH. The required Fe-chelate dosage to keep the target levels of Fe was much higher for HEEDTA than for DTPA and EDDHA, at high pH a higher dose of DTPA was needed than for EDDHA in particular with increasing Zn levels. The Fe uptake was not significantly affected by the treatments. The Zn uptake increased with the supply. Interaction of chelate type and pH was found. With EDDHA the Zn uptake was three to five times higher than with DTPA and HEEDTA at high pH. This was in line with the much higher Zn supply needed with EDDHA at high pH The results of these experiments draw the attention to the effects of different Fe chelates on the availability of several micro nutrients to plants, which apparently can be explained by a different behavior to complexation.