The effect of chelating Zn, Cu and Mn on plant Fe nutritional status of hydroponically grown tomato plants

Common practice in Dutch horticulture is the application of Fe chelates in combination with unchelated Zn, Cu and Mn salts. Past research has shown that this unchelated Cu, Zn and Mn can compete with Fe for the DTPA molecule, resulting in a loss of available Fe. It was hypothesized that in nutrient solutions based on the Dutch Nutrient Solution Manual (Bemestingsadviesbasis), this loss of Fe can be up to 30%. In order to investigate the implications of this modelled Fe loss in a practical situation, and to investigate whether supplying Cu, Mn and Zn as chelated micronutrients would help solve this issue, a greenhouse trial was set up. In this experiment Solanum lycopersicum was grown on rockwool slabs using nutrient solutions based on common grower practice. Fe was supplied continuously using Fe-DTPA at two concentrations: 10 and 7 µmol L(‑1) Fe while Zn, Mn and Cu were supplied as sulfate salts or as Zn, Mn and Cu-EDTA. Throughout the growing season SPAD-index was quantified as indicator for leaf chlorophyll content. Additionally, both the fertigation solution and the drain-water were regularly analysed for mineral nutrient content. Cumulative fruit yield was obtained upon termination; 265 days after the experiment start. On average, Fe concentration in the rockwool slab was 57 and 71% higher for respectively the 10 and 7 µmol Fe L(‑1) chelate treatments than the sulfate treatments. On average, leaf chlorophyll content was found to be higher in the chelate treatments. Final fruit yield was also higher in the chelate treatments, with a yield of 58.4 kg m‑2 in the 7 µmol Fe L(‑1) sulfate treatment and 67.1 kg m‑2 in the 7 µmol Fe L(‑1) chelate treatment. These results show that chelating Cu, Zn and Mn is an important strategy to minimize losses of plant available Fe from the nutrient solution.