FE-polyphosphate formulations as replacement for fe-chelates in nutrient solutions: preliminary experiments with cucumber (cucumis sativus l.) in hydroponics

One of the critical success factors for soilless culture is the sequestration of FeIII-ions, making Fe available for plant roots. Chelates are used primarily, but other compounds, like polyphosphates are also able to bind Fe-ions. A new group of Fe-polyphosphates have recently been developed by Prayon chemical company. Two experiments were conducted to evaluate the utility of these products in nutrients solutions for soilless systems using cucumber as test crop. Three different poly-phosphate formulations were compared with Fe-DTPA as reference, at two pH levels: low pH (5.2-5.6) and high pH (6.4-6.8). The plants were grown in substrate-free nutrient solution for six weeks, starting from the seedling stage. The first test was conducted in spring with 25 µmol Fe l-1 and the second test in summer with 15 µmol Fe l-1. In the first experiment, no differences in biomass production were found. Light chlorosis symptoms appeared throughout the growing season at all treatments, tending to be less at the reference treatment with high pH. Fe contents in plant tissue were lower at the high pH treatments for both the reference and the polyphosphate treatments, whilst the Fe contents were significantly higher at low pH with the polyphosphate treatments. In the second experiment the biomass production for all treatments was significantly lower at the high pH, the lowest for the polyphosphate treatments. Chlorosis was closely related with the biomass production and was most severe in the polyphosphate treatments at high pH. Fe contents in the tissue were not different among treatments at low pH, but at high pH these were clearly lower for the polyphosphate treatments compared to the reference. Interestingly, tip-burn symptoms, commonly ascribed to Ca deficiency, were observed and were significantly more severe in the reference treatments than at the polyphosphate treatments. Polyphosphates can be used as complexer agent for the delivery of Fe3+ ions to plant roots grown in nutrient solutions. However, the combination of high pH and low Fe supply concentrations should be avoided. Further investigation with this type of fertilisers is necessary to evaluate the applicability in commercial practice