Response of hydroponically grown gerbera to nutrient solution recycling and different nutrient cation ratios

The effects of recycling the drainage solution by adding nutrients to replenish it at three different K: (K + Ca + Mg) ratios (0.40, 0.48, 0.56, equivalent basis) on yield, flower quality and nutritional status of soilless-grown gerbera were investigated. The recycling treatments were compared also to a control with free drainage and a standard K: (K + Ca + Mg) equivalent ratio of 0.40 in the irrigation solution. The ratios between NH4 and total N, Ca and Mg, macronutrient anions, and micronutrients, were identical in all treatments when injecting concentrated solutions of fertilizers to prepare fresh nutrient solution, and similar to standard recommendations for open systems, regardless of recycling or not. Furthermore, the target electrical conductivity and pH in the irrigation solution were identical in all treatments. When the drainage solution was recycled, the highest yield was obtained with the highest K: (K + Ca + Mg) injection ratio. However, both the number of flowers per plant and the flower stem length were significantly lower in all recycling treatments compared to those obtained when the effluents were discharged. This was attributed to a profound decline of the NH4 concentration in the drainage solution and, thus, also in the irrigation solutions prepared by reusing it, which resulted in increased pH in the root environment and restricted supply of Mn to the plants. In addition to Mn, the leaf P and Fe concentrations were also reduced by the increased pH in the root zone when recycling was applied. Hence, to prevent nutrient imbalances in closed hydroponic systems, it is suggested to supply potassium and nitrogen at higher K: (K + Ca + Mg) and NH4/total-N ratios than those recommended for open systems. (C) 2002 Elsevier Science B.V All rights reserved.