Establishing optimal nutrient solution compositions for watermelon cultivation in open or closed soilless crops

Cultivation of watermelon in soilless systems can restrict infections by soil-borne pathogens and increase yield. However, there is limited information in international literature about the optimal composition of nutrient solutions for soilless cultivation of watermelon. In the current study, a systematic model approach was applied aiming to calculate an optimal nutrient solution composition for watermelon based on experimental data originating from watermelon cultivation in a soilless float system. Since in the float hydroponic system there was no substrate and no losses of nutrient solution through drainage, it was possible to quantify the net nutrient uptake in terms of both nutrient to water uptake ratios (uptake concentrations) and mutual ratios of nutrients in the plant tissues. These values were used to establish an optimal nutrient composition for the root solution. Subsequently, a mass balance model was applied to estimate a target composition for the nutrient solution supplied to substrate-grown watermelon crops, as a relationship of the target leaching fraction. The experimental results showed that the target concentration in the supply solution increases linearly with increasing target drainage fraction. However, the slope of increase was steeper for bivalent than for monovalent macronutrient ions. Furthermore, watermelon had a low demand for iron and manganese compared to other fruit vegetables. Based on the obtained results, optimal nutrient solution compositions for the root solution and the supply solution in watermelon crops cultivated in both open and closed-loop soilless cropping systems were computed.