NaCl accumulation and macronutrient uptake by a melon crop in a closed hydroponic system in relation to water uptake

• Na and Cl to water uptake by melon relate exponentially to external Na and Cl levels.
• Na and Cl accumulation in closed hydroponic melon relates to cumulative water uptake.
• Simulation of Na and Cl buildup in closed hydroponic melon was successfully tested.
• External NaCl up to 45 mM did not restrict uptake concentrations of nutrients by melon.

To optimize nutrient supply in melon (Cucumis melo L.) cultivated in closed-loop hydroponic systems under Mediterranean climatic conditions, the process of salinity build-up has to be better understood. To attain this objective, two experiments were conducted in two cropping seasons (winter–spring and spring–summer) in order to: (i) establish relationships between Na+ and Cl− concentrations in the root zone and uptake concentrations (UC) of Na+ and Cl−, respectively, i.e., Na+/water and Cl−/water uptake ratios, and (ii) test whether macronutrient UC in melon grown in closed hydroponic systems are influenced by the gradual salinity build-up. Three different NaCl concentrations in the irrigation water used to prepare nutrient solutions, i.e., 0.7, 2.5, and 5 mM, were applied. The UC of Na+ and Cl− increased over time but at a certain time point in the cropping cycle they converged to a plateau corresponding to the salinity treatment. Exponential relationships between the Na+ and Cl− concentrations in the root zone and the UC of Na+ and Cl−, respectively, were fitted to experimental results in both experiments. However, parameterization of the model with data from the high-transpiration season revealed superiority. The established model parameters corresponded well over the whole melon cultivation cycle and a wide range of climatic conditions. The NaCl-salinity up to the tested level had no significant effect on the UC of macronutrients (i.e., N, P, K, Ca and Mg). The mean UC of Ca and N were higher than those reported under northern-European climatic conditions. The obtained results may be used through on-line operating decision support systems to optimize nutrient supply and minimize salinity impacts in melon grown in closed hydroponic systems when the quality of the irrigation water is sub-optimal.