Development and validation of an innovative algorithm for sodium accumulation management in closed-loop soilless culture systems

In closed-loop soilless culture systems, the increase of the electrical conductivity (EC) caused by Na+ accumulation is a major bottleneck for recycling of the nutrient solution if water sources are used which contain Na+ at substantial concentrations. Currently, the recommended nutrient concentrations for the root zone of tomato exceed those for maximising yield, aiming to induce a moderate salinity stress to enhance fruit quality. A new strategy for Na+ accumulation management is proposed, which gradually reduces the target concentrations of macronutrients except for P in the root zone in proportion to the accumulation of Na+, considering a minimum safety threshold, while maintaining the mutual ratios between them constant. The algorithm developed to support this new strategy was applied via the Decision Support System NUTRISENSE (https://nutrisense.online/) in tomato grown in mineral wool in a closed-loop soilless cropping system. The new strategy was compared with the standard strategy of maintaining constant nutrient concentrations in the root zone. The results demonstrate the efficiency of the new strategy to control the EC in the root zone while ensuring a balanced nutrition of the plants. By applying this strategy, the losses of nitrate and phosphorus due to discharge of drainage solution (DS) were reduced by 25.5 % and 9.20 %, respectively, while avoiding yield losses due to salinity stress, and increasing water productivity (WP) and agronomic efficiency of nitrogen (AEN). Nevertheless, the new strategy could not fully eliminate the need to discharge DS when raw water with a Na+concentration of 4 mM was used to prepare nutrient solution.