Optimisation of nutrition management in greenhouse vegetables through an innovative decision support system and ion selective electrodes

The recycling of drainage solution (DS) in greenhouses by applying closed-loop soilless cropping systems (CLS) eliminates pollution of the aquifers by nitrogen and phosphorus emissions. Nutrient management faces two main challenges when the DS is recycled in CLS, the sodium (Na+) accumulation and the variable composition of the DS. If the water source used to prepare nutrient solution (NS) contains Na+ at substantial concentrations, Na+ will accumulate to levels imposing salinity stress, and this discourages farmers from recycling of the nutrient solution. To address this problem, a new nutrient management strategy is suggested to control the increase of the electrical conductivity (EC) in the root environment, thus avoiding salinity stress and minimizing the discharged DS. Currently, the recommended nutrient concentrations for the root zone of tomato exceed those for maximising yield, aiming at a moderate salinity stress to enhance fruit quality. The suggested strategy for Na+ accumulation management is to gradually reduce the target concentrations of macronutrients except for P in the root zone in proportion to the accumulation of Na+, by considering a minimum safety threshold, while maintaining constant the mutual ratios between them. The algorithm developed to support this new strategy was integrated to the Decision Support System NUTRISENSE (https://nutrisense.online/) and applied in a tomato crop grown on mineral wool in a CLS. The new strategy was compared with the standard strategy of maintaining constant nutrient concentrations in the root zone. The results demonstrated the efficiency of the new strategy to control the EC in the root zone while ensuring a balanced nutrition of the plants. Τhe new strategy could not fully eliminate the need to discharge DS with a Na+ concentration of 4 mM in the raw water. However, the new strategy reduced the losses of nitrates and phosphorus due to discharge of DS by 25.5% and 9.20%, respectively, avoided yield losses due to salinity stress, and increased the water productivity (WP) and agronomic efficiency of N (AEN). The variable nutrient composition of the DS poses a significant challenge to its recycling in CLS. To address this challenge, a strategy was developed, based on daily monitoring of macronutrient concentrations in the DS using ion-selective electrodes (ISE) coupled with a modelling approach applied through the DSS NUTRISENSE. The ISE measurements were input data for the DSS to calculate appropriate rates of different single-fertiliser concentrated solutions for automatic injection into a mixture of DS and raw water. The fertiliser injection rates were transmitted online to a fertigation system equipped with nine different single-fertiliser concentrated solutions and applied automatically in real time. The objective was to maintain the root-zone nutrient concentrations within an optimal range. To test this technology and to validate the modelling approach applied through this DSS, tomato was grown in a CLS with daily measurement of NO3-, K+, Ca2+, and Na+ concentrations in the DS using ISEs. The credibility of the ISEs was confirmed by comparing their measurements with those from standard laboratory procedures in the same samples. The model approach successfully maintained the target nutrient concentrations in the root zone and concomitantly increased the fruit yield by 7.6% and the agronomic efficiency of nitrogen by 23%. The proposed technology is expected to encourage the adoption of CLS by growers thus minimising environmental impacts from greenhouse crops. The algorithm applied via the NUTRISENSE DSS to support the use of ISEs in CLS was designed based on the assumption that the Na+ concentration in the raw water used to prepare NS is low (≥0.6 mM) and Na+ accumulation in the system will be negligible. However, if the Na+ concentration in the raw water is suboptimal (0.6 to 3 mM), its accumulation must be taken into consideration when applying ISEs, to avoid excessive EC increase causing salinity stress. To address this problem, a combination of the two above-mentioned strategies, Na+ accumulation management and use of ISEs to monitor the mineral composition of the DS, was tested in a cucumber crop grown in a CLS using raw water with a sub-optimal Na+ concentration (2.5 mM) to prepare NS. The K+, Ca2+, NO3- and Na+ concentrations in the DS were measured daily using ISEs and entered to the NUTRISENSE DSS, which calculated actual injection rates of concentrated fertiliser solutions and transmitted these on-line to a multi-tank fertigation system for automatic implementation. Three treatments were compared: an open system, a CLS with standard nutrient management, and a CLS with daily automatic readjustment of the nutrient supply by NUTRISENSE based on the ISE-measured K+, Ca2+, NO3- and Na+ concentrations of the day. This strategy maintained the target EC in the root zone, thus preventing exposure of the crop to salinity. Furthermore, no nutrient deficiency was observed in the ISE-NUTRISENSE treatment, and yield was similar to that obtained in the open soilless system. Water productivity and agronomic efficiency of nutrients were significantly improved in the ISE-NUTRISENSE treatment. This study showed that smart digital technologies supported by ISEs can eliminate salinity damage due to Na+ accumulation in CLS when the Na+ concentration in the raw water does not exceed 2.5 to 3 mM depending on the crop species.