On-site ion monitoring system for precision hydroponic nutrient management

Hydroponic solutions used in greenhouses or plant factories are usually evaluated based on their electrical conductivity (EC) and pH. However, EC and pH cannot provide sufficient information about ion imbalances in hydroponic solutions, and this may result in wastage of nutrients or poor yields. This paper reports on the development of an on-site ion monitoring system based on ion-selective electrodes (ISEs) that can automatically calibrate sensors and measure the concentrations of individual ions (NO3−, K+, and Ca2+) in hydroponic solutions. This enables farmers to effectively manage nutrients in reused solutions by rapidly identifying any imbalances that appear in the nutrient ratios. The measurement performance of the developed system was evaluated using hydroponic solutions prepared for growing paprika crops in greenhouses. An application test was conducted to investigate the feasibility of using the developed on-site ion monitoring system for the automated measurement of three macronutrients (NO3−, K+, and Ca2+) in a real greenhouse. The results showed that the developed system was able to measure NO3− concentrations, showing an almost 1:1 relationship with the results of a standard instrument, i.e., ion chromatography (slope of 0.99 and R2 of 0.99). Although the developed system overestimated and underestimated the K+ and Ca2+ concentrations with slopes of 1.17 and 0.75, respectively, the high coefficients of determination of 0.99 and 0.97 made it possible to use calibration factors to compensate for differences in estimation. In fact, relatively low RMSEs of < 20 mg L−1 over the range of 40-1200 mg L−1 were obtained from a comparison of the ISE method and standard analysis when tested in hydroponic samples taken on different days during the period of paprika growing. This indicates that ISEs could be applicable to measurements where there is a strong linear relationship between the ISE method and standard analysis. In the application tests, the system could monitor the temporal changes in ionic concentrations in hydroponic solutions effectively, showing sensitive responses to changes in the concentrations of the three ions with an acceptable level of performance.