Optimizing Nutrient Compensation Intervals Based on Ionic Monitoring in Drainage Water from Open and Closed Tomato Hydroponics

Closed hydroponics (recirculating) is increasingly recognized as a sustainable approach for conserving water and fertilizer resources. However, concerns remain among growers regarding ionic imbalances and yield instability during nutrient–solution recirculation. This study aimed to clarify these issues through continuous ionic monitoring of drainage water and optimization of nutrient compensation intervals in commercial tomato (Solanum lycopersicum L.) cultivation. Two greenhouse systems, an open (non-recirculating) and a closed (recirculating) system, were compared. Electrical conductivity (EC), pH, and major ions (NO3−, K+, Ca2+, Mg2+, SO42−, PO43−, and Na+) were analyzed using ion chromatography. Based on ionic fluctuation trends, compensation intervals of 0, 2, and 4 weeks were evaluated in the closed system. Contrary to expectations of growers, open hydroponics exhibited greater ionic imbalance due to uncontrolled leaching. Periodic compensation (every 4 weeks) stabilized ionic ratios, reduced fertilizer input by 67–69%, and decreased water use by 33–36% compared with the open system. These findings demonstrate that drainage-based ionic monitoring and interval-based compensation can improve the environmental and economic performance of closed hydroponics.