Load Cell-Based Estimation and Control of Substrate Volumetric Water Content for Automated Irrigation in Plug Production

Effective water management in plug trays is critical for high-quality young plant production. While soil moisture sensor-based irrigation can improve irrigation efficiency, its application to young plants is constrained by the small size and high density of plug tray cells, which hinder reliable moisture sensing. We developed a load cell-based automated irrigation system that estimates substrate volumetric water content (VWC) from plug tray weight dynamics and evaluated its applicability for plug production. The system continuously monitored tray weight and estimated VWC relative to a saturated reference tray weight, which was updated after each irrigation event to account for plant growth. Estimated VWC closely matched oven-dry measurements (R2 = 0.9888, RMSE = 0.029 m3·m−3). In a trial with torenia, irrigation was regulated using three VWC thresholds (0.30, 0.45, and 0.60 m3·m−3) and compared with a daily irrigation regime. The system regulated irrigation according to the target thresholds, with the 0.45 m3·m−3 threshold achieving the best balance between plant growth and irrigation efficiency, indicating it as the optimal irrigation threshold for torenia cutting propagation. This approach provides a practical decision-support tool for precision irrigation and for determining crop-specific VWC thresholds, supporting growers in improving water-use efficiency while ensuring high propagation performance in plug production.