Monitoring and Controlling Ebb-and-flow Subirrigation with Soil Moisture Sensors

Abstract

Subirrigation can reduce water loss and nutrient runoff from greenhouses, because used nutrient solution is collected and recirculated. Capacitance moisture sensors can monitor substrate volumetric water content (θ) and control subirrigation based on minimum θ thresholds, providing an alternative to timers. Our objectives were to automate an ebb-and-flow subirrigation system using capacitance moisture sensors, monitor moisture dynamics within the containers, and determine the effect of five θ thresholds (0.10, 0.18, 0.26, 0.34, or 0.42 m3·m−3) on hibiscus (Hibiscus acetosella Welw. ex Hiern.) ‘Panama Red’ (PP20,121) growth. Subirrigation was monitored using capacitance sensors connected to a multiplexer and a data logger and controlled using a relay driver connected to submersible pumps. As the substrate θ dropped below the thresholds, irrigation was turned on for 3 min followed by 3-min drainage. Capacitance sensors effectively controlled subirrigation by irrigating only when substrate θ dropped below the thresholds. Each irrigation cycle resulted in a rapid increase in substrate θ, from 0.10 to ≈0.33 m3·m−3 with the 0.10-m3·m−3 irrigation threshold vs. an increase in θ from 0.42 to 0.49 m3·m−3 with the 0.42-m3·m−3 irrigation threshold. Less nutrient solution was used in the lower θ threshold treatments, indicating that sensor control can reduce water and thus fertilizer use in subirrigation systems. The water dynamics showed that the bottom part of the pots was saturated after irrigation with θ decreasing quickly after an irrigation event, presumably because of drainage. However, the water movement among substrate layers was slow with the 0.10-m3·m−3 irrigation threshold with water reaching the upper layer 5.5 to 20 h after irrigation. The 0.10-m3·m−3 θ threshold resulted in 81% fewer irrigations and 70% less nutrient solution use compared with the 0.42-m3·m−3 θ threshold. However, the 0.10-m3·m−3 θ threshold also reduced hibiscus shoot height by 30%, shoot dry weight 74%, and compactness by 63% compared with the 0.42-m3·m−3 θ threshold. Our results indicate that soil moisture sensors can be used to control subirrigation based on plant water use and substrate water and to manipulate plant growth, thus providing a tool to improve control over plant quality in subirrigation systems.