Use of Capacitance Sensors for Development of Conservative Irrigation Regimes

Several experiments were conducted to further develop capacitance sensor-based automated irrigation systems. The first experiment tested whether the photosynthetic response to decreasing volumetric water content (VWC) differed among four species tested. A sigmoidal curve best described the relationship for all species (r² [r-squared]>0.86). The VWC that maintained maximum photosynthesis at 90% was selected as a potential conservative irrigation set point and values were not different between species, nor were 100% container capacity values. This indicates that a single set point is adequate to initiate irrigation and that a common upper threshold for VWC can be used for this group of taxa. This research also examined which of five sensor placements best estimates VWC and the effect of low VWC on sensor reading variability. Five sensor placements were tested; three sensors were horizontally inserted into the sidewall at 5 cm, 10 cm and 15 cm from the base of the container. The other two placements, vertical and diagonal, were inserted into the substrate surface. All positions showed a strong linear relationship (r²[r-squared]>0.92) making them all appropriate models of container substrate moisture. No placement proved better than the others, but choosing a vertical placement is most practical for sensor calibration, installation, and removal. Other trials were conducted to test two container nursery irrigation regimes on oakleaf hydrangea (Hydrangea quercifolia ‘Alice’) in both nursery and greenhouse environments. Plants were automatically irrigated by one of two substrate moisture sensor-based regimes: 1) a daily water use (DWU) system that delivered the exact amount of water lost in the previous 24 h and 2) an on-demand (OD) irrigation system based on the relationship between substrate moisture level and photosynthetic rate. In this system, irrigation was applied when the substrate moisture level fell below 33% container capacity, which corresponded to 90% maximum predicted photosynthetic rate. Both treatments used significantly less water than the industry standard of 2.5 cm/day. This research demonstrated that automating irrigation based on the relationship between photosynthesis and VWC may be practical for multiple species in a nursery setting and can attenuate water use to meet crop demands.