Relating Real-Time Substrate Matric Potential Measurements to Plant Water Use for Precision Irrigation

The nursery and greenhouse industries require a more precise method to schedule irrigation water applications, since current methods are subjective and contribute to excessive water and nutrient run off from production areas. More importantly, we need to know whether plants are in water stress, which is related to plant-available water (PAW) and substrate matric potential (Psi m). There are varying reports of reductions in photosynthesis in the literature at Psi m as low as -10 kPa. Plant-available water is coupled with substrate physical properties, but is likely influenced also by plant species, root density, root age and root volume. This is in addition to daily changes in environmental conditions, which drive the short-term (hourly) dynamics of water uptake from the container. Our research group is using Ech(2)0 capacitance sensors (Decagon Devices, Pullman, WA) to sense PAW, as these may provide a reliable and affordable method to develop sensor networks for large-scale growing operations. Using a modified tension table, we calibrated various Ech(2)0 sensors in five commercial soilless substrates, with varying physical properties and air contents, by applying pressures from 1-100 kPa, the range in which most PAW is held in soilless substrates. By measuring the water expressed from 10 replicate columns at each pressure and simultaneously measuring the output from a sensor embedded in each column, a series of calibration curves were developed for each substrate and sensor type. Fitted regression curves of sensor output (mV) versus pressure applied (kPa) had highly significant R-2 values (P<0.001; n=147 to n=360) except for the perlite substrate (R-2=0.29; P<0.05; n=187). Confident that these sensors can provide precise measurements of PAW between -1 and -20 kPa in most soilless substrates, we are continuing to test sensor performance under field conditions with wireless sensor networks.