Calibration and performance of moisture sensors in soilless substrates: ECH2O and Theta probes

Reliable and affordable moisture sensors for measuring the water content in soilless substrates are limited. In this study, we examined the efficacy of two moisture sensors (ECH2O-10 and Theta probe ML2X) for measuring water content in soilless substrates. We developed calibration equations and analyzed the effect of increasing electrical conductivity (EC) and substrate temperature on the voltage output of probes. We found that a single equation (one for each probe) could be used to adequately measure water content in different custom-made substrates maintained at low EC and a substrate temperature of ∼23 °C. The calibration equation developed for the Theta probe from substrates maintained at low EC could also be used in two commercial substrates with high EC (2.0-5.0 dS m-1). Further measurements in substrates maintained at different water contents and EC levels indicated that the output of the ECH2O-10 probe, but not the Theta probe, was significantly affected by substrate EC. The lack of a substrate EC effect on the Theta probe readings was surprising, because the probe was sensitive to EC when fertilizer solutions with different ECs were measured. Increasing the temperature of the substrate from 10 to 40 °C increased the voltage output of ECH2O-10 probes by 1.88 mV °C-1, or on average 0.0022 m3 m-3 water content per °C. There was no effect of increasing substrate temperature on the Theta probe. It was concluded that ECH2O-10 probes can be used in greenhouse operations requiring less measurement precision (like irrigation), however for accurate measurements of water content, the Theta probe is preferred. © 2006 Elsevier B.V. All rights reserved.