Field Evaluation and Performance Comparison of Soil Moisture Sensors

Agricultural producers who choose to supplement their crops’ water requirement are able to determine irrigation scheduling practices better when the soil water content of their fields is known. The objective of this study was to statistically evaluate numerous sensors for their ability to accurately estimate water content in a 90-cm soil profile, based on calibrated neutron probe measurements. The sensors tested were Irrometers, Watermarks, EnviroScan, Troxler Sentry, AquaTel, AquaFlex, Trime, AquaPro, and GroPoint. The sensors were field tested at different water content levels and a variety of irrigation frequencies over a 3-year period in a Warden silt loam soil (Coarse-silty, mixed, mesic, Xerollic Camborthids) planted to alfalfa. The default factor calibration was used to estimate the soil water content from all sensors except the neutron probe, which was calibrated for the soil using the gravimetric moisture content method. The Irrometer and Watermark sensors utilized a local soil water retention relationship in order to convert soil water potential into volumetric water content. The results suggest that most sensors were able to follow the general trends successfully as soil water content changed during the growing season, and there was significant correlation between the sensors and the neutron probe readings. Although sensor trends were similar, visual and statistical analyses indicated that the actual measured values varied significantly between the sensors and the calibrated neutron probe measurements. Therefore, a soil specific calibration of each sensor would have been necessary to obtain a high degree of absolute accuracy in soil water content measurements. The results suggest that irrigators can still use uncalibrated sensors to improve their watering schedules by setting irrigation trigger points that may relate only to a specific sensor in a specific soil. These trigger points cannot easily be related to different soils, different sensors, and other sources of information such as extension fact sheets and research publications, however, because the actual water content measurements may not be correct.