Measurement of soil water content with a 50 MHZ soil dielectric sensor

Authors: Seyfried, Mark; Murdock, Mark

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2004
Publication Date: 6/20/2004
Citation: Seyfried, Mark S., Murdock, Mark D., Measurement of Soil Water Content with a 50 MHZ Soil Dielectric Sensor, Soil Science Society of America Journal, 2004, v. 68, pp 394-403.

Interpretive Summary: The Hydra Probe is a relatively inexpensive and widely used soil water content( , m3m-3) sensor. It measures both the real (er) and imaginary (ei) components of the complex soil dielectric constant at 50 MHz. Our objectives were to: (i) evaluate HP dielectric measurement accuracy and precision and how it is affected by electrical conductivity, (ii) determine how soil type and temperature affect the er- relationship, and (iii) describe those effects in terms of measurement frequency relative to TDR in order to develop more general -er relationships. We evaluated Hydra Probe er measurement precision and accuracy in air, ethanol and water and a series of aqueous KCl solutions. Effects of soil type on calibration were evaluated with four soils. Temperature sensitivity was tested in air, oven-dry and nearly saturated soil. Each test was performed with three sensors. We found that, in fluids, the sensors were accurate (er within 0.5), precise (CV<1%) and that inter-sensor variability was generally low except in KCl solutions greater than 0.01 M. There was a strong correlation between and er for all soils tested but the -er relationship varied with soil. None of the three calibration equations supplied by the manufacturer effectively described the measured data. Deviations of measured -er from the Topp equation increased in magnitude with ei. The effect of temperature on er was negligible in oven dry soils and different for each soil when nearly saturated. Given a temperature change of 5 to 45°C, er changes corresponded to changes of from ¿0.03 m3m-3 to 0.06 m3m-3. In general, it appears that differences between Hydra Probe performance and TDR are related to differences in soil dielectric properties at the measurement frequencies of the two instruments.

Technical Abstract: Knowledge of soil water content is useful for those who schedule irrigation of crops or landscaping, estimate plant growth, manage water supply or are concerned with the rate of microbial breakdown and transport of organic pollutants. In recent years, electronic instrumentation, known as TDR for time domain reflectometry, has been developed that uses measured soil electrical properties that are strongly affected by water to calculate soil water content. These instruments are particularly valuable because data from them can be transmitted via radio or phone lines to interested managers or scientists. The high cost of TDR, which is prohibitive for many practical applications, has lead to the development of numerous, less expensive instruments that use other techniques to measure the same electrical properties of soil. Unfortunately, these instruments have received very little independent testing and critical information such as instrument accuracy and precision are not known. We tested one such instrument , the Hydra Probe, which is currently in widespread use, to determine the accuracy of the calibrations and how they are affected by temperature. We found that the manufacturer-supplied calibrations are seriously in error for the soils we tested and that other calibrations should be considered. We also found that temperature effects those calibrations. We showed how calibrations may be expected to vary from a standard calibration based on soil properties and that temperature effects may not be critical if acknowledged. Differences between Hydra Probe and TDR responses reflect how soil properties change when measured at different electrical frequencies. These results should be useful for those using or intending to use the Hydra Probe or other similar instruments by indicating the kinds of soil properties that affect calibration and the effect of temperature on calibration. Insights gained from the implications of soil property changes with measurement frequency may result in improved accuracy of future, relatively inexpensive, soil water content sensors.