Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/12/2009
Publication Date: 1/7/2010
Citation: Logsdon, S.D., Green, T.R., Seyfried, M.S., Evett, S.R., Bonta, J.V. 2010. Hydra Probe and Twelve-wire Probe Comparisons in Fluids and Soil Cores. Soil Science Society of America Journal. 74:5-12. Interpretive Summary: Soil moisture probes are used to assist in understanding and making decisions about the amount of water available in crop root zones. This study compared the commercially-available Hydra probe with a laboratory probe for soil samples from Iowa, Idaho, Colorado, Ohio, and Texas. Both the Hydra probe and the laboratory probe measure dielectric properties, which relate to soil water content, and measure electrical conductivity, which relates to salts and clays in the soil. The dielectric properties agreed well between the two probes, but the bulk soil electrical conductivity from the Hydra probe needed to be recalculated with alternate equations. This information is useful for scientists and consultants because it shows how to make the calculation adjustment for electrical conductivity.
Technical Abstract: Soil water content is determined from various permittivity probes. The Hydra Probe measures both the real and imaginary permittivity and estimates electrical conductivity (EC). The twelve-wire probe has been used with a vector network analyzer to determine real and imaginary permittivity and EC, and has been used with undisturbed soil cores. The purpose of this study was to compare results from the Hydra Probe and the twelve-wire probe for fluids as well as for soil cores taken from six locations in order to better understand these measurement systems. Undisturbed soil cores were collected from an Iowa prairie and adjacent soybean field, an Iowa forest and adjacent soybean field, Idaho burned and unburned sites, Colorado grassland and dryland wheat sites, Ohio grassed areas, and Texas rangeland and an irrigated field. For some of the Texas samples it was not feasible to measure electrical properties on undisturbed samples because dense soil restricted insertion of probes and wetting of samples. The measured EC and real and imaginary permittivity of fluids were compared between the two measurement systems and with theoretical values. Since the theoretical values for soils are not known, the measured values were compared between systems. Both probes showed EC and real and imaginary close to the theoretical values for fluids. In soils, we showed that an adjustment was needed in the calculated EC from the Hydra Probe to account for dielectric relaxation. The soil EC of the Hydra Probe better matched that from the twelve-wire probe after the adjustment was made.