Effects of Water Content and Salinity on Soil Electrical Properties at 50 MHz: Structural and Textural Interactions

The use of dielectric methods for estimating water content and electrical conductivity in saturated and partially saturated porous media is one of the major innovations in soil physics over the past decades. One example of a dielectric sensor is the 50 MHz Hydra Probe. The Hydra Probe is an impedance device that operates at a fixed frequency measuring both components of the complex soil dielectric permittivity response. The general objectives of this research were to: i. improve the understanding of the relationship between soil physical and electrical properties measured at 50 MHz using the Hydra Probe sensor, ii. evaluate the effects of texture, disturbance and salinity on the estimation of water content using the 50 MHz sensor, and iii. develop new models for predicting soil pore solution conductivity from soil electrical properties at 50 MHz. Disturbed and undisturbed duplicate samples from a range of soil textures (Clay, Silty Clay Loam, and Sandy Loam) were saturated with distilled-deionized water and saline solutions at four concentrations: KCl and CaCl₂ at 0.01 and 0.02 Mol L^-1 for three days and then air dried under laboratory conditions. Real and imaginary components of the dielectric permittivity were measured every 5 minutes by the Hydra Probe. Load cells recording changes in sample weight over time, which were later converted into volumetric water content, were also logged. Soil bulk apparent conductivity was calculated from the imaginary permittivity. I found that there was no benefit in including the imaginary dielectric permittivity, or a correction for the loss tangent, in models for estimating water content at 50 MHz. Based on the results, Clay soils should be assessed independently when developing calibration equations for the Hydra Probe. Furthermore, the sensor’s water content estimations are sensitive to soil disturbance. New models for estimating the pore solution conductivity were developed. These are dielectric equivalents of Rhoades type two-pathway models based on linear and power law solutions for the transmission coefficient. Overall the average soil solution conductivity predicted by the new models compared favorably to that of the saturating solutions for conductivities greater than about 1.23 dS m^-1_.