Measured and Modeled Dielectric Properties of Soils at 50 Megahertz

Commercial soil water sensors of the capacitance and impedance type generally operate at low (<200 MHz) frequencies where the soil dielectric properties are frequency dependent, especially for soils high in clay. In this work, we examined the dielectric properties of four soil materials (fine sand, sandy loam, loam, and clay) at 50 MHz using Hydra impedance sensor real and imaginary permittivity measurements and time domain reflectometry (TDR) apparent permittivity and bulk soil electrical conductivity (EC) measurements. The results showed that the Hydra sensor real and imaginary permittivity increased with increasing soil clay content and that relaxation loss was a significant part of the total energy loss, except for the fine sand, where the relaxation loss was zero. Model calculations showed that the relative contribution of surface conductivity to bulk soil EC increased over the contribution of pore water conductivity as clay content increased. Application of a soil permittivity model showed that the observed real permittivity epsilon' and relaxation loss epsilon(rel) for all soils except the fine sand could not be explained using simple mixing rules. The underestimation of the modeled epsilon' and epsilon(rel) for soils with non-negligible clay content was attributed to interfacial polarizations that were not included in the model. The modeled contribution of bound water to relaxation loss was found to be small.