Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 9, 2005
Publication Date: July 5, 2005
Citation: Logsdon, S.D. 2005. Soil dielectric spectra from vector network analyzer data. Soil Science Society of America Journal. 69(5)983-989.
Interpretive Summary: Knowing how much water is in the soil is necessary to understand how crops take up water and how chemicals are leached out of the soil. Soil water is often measured by its dielectric properties, which are high because water molecules have positive and negative ends that line up when an electrical field is applied. Often these measurements are sensitive to properties other than soil water content. This study compared the dielectric properties of six soils from different parts of the country. The dielectric properties were related to the type of clays and soluble materials in the soil as well as the water content. This research is useful for scientists who want to measure soil water content, and for manufacturers of electrical equipment that measure soil properties.
For over two decades, dielectric properties have been used for soil water content measurements, but spurious results have been shown for saline soils or soil high in smectite clays. Permittivity is complex with real and imaginary components, and varies with frequency. The frequency-dependence is due to dipole rotation and charge migration processes under alternating current. Parameters can be derived from the frequency-dependent spectra, and these parameters can be related to soil properties. The purpose of this study is to develop a procedure for determining complex permittivity spectra for soils, to determine how spectra or derived parameters are related to soil properties, and to use dielectric spectra to help explain spurious results when dielectric properties are used for water content determination. Six soils with a range of mineralogies were pre-equilibrated at four replicated water contents. They were packed into truncated coaxial cells, and the reflection scattering parameter was measured for frequencies between 300 kHz to 3 GHz. The complex permittivity was difficult to use for deriving unique parameters because of overlapping influence of electrical conductivity and multiple dielectric relaxation processes that extended beyond the measured frequency range. The complex resistivity was easier to interpret, clearly showing one major relaxation process. The two smectitic soils and the soil with free carbonates showed the greatest increase in square root of apparent dielectric at low frequencies and high water contents, but all soils showed an increase as frequency decreased.