The dielectric properties of soil‐water mixtures at microwave frequencies

Recent measurements of the dielectric constants of soil-water mixtures have shown the existence of two frequency regions in which the dielectric behavior of these mixtures was quite different. At the frequencies of 1.4–5 GHz there was strong evidence that the variation of the dielectric constant e with water content W depended on soil type. While the real part of e, e′, for sandy soils rose rapidly with the increase in W, e′, for the high clay content soils rose only slowly with W. As a consequence, e′ was generally higher for the sandy soils than for the high clay content soils at a given W. On the other hand, most of the measurements at frequencies < 1 GHz indicated the increase of e with W to be independent of soil types. At a given W, e′ (sandy soil) ≅ e′ (high clay content soil) within the precision of the measurements. These observational features could be satisfactorily interpreted in terms of a simple dielectric relaxation model suggested by Schwarz (1962), with an appropriate choice of the mean relaxation frequency ƒm and the range of the activation energy β. It was found that smaller ƒm and larger β were required for the high clay content soils than for the sandy soils in order to be consistent with the measured data. These requirements on ƒm and β were strongly supported by the observed dependence of the soil surface area per unit weight on the soil types.