Extended Dual Composite Sphere Model for Determining Dielectric Permittivity of Andisols

A dielectric mixing model was used for understanding the dependency of dielectric permittivity on water content and soil physical properties. We used a mixing model to interpret aggregate structural effects on the relationship between volumetric water content (θ) and dielectric permittivity (ε) of Andisols. Our objective was to determine the applicability of a mixing model to describe the dielectric permittivity of aggregated soil. A dual composite sphere model, proposed by Friedman in 1998, was extended by taking into account water distribution in aggregated soil and the processes of water filling in intra‐ and interaggregate pores. Hence, our extended model includes four‐phase composite spheres and sigmoidal functions as a weight function in the model. Experimental data for two wet aggregates (0.1–0.25 and 1.0–2.0 mm in diam.) and two Andisols taken from Kumamoto and Miyazaki in Japan were used for the demonstration of model applicability. The addition of an additional layer in the composite sphere model improved the predictability of the model for the θ – ε relationship in a moisture range of less‐than‐critical water content. The θ –d ε/ d θ curves estimated by the model were in better agreement with the experimental data than those of Friedman's model. In particular, applying the sigmoidal weight functions improved the estimation of θ –d ε/ d θ curves in a moisture range higher than that of critical water content. Our adjusted model serves to improve the understanding of the relationship between the physical properties of aggregated soils and their dielectric permittivity.