Measurement of solid phase permittivity for volcanic soils by time domain reflectometry

Dielectric properties of soils are widely used to estimate their water content. Andisols are unique soils in terms of aggregate structure and show dielectric properties different from other mineral soils. To understand the relationship between the dielectric properties and soil physical properties, multi‐phase dielectric mixing models are often used. However, solid phase permittivity (?_s) for Andisols, which is required for calculating the model output, has not been directly measured yet. Therefore, the objectives for this study were to measure ?_s for Andisols in Japan. In addition, the measured ?_s values were compared with those calculated from the traditional method, which applies two‐phase mixing models to estimate ?_s values based on effective permittivities of repacked soil samples. The applicability of the traditional method to estimate such values for Andisols is also discussed. The effects of bound water and aggregate structure on measured ?_s values were evaluated prior to their measurements. We found that the aggregate structural effects were negligible. However, the amount of bound water caused overestimates of ?_s. Four Andisols from the A and B horizons of soils in Memuro Hokkaido (northern part of Japan), from the A horizon of a soil in Tsukuba Ibaraki (central part of Japan), and from the A horizon of a soil in Koshi Kumamoto (southern part of Japan) were used in this study. The ?_s values obtained fall between 5.6 and 6.1, and deviated from the estimated ?_s values derived from the traditional method. Therefore, the traditional method is probably unsuitable to estimate ?_s values for Andisols.