Assessment of clay and humus content in soils using the dielectric method

The paper presents the results of measuring the content of clay, humus, and complex permittivity of 23 soil samples in the frequency range from 10 kHz to 8–10 GHz at different moisture. The content of clay (particles smaller than 0.01 mm) in the samples varied from 1.7 to 47.5%, the humus content – from 0.7 to 8%. The processes of dielectric relaxation were studied, correlations between the process parameters and the clay content were found. To estimate the clay content, the intensity of the low-frequency relaxation process and the complex permittivity at a frequency of 100 kHz, measured in a state close to complete capillary saturation, were used. The square of the correlation coefficient of the found average values with those measured by the sedimentation method was 0.89, the average deviation was – 14% with a maximum deviation of 50–60% for two soil samples. In order to estimate the humus content, the values of clay and the maximum amount of bound water found by the dielectric method were used. The maximum amount of bound water was determined by modeling the dielectric spectra using the relaxation-refraction model as the average of the values found at different moisture of the soil samples. The square of the correlation coefficient of the found values with those measured by the Walkley–Black acid digestion method was 0.72, the average deviation was 28% with a maximum deviation of 200% for one soil sample. The reasons for the high errors were discussed.