Estimating Soil Water Content from Permittivity for Different Mineralogies and Bulk Densities

Time domain reflectometry (TDR) exploits the difference in dielectric permittivity between the soil solid, gas, and liquid phases and has become increasingly popular for the determination of soil water content. The electromagnetic determination of water content in soils with high clay contents requires a good understanding of the main factors that affect the relationship between the dielectric permittivity of clay minerals and soil water content. The objectives of this study were to: (i) investigate the relationships among soil bulk density, clay content, clay mineralogy, and the soil dielectric permittivity, and (ii) evaluate the performance of the approach to correct the dissipation effects due to high clay contents. Samples were taken from five areas with different textures and bulk densities, both topsoils and subsoils. Soil physical properties, i.e., organic matter, clay, silt, and sand contents, and clay mineralogy were determined. High clay contents caused an underestimation of soil water content in the low moisture range and its overestimation in the high moisture range. Results also demonstrated that soils with similar clay contents but different mineralogies showed different permittivity values. For soil samples with equal water contents, soils with higher bulk density, or lower porosity, had a larger permittivity than soils with low bulk density and high porosity. This behavior was the result of the increase in the ratio of solid particle volume to air because the solid particles have a larger dielectric permittivity than air. Soil bulk density and clay content impact the accuracy and calibration of TDR. Smectites also cause signal attenuation and dispersion, resulting in smaller signal amplitude and longer rise time. In addition, smectites cause dispersion of the reflected signal, resulting in a longer rise time.