Dielectric models for moisture determination of soils with variable organic matter content

An accurate calibration function relating soil dielectric permittivity to its volumetric water content is essential for reliable measurements of soil moisture with the use of dielectric sensors. In the operational frequency range of popular capacitance and impedance sensors, various soil properties may influence the relation between the soil moisture and dielectric permittivity, which may worsen the measurement accuracy of a sensor. The aim of this paper is to investigate the relations among the organic matter content, dry bulk density, volumetric water content and dielectric permittivity in the 10–500 MHz frequency range, which covers the frequency range of operation of common soil moisture dielectric sensors operating in the frequency domain. Samples of soil material of belonging to sand, sandy loam and silt loam texture classes with varied organic matter content were measured with the use of a coaxial-like open-ended seven-rod probe connected to a vector-network-analyzer. A new simplified dielectric model is presented in order to accurately describe the obtained spectra. Next, several models connecting the real part of dielectric permittivity and volumetric water content, taking account of the impact of bulk density, were examined. It was found that two linear segmented models fitted to the obtained data better in the case of soil materials of sandy loam and silt loam textures. The best segmented model exhibited RMSE of about 0.023 m3 m−3 in comparison to about 0.04 m3 m−3 in the case of a simple linear model without density correction.