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Many popular soil moisture meters are based on dielectric measurements. Volumetric water content is the main contributor to soil dielectric permittivity. However, other soil properties such as texture, salinity and organic matter content may also have a significant influence. Consequently, if the impact of these factors is not taken into account, soil moisture measurement accuracy may be deteriorated. This is especially important at frequencies below about 500 MHz, at which the impact of soil-specific interfacial dielectric relaxation phenomena may be visible. Most of the relatively inexpensive capacitance and impedance sensors operate at a single frequency not higher than 300 MHz, which makes them especially prone to such effects.
The aim of the presented research is to model the impact of organic matter content of mineral soils on complex dielectric permittivity spectra in the 10 – 500 MHz frequency range. Sand, black soil, garden soil and artificial soil material with additions of organic matter were examined. For each material, samples of various water content were prepared and measured with a short seven-rod coaxial-like open-ended probe connected to a vector-network-analyzer. The obtained dielectric spectra were modeled with the use of dielectric models with Debye and Cole-Cole poles. The impact of organic matter on the spectra were analyzed and the implications for soil moisture measurements were discussed.