Influence of Temperature on Soil Dielectric Spectra in the 20 MHz–3 GHz Frequency Range

Measurement of soil dielectric permittivity is frequently used as a basis for soil water content estimation by many commercial and experimental sensors. However, the relations between dielectric permittivity and soil water content may be affected by various other factors, including texture, salinity, and temperature. In this present article, spectra of complex dielectric permittivity of samples of loamy sand, sandy loam, and silt loam soils of various water content and bulk electrical conductivity were measured with the use of a six-cell coaxial transmission-line system in the 20 MHz–3 GHz frequency range at temperatures from 0.5 °C to 40 °C. The analysis focused on the impact of temperature on: 1) the real part of complex dielectric permittivity; 2) the relations between the real part of dielectric permittivity and volumetric water content; and 3) bulk electrical conductivity estimated from dielectric modeling. The results showed complicated temperature behavior of soil dielectric spectrum, depending on water content, salinity, frequency, and texture. Also, the parameters of a permittivity-moisture conversion function were established at various temperatures and frequencies.