Impact of soil salinity, texture and measurement frequency on the relations between soil moisture and 20 MHz–3 GHz dielectric permittivity spectrum for soils of medium texture

Abstract

Dielectric sensors are a popular choice for soil moisture determination. However, the output of many of these devices, especially those operating at frequencies in the MHz range, can be significantly affected by the influence of soil salinity and texture. The aim of this paper is to examine the impact of salinity, texture and measurement frequency on dielectric permittivity – soil moisture calibration curves for ten soils of medium texture in the frequency range from 20 MHz to 3 GHz based on the complex dielectric permittivity spectra measured with the use of a coaxial transmission line cell connected to a vector-network-analyzer. The use of laboratory-measured spectra ensures that at all frequencies dielectric permittivity is measured at the same conditions with no influence of factors such as different sensitivity zone or various installation conditions, as in the case of comparing individual sensors operating at various frequencies. The obtained relations were comparable to the calibration curves of several capacitance/impedance sensors operating at corresponding frequencies. Calibration curves for apparent permittivity, which were obtained with the use of simulations in the time domain based on the measured spectra, were also determined for the tested soils. Even though the tested soils did not include soils with high clay content, the impact of soil texture on the calibration curves was observed. Results confirmed that the influence of soil salinity and texture on dielectric permittivity was the most significant in the lower part of the frequency spectrum, while the smallest impact of these factors was observed at frequencies of several hundred MHz and above.