Maxwell-wagner-sillars, adsorbed water, and free-water dielectric relaxations within a hydrated arid-zone calcic soil

We measured the broadband (40 Hz-20 GHz) relative dielectric permittivity of sieved surficial samples of an arid-zone soil. The soil samples were equilibrated at 25 °C with relative humidities of 43, 54, 84, and 97 %. Debye-like Maxwell-Wagner-Sillars (MWS) polarization relaxation times of 309, 227, and 73 ns were calculated for the soil samples equilibrated at relative humidities of 43, 54, and 84 %, respectively. Due to electrode polarization, MWS polarization relaxations could be not be discerned on the soil sample equilibrated at 97 % relative humidity. The measured differences between the static and high-frequency relative permittivity (ε_s - ε_∞) due to MWS polarization were between 25 and 45 times that of the oven dry soil. This would cause the MWS relaxation to increase radar-wave attenuation rates at frequencies as high as 1 GHz. We found Debye-type relaxation times associated with adsorbed water of 13, 15, 17, 19, and 22 ps for an oven-dry sample and ones equilibrated at at relative humidities of 43, 54, 84, and 97 %, respectively. These frequencies are higher than previously thought. This progression with water contents may be due to the increase in soil solution salinity with increasing water contents. They were not likely caused by free water since a weakly resolved relaxation could be discerned corresponding to a relaxation time of approximately 8 ps, as expected for free-water.