Soil permittivity response to bulk electrical conductivity for selected soil water sensors

Bulk electrical conductivity (σa) can dominate the low frequency dielectric loss spectrum in soils, causing changes in the permittivity and errors in estimated water content. We examined the dependence of measured apparent permittivity (Ka) on σa in contrasting soils using time-domain reflectometry (TDR), a digital time-domain transmission (TDT) sensor, and a capacitance sensor (5TE) during near saturated solute displacement experiments. Sensors were installed in columns packed with fine sand or a clay loam soil. Displacement experiments were completed by first equilibrating columns with 0.25 dS m-1 CaCl2, introducing a step pulse of ∼4.7 dS m-1 CaCl2 and, after equilibration, displacing the resident solution with 0.25 dS m-1 CaCl2. Using TDR, measured Ka increased with increasing σa; however, the slope of this response averaged 3.47 m dS-1 for clay loam compared with 0.19 m dS-1 for sand. The large response in the clay loam was attributed to relaxation losses that narrowed the effective bandwidth from 821 to an estimated 164 MHz. In contrast, the effective frequency in sand averaged 515 MHz. Permittivity measured using the TDT probe exhibited little or no sensitivity to σa (