Hideki Miyamoto, Faculty of Agriculture, Saga University, Saga, JAPAN, Masaaki Uemura, Graduate school of Agriculture, Saga University, Japan, Saga, Japan, Markus Tuller, PO Box 210038, University of Arizona, Tucson, AZ and Ty Paul Ferre, University of Arizona, Tucson, AZ
Digital time domain transmission (TDT) sensors have become widely accepted as accurate and economical alternative to time domain reflectometry for moisture content measurements in moderately conductive soils. However, their applicability for dielectrically lossy clays has not been investigated thoroughly. Two types of coatings, a silicone coating (SC) and a high-dielectric coating (HDC), were applied to commercially available TDT sensors. The sensors were used for calibration experiments in highly dispersive bentonite slurries and less dispersive kaolinite slurries at a range of moisture contents and electrolyte concentrations. Because the apparent permittivity measured with the coated sensors consists of two components (permittivity of the soil and permittivity of the coating), the standard Topp relationship cannot be applied directly to infer water content from apparent permittivity. Individual calibration of the coated sensors yielded good results for kaolinite slurries with gravimetric water contents higher than 0.37 kg/kg. But, dielectric dispersion in the bentonite-water mixtures resulted in reduction of the effective measurement frequency, making it impossible to obtain a satisfactory calibration. As a result, we conclude that coated TDT sensors can be calibrated for moisture content measurement in heavy natural clay soils if the dominating clay mineralogical fractions do not exhibit highly dispersive behavior.