Estimation of soil solution electrical conductivity from bulk soil electrical conductivity in sandy soils

Studies of solute transport through soil and attendant environmental impacts are hampered by the lack of methods for continuous monitoring of solute concentration. Measurement of bulk soil electrical conductivity (ECb) using time domain reflectometry (TDR) is a promising technique, but it is indirect, and estimation of solute concentration from such measurements requires a model relating soil solution electrical conductivity (ECw) to ECb. Several models of varying complexity exist, but further testing is required to determine their relative merits and applicability. This study was conducted to determine the efficiency of various models that use different methods of estimating the tortuosity factor, F-g, in order to estimate ECw from ECb. All models assume that the ratio of ECb/ECw is proportional to soil water content, theta, with F-g as the coefficient of proportionality. In this study, two types of models were compared, those in which F-g is obtained from soil hydraulic properties and those in which F-g is estimated as it is in gas diffusion models, except with theta rather than porosity as the independent variable. Measurements were conducted in a sandy soil, across a range of ECw from 0.10 to 0.56 S m(-1). The models in which F-g is obtained from soil hydraulic properties did not perform as expected. The results with the gas diffusion analog models were variable; the most successful of these was based on the 1959 model of Marshall, in which F-g is a power function of theta, theta (b). Optimal results were obtained with b = 0.58, not far from Marshall's suggested value of 0.5 for gas diffusion. We conclude that there is no benefit to the use of soil hydraulic properties in estimating ECw from ECb measurements, at least for sandy soils, where simpler relationships appear to provide superior results.