New semi-empirical formulae for predicting soil solution conductivity from dielectric properties at 50 MHz

Pore solution electrical conductivity (σw) is an important measurement for agricultural and environmental applications. Salt concentration can be predicted from σw and used to trace and monitor the transport of ionic solutes. Most models for predicting σw rely also on measurements of volumetric water content (θv). However, θv dependent estimations of σw are difficult to obtain because θv measurements are not always available and because of the complex nature of physicochemical interactions between soil and water. Electromagnetic sensors offer an alternative approach because estimations of σw can be obtained without direct knowledge of θv. We developed two semi-empirical formulae for predicting σw that are mathematically independent of θv. The new models are dielectric equivalents of the Rhoades type two-pathway models that are based on linear and power law solutions for the transmission coefficient. The models were fitted by nonlinear regression to a data set collected from different soil textures, and disturbance treatments, to predict known electrical conductivities of ∼0, 1.23, 2.41, 2.02 and 3.96 dS m−1. Average σw predicted by the new models compared well to known saturating solution conductivities (average R2 = 0.82 and average root mean square error of 0.80 dS m−1). The new models also compared well to models reported in the literature. The precision and accuracy of the estimates increased as σw increased, which might be related to a reduction of the influence of soil solid phase components on the estimates as σw becomes much higher than the solid phase conductivity.