Determining soil salinity from soil electrical conductivity using different models and estimates

The appropriateness of two versions of a model describing electrical current flow in undisturbed soil was evaluated for purposes of diagnosing and mapping soil salinity (ECe). Different methods of measuring bulk soil electrical conductivity (EC.) and different ways of obtaining the values of other parameters required by the model were also evaluated using three different sets of data. The reliability of the variously predicted salinities were evaluated by comparing them against conventionally measured salinities using linear regression analyses, ranking tests, and a procedure based on the weighted sums of squared differences. It was found that salinity-prediction accuracy, though relatively good, was underestimated for the instrumental/model techniques because of sample-size differences and spatial-variability effects. Conclusions arrived at from earlier sensitivity analyses about the suitability of the parameter-estimation procedures were borne out by these results, i.e., that soil salinity appraisal and mapping can be quite adequately made using field measurements of EC. (by any one of three methods) and estimates of soil water content, bulk density, and surface conductance determined by "feel" methods. R a new model of electrical current flow in undisturbed soil has been presented for the determination of soil salinity (Rhoades et al., 1988a). Soil salinity in this model is expressed as the electrical conductivity of the saturation extract (EC? ). The practical use of this method of salinity appraisal requires the measurement of bulk soil electrical conductivity (ECJ and estimates of the volumetric content of soil (0S), the volumetric content of soil water in the seriescoupled pathway (0WS), essentially the water in the fine pores, or so-called immobile water, the average electrical conductivity of the soil particles (ECS), the bulk density of the soil (pb), and the average density of the soil particles (ps). The volumetric content of soil water (0W) must also be known, either from measurement or estimate. Empirical relations have been established between 0WS and 0W and between ECS and saturation percentage (SP) or percent clay content of the soil (%C), to estimate 0WS and ECS, respectively. Estimates of the other unmeasured parameters must be made for the practical use of this model. Sensitivity analyses of this model, and the empirical relations used to determine ECe, were made to ascertain which of the model parameters must be known accurately (hence must be measured) and which can be estimated well enough so as not to produce significant error in the field appraisal of soil salinity (Rhoades et al., 1988b). It was concluded from this study that reliable determinations of ECe should be obtainable from measurements of ECa and with reasonable estimates of the other model parameters, such as the average field soil scientist should be able to USDA-ARS, U.S. Salinity Laboratory, 4500 Glenwood Drive, Riverside, CA 92501. Received 7 Sept. 1988. "Corresponding author. Published in Soil Sci. Soc. Am. J. 54:46-54 (1990). make. This paper tests this conclusion and, additionally, it evaluates the relative appropriateness of the different methods of measuring ECa, of various ways of estimating the values of the required model parameters, and of a limited version of the model. MATERIALS AND METHODS The equations used to determine ECe from measured and estimated parameters were as follows ECa = (0S + 0WS) ECWS ECJ (0S) EC ECJ