An empirical equation to calculate soil solution electrical conductivity at 25 degrees C from major ion concentrations
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The electrical conductivity at 25 degrees C (EC(25)) of soil solutions or irrigation waters is the standard property for assessing salinity. Many models for soil salinity prediction calculate the major ion composition of the soil solution. The electrical conductivity of a solution can be determined from its composition through several different empirical equations. An assessment of these equations is necessary to incorporate the most accurate and precise equations in such models. Twelve different equations for the EC(25) calculation were calibrated by means of regression analyses with data from 133 saturation extracts and another 135 1: 5 soil-to-water extracts from a salt-affected agricultural irrigated area. The equations with better calibration parameters were tested with another data set of 153 soil solutions covering a wide range of salt concentrations and compositions. The testing was conducted using the standardized difference t-test, which is a rigorous validation test used in this study for the first time. The equations based on the ionic conductivity decrement given by Kohlrausch's law presented the poorest calibration parameters. The equations founded on the hypothesis that EC(25) is proportional to analytical concentrations had worse calibration and validation parameters than their counterparts based on free-ion concentrations and ionic activities. The equations founded on simpler mathematical relationships generally gave improved validation parameters. The three equations based on the specific electrical conductivity definition presented a mean standardized difference between observations and predictions indistinguishable from zero at the 95% confidence level. The inclusion of the charged ion-pair concentrations in the equation based on free-ion concentrations improved its predictions, particularly at large electrical conductivities. This equation can be reliably used in conjunction with chemical speciation software to assess EC(25) from the ion composition of soil solutions.