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Computation of Soil Solution Composition Variation with Water Content for Desaturated Soils¹

ABSTRACT

Mathematical models were evaluated which calculate the change in soil solution composition and electrical conductivity as water content is changed by evaporation or extraction by plants. Two models included consideration of precipitation of salts, cation exchange, soil pH buffer capacity, ionic activities, sulfate, bicarbonate and carbonate ion pairs of calcium, sodium, and magnesium, and partial pressure of carbon dioxide. These models differed in the form of the Debye-Huckel equations used to calculate ionic activity coefficients. A third model included only the ion pair calcium sulfate. The reliability of the models was evaluated by comparing electrical conductivities measured by in situ salinity sensors with calculated conductivities. The calculated values were based on chemical data obtained from saturation extracts of soil samples taken from locations at which the sensors were installed. The best model considered the maximum number of ion pairs and used a form of the Debye-Huckel equation with individual ion parameters. This model resulted in an average difference between calculated and measured values of 1.1 mmho/cm, which represented an error of 6% when compared to the average measured value of 17.1 mmho/cm.

¹ Contribution from the US Salinity Laboratory, SWC, ARS, USDA, P. O. Box 672, Riverside, Calif. 92502. Presented as part of the symposium "The Soil Solution" before a joint meeting of Div. S-2, S-1, S-4, and S-9, Soil Science Society of America, Aug. 24, 1970, at Tucson, Ariz.

² Research Soil Scientists. The junior author is now with the Department of Agronomy and Soils, Washington State University, Pullman, Wash. 99163.

Received for publication November 19, 1970. Accepted for publication January 14, 1971.