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Independent Measurement of Matric and Osmotic Potential of Soil Water¹

ABSTRACT

A technique is described that uses the operating principles of the thermocouple psychrometer and the porous plate apparatus to measure the matric and osmotic potentials of soil water without extraction of a significant amount of soil solution. The technique was tested on a Na-saturated Gila soil to which NaCl solutions with osmotic potentials of –1, -2, and –6 bars were added. The standard error of measurement of the soil water potential components was ±0.04 bar. The measured osmotic potentials of the soil were lower than those of the added solutions, which can be explained by salt exclusion although mineral dissolution could contribute to these results. The technique also permitted measurement of the partial molar volume of soil water with an accuracy of about ±1%. Within this precision, the partial molar volume of soil water in this study was shown to be the same as that of pure water.

Contribution from the US Salinity Laboratory, SWC, ARS, USDA, P.O. Box 672, Riverside, Calif. 92502. This work was supported in part by the Atmospheric Sciences Laboratory, Research Division, US Army Electronics Command, Fort Huachuca, Ariz. 85613.

² Research Soil Scientists and Chemist, respectively.

Received for publication May 20, 1968. Accepted for publication August 14, 1968.