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Some Thermodynamic Properties of Soil Water¹

ABSTRACT

The partial specific Gibbs free energy or moisture potential of soil water has been measured by methods that are widely accepted. There are several other thermodynamic properties that are of significance in soil moisture relationships that are not so readily measured and are not well understood. The partial specific entropy and enthaply (heat content) of soil water are two such properties of interest. They have been calculated from the temperature dependence of the moisture potential.

The moisture content, solute content and pressure of soil samples were held constant. The temperature of the system ranged between 2° and 45° C. It was changed periodically and controlled at well-defined intervals. After each change in temperature, the system was allowed to come to equilibrium and the moisture potential, as measured by tensiometers, was determined. From the temperature dependence of the moisture potential, the partial entropy change of soil water was found to be more negative than that of free water. Significant entropy differences were found in soils that contained water at a moisture potential of 0.008 joules per g. (0.08 bars suction) at about 20° C. The partial specific entropy and enthalpy of soil water were found to be dependent upon moisture content and temperature.

¹ Research reported here was done as part of Western Regional Research Project W-29 with twelve Western states and the Agricultural Research Service cooperating. Presented before Div. I, Soil Science Society of America, Nov. 19, 1959, at Cincinnati, Ohio. Approved for publication by the director of the Utah Agr. Exp. Sta. as Journal Paper No. 112.

² Professor of Soil Physics, Utah State University, Logan; and former graduate assistant, Utah Agr. Exp. Sta., now graduate assistant Washington State University, Pullman, respectively.

Received for publication October 16, 1959. Accepted for publication February 3, 1960.