THE MOISTURE POTENTIAL OF SOILS

Abstract

The moisture potential has been defined as the chemical potential of water in ergs per gram. It is identical with the partial molal free energy of water in all respects except for the unit of mass employed. In accordance with previous terminology the moisture potential of an aqueous solution has been called the "osmotic potential." A general relationship between composition and osmotic potential has been presented and methods of calculation have been discussed. The moisture potential of a soil may be calculated from tensiometric, centrifugal, or pressure-membrane methods, provided the osmotic potential and the partial specific volume of water in the liquid phase are known. This relationship is important where it is desired to compare these types of data with vapor-pressure or freezing-point data. The theory of the cryoscopic method has been developed and an equation similar to that of Schofield has been derived. A comparison of cryoscopic data with data from static vapor-pressure measurements has demonstrated that the freezing method must be further improved before results completely consistent with thermodynamics can be obtained. A possible explanation of the discrepancies is that irregular freezing results in excessive withdrawal of water from the liquid phase in the localized areas in which ice crystals have formed.