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Water Retention by Capillary Forces in an Ideal Soil¹

ABSTRACT

In a system of air, water and solid spheres of uniform size the water content by volume may be expressed as a function of the pressure drop across an air-water interface. Such an expression has been obtained by R. A. Fisher (1926).

The validity of the Fisher theory was tested by making water extractions from saturated packs of equal size (monodisperse) glass beads. The extractions were made by means of pressure plate, centrifuge, osmotic solution and pressure membrane techniques. The glass spheres were Minnesota Mining and Manufacturing Co. "Superbrite" glass beads of 51, 99, and 203µ diameter.

The data show that at all values of suction the amount of water retained by the pack of glass beads greatly exceeds the amount predicted by the Fisher capillary theory.

This result indicates that a mechanism in addition to the pressure drop across the air-water interface is involved in the retention of water in coarse-grained, "noninteracting" materials. The lack of agreement may be a consequence of the omission of an adsorptive mechanism in this application of capillary theory.

¹ Contribution from the Department of Soils and Plant Nutrition, University of California, Davis.

² Laboratory Technician IV, Instructor, and Assistant Professor of Soil Physics, University of California, respectively.

Received for publication September 30, 1960. Accepted for publication February 13, 1961.