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Materials for Retainer Plates and Their Use for Retentivity Measurements¹

ABSTRACT

The selection of ceramic materials suitable for use as sample retainer plates is discussed. Methods for measuring the permeability, air-bubbling pressure, and air-entry pressure of retainer plates are given. The transfer conductance between retainers and a ceramic suction control surface is satisfactory if the contacting surfaces are reasonably flat and clean. Several tests have consistently indicated, however, that better results are obtained if a capillary contact medium is used when ceramic retainer plates are used on cellulose membranes. The retentivity at high suctions for cores may be significantly different than for air-dried and fragmented soil. For example, the average 15-bar percentage for 7 cores of sandy loam soil was 5.96, whereas the average 15-bar percentage for the soil in these cores after air drying and fragmenting was 5.48, the difference being highly significant. Tests indicate that the water-release curves for cores of a fine sandy loam soil that were wetted in the field by flood irrigation are the same as when the same cores are wetted to zero suction in the laboratory.

¹ Contribution from the U. S. Salinity Laboratory, Soil and Water Conservation Research Branch, A.R.S., U.S.D.A., Riverside, Calif., in cooperation with the 17 Western States and the Territory of Hawaii.

² Principal Soil Scientist and Assistant Soil Scientist, respectively.

Received for publication May 13, 1955.