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Psychrometric Measurements of Soil Samples Equilibrated on Pressure Membranes¹

ABSTRACT

The osmotic pressure or solute suction (SS) of the extract from a "saturated" soil paste can be measured with a thermoelectric psychrometer which is calibrated by use of standard osmotic solutions. The same psychrometric procedure can be used to measure the total soil-moisture stress or total suction (TS) of water in soil that has been equilibrated with a pressure membrane at known soil-moisture tension or matric suction (MS). From membranous and psychrometric measurements, the difference S = TS - (MS + SS) was evaluated for 134 samples of Pachappa soil at three SS levels (0.3, 2.5, and 5 bar) attained by leaching with calcium chloride solutions and three MS levels (5, 10, and 15 bar). Mean S values at 5, 10, and 15 bar MS were, respectively, –0.6, –0.4, and –0.1 bar. Mean S values at 0.3, 2.5, and 5 bar SS were, respectively, –1.0, +0.04, and +0.5 bar. Standard deviation values for MS, SS, and TS, respectively, were 0.07, 0.1, and 0.5 bar.

¹ Contribution from the U. S. Salinity Laboratory, Soil and Water Conservation Research Division, ARS, USDA, Riverside, Calif., in cooperation with 17 western states and Hawaii. This work was conducted with the cooperation of the U. S. Department of Defense. The authors are indebted to J. R. Hadley for assistance with the experimental work.

² Soil Scientists, U. S. Salinity Laboratory, Riverside, Calif.

Received for publication January 16, 1961. Accepted for publication January 24, 1961.