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Measurement of the Total Suction of Soils by a Thermistor Psychrometer¹

ABSTRACT

A detailed description of a thermistor psychrometer is presented. The psychrometer depends on differential cooling due to unequal evaporation rates of water droplets suspended from matched thermistors which are located above the sample and reference. The resulting difference in resistance between the thermistors is measured by a Wheatstone bridge and referred to a calibration curve for the determination of the total suction of the sample.

Results obtained by the psychrometer on a Brookston silt loam are shown to agree with those obtained by a pressure plate. However, results obtained by the psychrometer on a pure clay-water system are shown to differ from those obtained by a tensiometer. A possible reason for this difference is given.

¹ Journal Paper no. 3728, Purdue University Agr. Exp. Sta., Lafayette, Ind. Contribution from the Department of Agronomy. This research was supported by grants from the National Aeronautics and Space Administration.

² Graduate Assistant and Professor of Agronomy, respectively. The first-named author is now located in the Department of Soil Science, University of Guelph, Guelph, Ontario, Can.

Received for publication August 4, 1969. Accepted for publication December 31, 1969.