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Design and Performance of an Osmotic Tensiometer for Measuring Capillary Potential¹

ABSTRACT

An osmotic tensiometer makes use of a confined aqueous solution, instead of pure free water, as the reference state in measuring soil-water potentials. The membrane separating the confined solution from soil water is highly impermeable to the confined solute (polyethylene glycol, molecular weight 20,000), but exchange of much smaller molecules and ions occurs. Thus the capillary potential of soil-water is measured unless soil solutes are excluded from the instrument by a vapour gap. The advantage of this instrument over conventional tensiometers is that measurements of capillary potential can be made throughout the range 0- to –15-bar. Tests and observations show that osmotic tensiometers can be useful for measurements of capillary potential at depths greater than about 10 cm during the redistribution of soil water, and its uptake by plants. For accurate data, zero checks should be made when possible, and instrument temperature should be recorded to enable correction for its effect on each reading.

¹ Contribution from the CSIRO Division of Plant Industry, Canberra. ACT 2601, Australia.

² Senior Research Scientist and Experimental Officer, respectively.

Received for publication September 4, 1968. Accepted for publication December 10, 1968.