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SOIL PHYSICS

Measurement of Low Matric Potentials with Porous Matrix Sensors and Water-Filled Tensiometers

W. R. Whalley^a,*, G. Lock^b, M. Jenkins^b, T. Peloe^b, K. Burek^b, J. Balendonck^c, W.A. Take^d, .H. Tuzel^e and Y. Tuzel^e

^a Rothamsted Research, West Common, Harpenden, St Albans, AL5 2JQ UK
^b Delta-T Devices, 130 Low Road, Burwell, Cambridge, CB25 OEJ, UK
^c Wageningen UR, Greenhouse Horticulture, PO Box 644, 6700 AP Wageningen, Droevedaalsesteeg 16708 PB Wageningen, the Netherlands
^d Department of Civil Engineering, Queen's University, Kingston, ON, K7L 3N6, Canada
^e Ege University, Faculty of Agriculture, Bornova, Izmir, Turkey

^* Corresponding author (richard.whalley{at}bbsrc.ac.uk).

Water-filled tensiometers are widely used to measure the matric potential of soil water. It is often assumed that, because these give a direct reading, they are accurate. With a series of laboratory tests with model laboratory systems of increasing complexity we show that the output of water-filled tensiometers can, particularly in drying soils, be in serious error. Specifically, we demonstrated that water-filled tensiometers can indicate a steady matric potential, typically between –60 and –90 kPa, when the soil is much drier. We demonstrate the use of water-filled tensiometers that can measure matric potentials smaller than –100 kPa in the laboratory and in the field. The physics of the failure of water-filled tensiometers is discussed. When the matric potential was greater than –60 kPa, in laboratory and field tests water-filled and porous matrix sensors were in good agreement. In the field environment the porous matrix sensor was useful because it allowed early detection of the failure of water-filled tensiometers. In dry soils (matric potential < –60 kPa) the porous matrix sensor was more reliable and accurate than the water-filled tensiometer.