RAPID MEASUREMENT OF LOW SOIL-WATER POTENTIALS USING A WATER ACTIVITY METER

Water potential is one of the most important physical properties of soil but remains among the most difficult to measure reliably. We describe a water activity meter that can be conveniently used to measure water potentials in soils ranging in wetness from near saturation to air dry (water potentials from - 0.4 to < - 100 MPa). The meter senses the dew point of water vapor with a miniature cooled mirror located above a soil sample in a closed chamber Readings for individual samples are obtained rapidly (2-5 min). Accuracy of the measurement is checked periodically using saturated soil solutions. Repeatability of measurements was found to be equal or better than the rated precision (+/- 0.003 activity units). Water potentials (calculated from water activities using the Kelvin equation) were obtained for soils ranging in texture from sand to clay. The meter was used on soil samples taken from sites near Richland, WA, and summertime water potentials were found to be as low as -264 MPa.