Moisture Sorption at High Temperature and Low Water Potential Affects Water Content Measurements.

It is well known that the soil moisture characteristic is a temperature dependent function.  The water potential of a soil at constant water content increases with temperature.  Temperature effects typically have been investigated at high water potential and for temperatures between 0 and 40 C.  We investigated these effects at much lower water potentials and higher temperatures.  A sealed chamber was constructed with a calibrated humidity sensor inside.  Soil samples were pre-equilibrated at humidities between 10 and 76%.  Water activity measurements were then made at temperatures between 20 and 120 C.  The results were consistent with the predictions of the Clausius-Clapeyron equation.  The heat of sorption was proportional to, but not equal to the water potential.  These results are relevant to the accuracy of water content determination by oven drying.  The high oven temperature both increases vapor pressure difference between sample and air, and “loosens” the water in the soil.  Both determine the water content of a sample when it is said to be “oven dry”.