Water Potentials Produced by Oven‐Drying of Soil Samples

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Soil water content or potential can be adjusted in the laboratory by equilibrating the soil at different relative humidities. These relative humidities can be produced by the use of different saturated salt solutions or by heating the soil to different temperatures in a convection oven. Usually, the conditions for equilibrating the soil are expressed as relative humidities if salt solutions are used or as the temperature of drying if a convection oven is used. This dichotomy does not allow all the results to be compared or plotted on a common axis. In this paper, the Magnus–Tetens and the Arden Buck equations are used in combination with the Kelvin equation to show how the results from both types of equilibration can be presented on the same scale (e.g., water potential or pF). Two new equations show how the results from oven drying depend not only on the oven temperature, but also on the temperature and relative humidity of the air in the laboratory. For example, drying soil in an oven in one set of atmospheric conditions can produce a significantly different water potential than in different atmospheric conditions. The new equations enable the soil water potentials (or pF values) produced by oven drying to be calculated for all combinations of oven temperature and atmospheric conditions. Predictions from the equations are evaluated by comparing water retention data obtained by oven drying and by equilibration over saturated salt solutions. It is shown that the mean absolute error in gravimetric water content is 2.3 g kg−1 Equations are presented for the mean thickness of water films adsorbed on the surfaces of soil particles under different drying conditions. The amount of water adsorbed on particle surfaces is predicted to be zero at pF6.6.