Wettability and Hysteresis Effects on Water Sorption in Relatively Dry Soil

The soil water retention curve (SWRC) is a key tool for understanding the fundamental relationship between soil moisture content and its associated energy. The objective of this study was to measure soil water retention including hysteresis at the dry end of the SWRC and to examine the effect of wettability on the SWRCs of two wettable soils and their hydrophobized counterparts. Two conditions, wettable and hydrophobic, were measured for each soil. The method used to measure the SWRCs was vapor equilibration over salt solutions of known osmotic potentials. Free water in the form of individual droplets was found to be present at the surface of the unwashed hydrophobic soils due to decreases in the osmotic potential during the hydrophobizing process. Water droplets did not form on the hydrophilic and washed hydrophobic soils. Soil wettability was found to affect soil water retention in relatively dry soil. The hydrophilic soils used in this study exhibited significant hysteresis in the water potential range of −2.3 to −19.2 MPa. Soil wettability and hysteresis should be considered when studying water sorption and desorption in relatively dry soil.