Moisture-Dependent Wettability of Artificial Hydrophobic Soils and Its Relevance for Soil Water Desorption Curves

Soil wettability is strongly affected by the soil water content. In turn, wettability influences water distribution in soil pores and thereby soil water retention characteristics. In this study, soil water desorption curves (SWDC) for naturally occurring hydrophilic sand, loam, and silt loam samples and for corresponding artificially hydrophobized sand, loam, and silt loam samples were measured in the pF range from 0 to 4.2. Moisture-dependent wettability was described with the contact angle (CA) method and the water drop penetration time (WDPT) test. One objective was to directly measure the phenomenological CA of the soils at different water contents and to compare the results with that from the WDPT test. A second objective was to show differences of soil water desorption curves for the hydrophilic materials as compared to the silanized hydrophobic materials. All experiments were done on soil samples with time-stable water repellency to study directly the impact of water content without further time dependent factors. Soil wettability as a function of water content was generally comparable, indicating that CA and WDPT could be used to indicate the moisture-dependent wettability. The SWDCs were separated into three domains. Domain I was characterized by the Wenzel wetting regime, and Domains II and III were described by the Cassie–Baxter wetting regime. Water repellency showed insignificant impacts on the SWDCs in Domains I and III, but it reduced soil water retention capacity in Domain II, which was supported by the monotonical increase of CA from zero to a maximum as soil water content decreased to a critical point near the permanent wilting point. Water repellency had clear and important influences on soil water desorption curves.