Soil water repellency: Dynamics of heterogeneous surfaces

Processes controlling the dynamics of the soil–water interfacial properties have a high impact on habitat, filter, buffer, storage, and transformation functions of a soil. Besides surface roughness and chemical heterogeneity, also the dynamics of surfaces properties in soils limit the application of well established methods of wettability determination for solid materials. Numerous studies investigated the influence of changing environmental conditions, like water content, pH and drying and wetting temperature on the repellency of soils. The present paper presents an integral approach linking some of the individual results of several studies. Two hypothetical models are suggested in order to explain differences in the nature of repellency between two types of sites and between wettable and repellent samples within each type of site. The chemical nature dominating the soil water repellency at the one type of sites can be best explained by hydrolysis–condensation reactions. The physico-chemical nature dominating the repellency on the other type of site is probably controlled by micelles- or reverse-micelles-like arrangement of amphiphilic molecules during drying. Wetting properties of the surface layers of organic coating on the soil particles then depend on number and size of amphiphilic molecules, pH and ionic strength in the soil solution. It is concluded that local site properties, e.g., soil-type, climate, or land-use, determine which mechanism controls the dynamics of repellency. Future research has to verify the suggested mechanisms before critical environmental conditions can be identified in order to prevent the negative impacts of soil water repellency.