Wettability of mineral soils

Abstract

Throughout the literature the problem of soil wettability is usually associated with hydrophobicity and water repellency. These phenomena occur, as a rule, in soils of high organic matter content. However, mineral soils, usually considered as well wettable, may markedly differ in wettability that may be particularly important for water transport in the unsaturated zone and for distribution of the liquid/gas phases in the soil matrix. To evaluate these differences, wettability of representative Polish arable mineral soils and of a few meadow organic soils (used for comparison) was characterized by the thin column wicking (TCW) method, which has been satisfactorily applied elsewhere for studies of high energy readily wettable solids. Contrary to the other methods, the TCW estimates surface free energy components of the solid phase that allows for characterizing the nature of the solid and of the solid–liquid interactions. The results were compared with standard soil wettability tests such as water drop penetration time (WDPT), molarity of ethanol drop (MED) and sessile drop (SD) methods. WDPT and MED tests differentiated only strongly hydrophilic (mineral) and extremely hydrophobic (organic) soils, however TCW and SD tests detected differences between well wettable mineral soils. Contact angles calculated from TCW data and measured by SD differed and were not related to each other. No correlations between surface energetic parameters governing wettability (surface free energy, their polar and nonpolar components, water contact angle and work of spreading calculated from TCW data) and other soil characteristics like pH, organic matter content and granulometric composition were observed.