Mechanisms Explaining Increasing Soil Water Repellency With Water Content Below the Permanent Wilting Point: A Review

Abstract

Soil water repellency (SWR) often displays a non-linear relationship with soil water content (SWC) below the permanent wilting point (−1500 kPa or pF 4.2). Rather than declining as soils wet, SWR frequently intensifies as SWC increases from oven-dry conditions to the permanent wilting point (or extremely dry range), reaching a peak before collapsing at SWC greater than the permanent wilting point. Several mechanisms based on pore-blocking or porosity-based processes, transitions from Cassie-Baxter to Wenzel states and soil microbial activity have been proposed to explain this behaviour; yet, none provide quantitative support for the observed changes in SWR under dry soil conditions. This review synthesises empirical observations for SWR behaviour in the extremely dry range and examines the physicochemical processes directly relevant to explaining its intensification with increasing SWC. The hydration and evolution of interfacial water films on hydrophilic, hydrophobic and heterogeneous non-soil surfaces are examined to provide insight into how water vapour is adsorbed on the surfaces of water-repellent soil particles, where its interfacial water film is particularly complicated if salts and organic compounds are present. The synthesis shows that the increase in SWR coincides with the onset of capillary condensation and the transition of adsorbed water from rigid, ice-like interfacial layers to more mobile, liquid-like thin films. Within this narrow hydration regime, ionic redistribution and hydration-dependent reorientation of amphiphilic organic compounds modify interfacial configurations without producing continuous wetting, thereby amplifying apparent hydrophobics. By integrating adsorption behaviour with existing evidence on surface free energy in heterogeneous soil systems, this review establishes a physicochemical basis for evaluating existing hypotheses and developing a coherent mechanistic explanation for the SWR-SWC relationships under dry soil conditions.