Hydration kinetics of wettable and water-repellent soils

The hydration kinetics of soil organic matter (SOM) are influential factors for transport and sorption processes in soil. Nevertheless, our knowledge about wetting and swelling processes, which both control the overall hydration kinetics, is limited. In this study, we observed the hydration process of actually water-repellent and wettable soil samples by three independent methods. The rate of water uptake by water-repellent samples was distinctly lower than that of wettable samples when the water was supplied in the liquid phase, but was comparable when water was supplied in the gas phase. Gravimetric measurements of the water uptake and H-1 nuclear magnetic resonance relaxometry showed that wetting of water-repellent soils may last up to 3 wk. This duration is distinctly longer than the water drop penetration time of the samples, which consequently only reflects the first wetting step of the soil surface and does not consider infiltration of larger amounts of water. Since the achievement of equilibrium conditions is a slow process, the hydration kinetics of SOM may control transport and sorption kinetics in water-repellent soils. Differences in the freezing, melting, and evaporation processes of soil water of actually water-repellent and wettable soil samples, observed by differential scanning calorimetry (DSC), are probably only a consequence of different water contents under field conditions. Consequently, the DSC measurements provided no evidence of a general difference in the way water binds in water-repellent and wettable soil samples.