Soil Water Repellency: Effects of Water Content, Temperature, and Particle Size

Water repellency (WR) of soils is a common problem in many countries. It can cause a reduction in the rate of water infiltration into soils as well as an unstable water flow within the soil matrix. Water repellency has typically been related to dry soils. We investigated the effect of soil water content and soil pretreatment temperature on the degree of WR in soils and soil size fractions. Water repellency was measured with an ethanol test as the minimum liquid surface tension of an aqueous ethanol droplet that can stay on the soil surface for at least 5 sec. The WR varied greatly with water content. The lowest water contents were achieved by drying soil at different temperatures between 40 and 105°C. Hence, the apparent effect of water content on WR for low water contents may be a combination of a direct water content effect and the effect of pretreatment temperature. High WR was observed at soil water contents up to 0.08 g g-1. Out of 14 soil samples, three did not show WR regardless of temperature treatment or soil water content, four had single peaks in WR as a function of water content, and seven had double peaks in WR, one peak at very low and one peak at higher soil water content. Results from comparison experiments with freeze-dried soil samples implied that the WR peak at lower soil water contents was caused mainly by temperature effects, while the peak at higher soil water contents was related to water content only. In water repellent soil the smaller soil size fractions exhibited the highest degree of water repellency. This can partly be explained by higher organic matter content in the fractions with smaller particle size. As water repellency is dependent on soil water content, performing the WR test solely on dry soils can lead to the wrong classification regarding whether a soil is water repellent or not.