Evaluation of surfactant effectiveness on water repellent soils using electrical resistivity tomography

Soil water repellence is a major limitation on crop production that can be ameliorated by surfactant application. Variable success has been reported from field trials and therefore this study employs a novel approach to determine the effectiveness of surfactants at modifying infiltration into water repellent soils. Using a physical catchment model (0.6 m × 0.6 m × 0.2 m), with soils arranged in a ridge and furrow topography, irrigation, surface runoff and subsurface flow rates were quantified. Electrical resistivity tomography (ERT) was used to measure changes in soil moisture patterns. Two sandy soils with contrasting severity of water repellence, as measured by the Molarity of Ethanol Droplet (MED) test, were assessed. The impact of two surfactants, at an equivalent rate of 1 L ha−1, and an untreated control were monitored over multiple wetting cycles. The very severely water repellent soil (MED 4.2 M) showed an increase in infiltration of up to 31%, which was concentrated in the area of surfactant application in the furrow. Volumetric water contents beneath the furrow increased up to 40% below 20 mm depth. Water infiltration into the untreated soil with low water repellence (MED 1.0 M) was 98%, and this did not significantly increase with surfactant application. This physical catchment model, combined with hydrological and geophysical monitoring provides a useful tool to assess the effectiveness of surfactants in increasing water infiltration and subsurface soil moisture in water repellent soils.