Surfactant and irrigation effects on wettable soils: runoff, erosion, and water retention responses

Surfactants are chemical compounds that change the contact angle of water on solid surfaces and are commonly used to increase infiltration into hydrophobic soil. Since production fields with water-repellent soil often contain areas of wettable soil, surfactants applied to such fields will likely be applied to wettable soil, with unknown consequences for irrigation-induced erosion, runoff, or soil water relations. We evaluated surfactant and simulated sprinkler irrigation effects on these responses for three wettable, Pacific Northwest soils, Latahco and Rad silt loams and Quincy sandy loam. We studied three surfactants: an alkyl polyglycoside in solution at a concentration of 18 g a.i./kg, a block copolymer at 26 g/kg, and a blend of the two at 43 g/kg. From 2005 to 2009 in the laboratory, each surfactant was sprayed at a rate of 46.8 L/ha onto each soil packed by tamping into 1.2- by 1.5-m steel boxes. Thereafter, each treated soil was irrigated twice at 88 mm/h with surfactant–free, well water. Runoff and sediment loss were measured for each irrigation and soil samples were collected after each irrigation. While measured properties differed among soils and irrigations, surfactants had no effect on runoff, sediment loss, splash loss, time to runoff, or tension infiltration, compared to controls. Across all soils, however, the alkyl polyglycoside increased volumetric water contents by about 3% (significant at P less than or equal to 0.08) at matric potentials from 0 to -20 kPa, compared to controls. With a decrease in the liquid-solid contact angle on treated soil surfaces, surfactant–free water appeared able to enter, and be retained in pores with diameters greater than or equal to 15 mm. All in all, surfactants applied at economic rates to these wettable Pacific Northwest soils posed little risk of increasing either runoff or erosion or harming soil water relations. Moreover, by increasing water retention at high potentials, surfactants applied to wettable soils may allow water containing pesticides or other agricultural chemicals to better penetrate soil pores, thereby increasing the efficacy of the co-applied materials