Surfactant and Irrigation Effects On Runoff, Erosion, and Water Retention of Three Wettable Soils.

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 irrigation effects on these responses for three wettable, Pacific Northwest soils, Latahco and Rad silt loams and Quincy sandy loam. We studied an alkyl polyglycoside in solution at a concentration of 18 g a.i. kg^‑1, a block copolymer at 26 g kg^‑1, and a blend of the two at 43 g kg^‑1. From 2005 to 2009, each surfactant was sprayed at a rate of 46.8 L ha^‑1 onto each soil packed into steel boxes. Thereafter, each soil was sprinkler irrigated twice with surfactant–free, well water. Runoff and sediment loss were measured 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 ≤ 0.08) at matric potentials from 0 to -20 kPa, compared to controls. With the liquid-solid contact angle decreased on treated soil, surfactant–free water entered, and was retained in pores with diameters ≥ 15 µm. Surfactants applied at economic rates to these wettable soils posed little risk of increasing either runoff or erosion or harming soil water relations. Moreover, by increasing water retention in wettable soils, surfactants may allow water containing pesticides or other agricultural chemicals to better penetrate soil pores, thereby increasing the efficacy of the co-applied materials.