Surfactant effects on soil aggregate tensile strength

Little is known regarding a soil aggregate's tensile strength response to surfactants that may be applied to alleviate soil water repellency. Two laboratory investigations were performed to determine surfactant effects on the tensile strength of 1) Ap horizons of nine wettable, agricultural soils collected from across the continental U.S., and 2) two of the nine soils (Latahco and Rad silt loams from the Pacific Northwest) that were sampled at two depths (5 and 15mm) after being sprinkler irrigated. Along with an untreated control, three surfactants (an alkyl polyglycoside, an ethylene oxide/propylene oxide block copolymer, and a blend of the two) were spray applied by hand at rates of 0, 1, 1.63, 3.35, 4.79, or 8.14kg active ingredient ha−1 to 1) air-dry, loose soil in Study 1 and 2) field-moist, tamped soil in Study 2 before being irrigated with surfactant-free water at 88mmh−1 twice, once for 0.33h, then about 8d later for 0.25h. Tensile strength was measured on oven-dry, 4- to 6.35-mm-diameter aggregates (18≤n≤37) of known mass for each treatment using a load cell with an attached flat-tip probe moving at a constant 0.27-mms−1 rate that applied continuous strain to each aggregate until it failed. In Study 1, tensile strength ranged widely, from 27kPa for Adkins loamy sand to 486kPa for Bolfar loam, averaged across surfactant treatments. Tensile strength for all nine surfactant-treated soils averaged 164kPa, 7% greater (P=0.099) than the control. In Study 2, surfactants significantly affected the tensile strength of Latahco but not Rad aggregates, when averaged across irrigations and sampling depths. After irrigation, aggregate tensile strength averaged 26% less (P