Aggregate Tensile Strength As Affected by Three Surfactants.

Little is known regarding a soil aggregate’s tensile strength response to surfactants that may be applied to alleviate soil water repellency.  In the laboratory, we determined surfactant effects on the tensile strength of aggregates from 1) the Ap horizons of nine wettable, U. S. agricultural soils, and 2) the 5- and 15-mm depths of two Pacific Northwest soils (Latahco and Rad silt loams) 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 8.14 kg 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 either once or twice with surfactant-free water.  Each treatment’s tensile strength was measured on at least 18 oven-dry, 4- to 6.35-mm-diameter aggregates of known mass using a load cell with an attached flat-tip probe moving at a constant 0.27-mm s^-1 rate that applied continuous strain to each aggregate until it failed.  In Study 1, tensile strength ranged widely, from 27 kPa for Adkins loamy sand to 486 kPa for Bolfar loam, averaged across the three surfactants and the control.  Tensile strength for all nine surfactant-treated soils averaged 164 kPa, 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.  Aggregate tensile strength averaged 26% less (P < 0.001) at the 5- than 15-mm depth, likely due to irrigation which resulted in either surfactant leaching or droplet kinetic energy fracturing near-surface, intra-aggregate bonds.  Overall, tensile strength varied more by soil series and depth than by surfactants.