Surfactant effects on soil aggregate tensile strength

Authors: Lehrsch, Gary; SOJKA, R.E. - Retired ARS Employee; Koehn, Anita

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2012
Publication Date: 8/30/2012
Citation: Lehrsch, G.A., Sojka, R., Koehn, A.C. 2012. Surfactant effects on soil aggregate tensile strength. Geoderma. 189-190:199-206.

Interpretive Summary: A soil aggregate’s ability to withstand fracturing is termed its tensile strength, important because it is a measure of how well a soil resists compaction from heavy agricultural equipment. We know little about how tensile strength responds to surfactants (wetting agents) that may be applied to soil to alleviate water repellency. Two laboratory investigations were performed to determine surfactant effects on tensile strength, the first study of nine continental U. S. soils, and the second of two Pacific Northwest silt loam soils. In Study 1, we sprayed two surfactants onto loose soil while in Study 2, we sprayed three surfactants onto tamped soil, then irrigated twice. Tensile strength was measured by crushing dry, 5-mm-diameter aggregates. In Study 1, tensile strength varied up to 18-fold from soil to soil. As a group, the nine soils were 7% stronger when surfactant-treated than untreated. In Study 2, after irrigation, tensile strength averaged 26% less near the surface than 15 mm below it, likely because surface aggregates were weakened by water droplet impact. All told, tensile strength varied more by soil series and depth than by surfactants.

Technical Abstract: 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 15 mm) 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 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 88 mm/h twice, once for 0.33 h, then about 8 d later for 0.25 h. Tensile strength was measured on oven-dry, 4- to 6.35-mm-diameter aggregates (18<=<=37) of known mass for each treatment using a load cell with an attached flat-tip probe moving at a constant 0.27-mm/s 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 surfactant treatments. 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. After irrigation, aggregate tensile strength averaged 26% less (P<0.001) at the 5- than 15-mm depth, likely due to droplet kinetic energy fracturing near-surface, intra-aggregate bonds or surfactant leaching. All told, tensile strength varied more by soil series and depth than by surfactants.