A current trend in weed control for annual crops is towards increased use of postemergence herbicides, mostly sulfonylureas, with very low application rates. There is, however, a lack of information on the behavior of the new sulfonylurea herbicides in soils. In this study, sorption of a relatively new sulfonylurea herbicide, primisulfuron (methyl 2-[[[[[4,6-bis(disfluoromethoxy)-2-pyrimidinyl]amino]-carbonyl]amino]sufonyl]benzoate; Beacon), on six oxides (alumina, ferrihydrite, goethite, Mn-substituted goethite, buserite, and silica), two clays (Ca- and Al-montmorillonite), and 23 soils was quantified for primisulfuron concentrations ranging from 0 to 7.5 mg L-1. Adsorption/desorption isotherms were linear for concentrations not exceeding the solubility of primisulfuron. For oxides and clays, linear adsorption distribution coefficients (Kd), measured at pH 6 ± 0.1, ranged from 1.16 for buserite to 5.33 for alumina. Adsorption greatly decreased with increasing pH, and there was no significant effect of temperature on adsorption. Adsorption on soils was negatively correlated with pH and positively correlated with clay content and extractable Fe and Al. The pH dependance may be attributed to the decreased solubility and increased hydrophobicity of primisulfuron with decreasing pH. Adsorption Kd values ranged from 0.02 to 2.7 and could best be described by a nonlinear fit: Kd = 10(2.93-0.59pH), r2 = 0.85. The organic matter content of soils was not correlated with primisulfuron sorption. Three mechanisms are proposed for primisulfuron sorption on minerals and soils: hydrophobically driven adsorption, dipole-dipole interactions, and anion exchange by outersphere complex formation on pH dependent charge sites. Primisulfuron is likely to be retained in soils of low pH and in fine-textured soil horizons.
Journal Paper no. J-16272 of the Iowa Agric. and Home Economics Exp. Stn., Ames, IA, Project no. 3254.