Average concentrations of surface-applied herbicides to soil may be as low as 1 to 4 mg kg21 (Moorman 1989). These concentrations, however, are usually calculated based on a 15-cm furrow slice. Concentrations based on 15-cm soil depth may be misleading because soil penetration of glyphosate may be only a few millimeters because of its high adsorptivity (Sprankle et al. 1975). Calculations based on a 15-cm soil depth may substantially underestimate the glyphosate concentration that soil microbes are exposed to in the shallow zone of herbicide penetration.