The increasing use of glyphosate-tolerant crops has increased concerns regarding the potential environmental effect of glyphosate. Glyphosate is a nonselective, foliar-applied herbicide used to control weeds preplant or postemergence in tolerant crops or using shielded sprayers. Glyphosate’s mode of action is inhibition of 5-enolpyruvylshikimate-3-phosphate synthase, resulting in the depletion of essential aromatic amino acids needed for plant survival (Ahrens 1994). Glyphosate has been shown to be readily adsorbed by clay minerals and hydrous oxides (Glass 1987; McConnell and Hossner 1985). Kd values range from 33 to 660 mL g21 (Glass 1987; USDA 1990). Glyphosate adsorption correlates with the amount of vacant phosphate sorption sites and may occur through binding of the phosphonic acid moiety (Ahrens 1994); yet, glyphosate is degraded microbially in soil and water (Ahrens 1994). It has a reported field half-life of 47 d and a laboratory half-life of , 25 d (Ahrens 1994). However, it is not known what effect glyphosate has on the microbial population.