It is known that glyphosate adsorbs strongly in the soil, through hydrogen bonds in organic matter and by anion exchange processes in the clay minerals, iron and aluminium oxides (Aubin & Smith, 1992; Haney et al., 2000; Veiga et al., 2001). The role of adsorption on the glyphosate bioavailability and biodegradation is not completely conclusive, although Veiga et al. (2001) state that the adsorbed glyphosate is more persistent in the soil, because its degradation is slower than that of the free herbicide.