SummaryAdsorption of simazine (2‐chloro‐4,6‐bisethyl‐amino‐1,3,5‐triazine) was 2.2–4 times greater than that of terbacil (5‐chloro‐6‐methyl‐3‐t‐butyl‐uracil) in the same soils and adsorption of both herbicides was 2–4 times greater in the topsoils than subsoils. Adsorption was inversely correlated with herbicide movement in a thick‐layer chromatography system. One year after application of 3 kg/ha to field plots, simazine residues were highest near the soil surface, whereas terbacil residues increased with soil depth in the sandy and sandy loam soils. Total residues recovered from the upper 25 cm of soils was 5% or less of the simazine originally applied, and 10% or less of the applied terbacil. In an oat seedling bio‐assay, the GR50values were generally 1.5–3 times higher for simazine