Fig. 4. Sorption isotherms of herbicides ([a] metolachlor, [b] atrazine, [c] 2,4-D, [d] picloram) on the Linne soil in the presence of oxamic acid. The sorption amount of oxamic acid on the soil was 42 mg kg^-1. Each data point is the mean of three replicates and the error bar is its standard deviation.