Fig. 4. Distribution coefficients (K_d) for the adsorption of p-aminobenzoic acid and sulfonamide antibiotics to soil samples calculated by a quantitative structure–property relationship (QSPR) model equation in relation to the experimental values obtained from batch trials (open symbols) and to independent data (filled symbols, from Langhammer, 1989; Thiele-Bruhn, unpublished data). Solid line = 1:1 ratio; error bars not shown are smaller than symbols.