| Abstract: | On the relation of herbicide adsorption and soil organic fraction Freundlich adsorption isotherms were measured for four herbicides (atrazine, terbuthylazine, chlorotoluron, isoproturon) and 24 soil horizons, which are typical of Schleswig-Holstein, northern Germany. The relationship between the adsorption constant (KFr) of a chemical and the fraction of organic carbon (fOC) of the soil horizons was evaluated. Chemical specific K'OC-values, that are independent of soil organic carbon fraction varied considerably for each herbicide (CV about 50%); therefore these values as well as KOC-values, that are determined by linear regression of KFr and fOC should be evaluated critically before application. K'OC- and KOC-values derived from experiments showed decreasing affinity of the herbicides to soil horizons in the order terbuthylazine > chlorotoluron > atrazine ? isoproturon. Calculation of KOC from KOW or water solubility of each herbicide, however, led to changes in the above mentioned ranking. Hence, especially KOW or water solubility based KOC-values may lead to false conclusions concerning chemical mobility. For atrazine, terbuthylazine and isoproturon Freundlich adsorption constants increased overproportionally with increasing organic carbon content of soils. The relationships between KFr and fOC therefore were described better by a non-linear equation (second order polynom) than by a linear approach. Contrary, for chlorotoluron a linear relation between KFr and fOC holds at least for the range of fOC investigated in this study. |
