ON THE RELATION OF HERBICIDE ADSORPTION A ND SOIL ORGANIC FRACTION

Freundlich adsorption isotherms were measured for four herbicides (atr azine, terbuthylazine, chlorotoluron, isoproturon) and 24 soil horizon s, which are typical of Schleswig-Holstein, northern Germany. The rela tionship between the adsorption constant (K-Fr) of a chemical and the fraction of organic carbon (f(OC)) 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 K-OC-values, that are determined by linear regression of K-Fr and f(OC) should be evaluated critically be fore application. K-OC'- and K-OC-values derived from experiments show ed decreasing affinity of the herbicides to soil horizons in the order terbuthylazine > chlorotoluron > atrazine congruent to isoproturon. C alculation of K-OC from K-OW or water solubility of each herbicide, ho wever, led to changes in the above mentioned ranking. Hence, especiall y K-OW or water solubility based K-OC-values may lead to false conclus ions concerning chemical mobility. For atrazine, terbuthylazine and is oproturon Freundlich adsorption constants increased overproportionally with increasing organic carbon content of soils. The relationships be tween K-Fr and f(OC) therefore were described better by a non-linear e quation (second order polynom) than by a linear approach. Contrary, fo r chlorotoluron a linear relation between K-Fr and f(OC) holds at leas t for the range of f(OC) investigated in this study.