Adsorption of isoproturon, diuron and metsulfuron‐methyl in two soils at high soil:solution ratios

Rates of degradation of isoproturon, diuron and metsulfuron‐methyl were measured in two soils incubated at two temperatures (5 and 25 °C) with soil moisture at a matric potential of −5 kPa. Rates of change in soil solution concentration were also measured after extraction of water from the soil using a centrifugation technique. The data, in general, indicated a more rapid rate of decline in aqueous‐phase concentrations of herbicide than in total soil concentrations, and hence a progressive increase in partition coefficient in favour of the adsorbed phase . In all of the incubations, however, adsorption of the herbicide was initially less than that measured using standard equilibration techniques that involved shaking with large volumes of solution relative to weights of soil. This may be explained by the ready availability of more adsorption sites in the shaken systems. With isoproturon and diuron, the changes in adsorption with time were similar at the two incubation temperatures. This indicates that the apparent changes in adsorption with these two compounds were not caused by preferential degradation in the soil solution, but by a slow equilibration with adsorption sites. The results with the weakly adsorbed compound metsulfuron‐methyl, however, suggested the possibility of preferential degradation in the solution phase because, when degradation was slow, the absolute amounts adsorbed remained constant or increased slightly, even although solution concentrations declined. Implications of the results for pesticide behaviour in soils in the field are discussed.