Changes in pesticide adsorption with time at high soil to solution ratios

Adsorption of six pesticides (2,4-D, dicamba, fluroxypyr, fluazifop-P, metsulfuron-methyl and flupyrsulfuron-methyl) in nine contrasting soils was measured using two techniques: (i) a classical batch method and (ii) a centrifugation method that allowed the measurement of adsorption at a realistic soil to solution ratio after one and seven days. Although the batch method gived significantly higher values of Kd than the centrifugation method for the more strongly sorbed molecules in the more sorptive soils, it tended to give lower adsorption coefficients compared to the centrifugation method when adsorption was lower. Discrepancies between the two methods were probably mainly due to the vigorous shaking applied in the batch technique that artificially enhances the availability of adsorption sites. This implies that shortly after application, more pesticide may be present in the soil solution and thus be available for degradation, plant uptake or leaching than will be predicted from adsorption coefficient determined using the batch method. Adsorption significantly increased between one and seven days and the extractability of total residues decreased with time. The increase in adsorption was not directly related to the level of adsorption although it was more important in soils containing more organic carbon (p = 0.022). These results confirm the importance of time-dependent processes and the necessity to include them in risk assessment procedures. The centrifugation technique is a useful method to measure adsorption of pesticides at realistic soil moisture contents and seems to be an adequate technique to characterise the fraction of pesticide that is available for leaching at a given time after application.