THE EFFECTS OF PARTICLE-SIZE, ORGANIC-MATTER CONTENT, CROP RESIDUES AND DISSOLVED ORGANIC-MATTER ON THE SORPTION KINETICS OF ATRAZINE AND ISOPROTURON BY CLAY SOIL

Reliable predictions of the fate and behaviour of pesticides in soils is dependent on the use of accurate 'equilibrium' sorption constants a nd/or rate coefficients. However, the sensitivity of these parameters to changes in the physicochemical characteristics of soil solids and i nterstitial solutions remains poorly understood. Here, we investigate the effects of soil organic matter content, particle size distribution , dissolved organic matter and the presence of crop residues (wheat st raw and ash) on the sorption of the herbicides atrazine and isoproturo n by a clay soil. Sorption Kd's derived from batch 'equilibrium' studi es for both atrazine and isoproturon by <2 mm clay soil were approxima tely 3.5 L/kg. The similarity of K-oc's for isoproturon sorption by th e <2 mm clay soil and <2 mm clay soil oxidised with hydrogen peroxide suggested that the sorption of this herbicide was strongly influenced by soil organic matter. By contrast, K-oc's for atrazine sorption by o xidised soil were three times greater than those for <2 mm soil, indic ating that the soil mineral components might have affected sorption of this herbicide. No significant differences between the sorption of ei ther herbicide by <2 mm clay soil and (i) <250 mu m clay soil, (ii) cl ay soil mixed with wheat straw or ash at ratios similar to those obser ved under field conditions, (iii) <2 mm clay soil in the presence of d issolved organic matter as opposed to organic free water, were observe d. Copyright (C) 1996 Elsevier Science Ltd