ADSORPTION OF METHOMYL BY SOILS OF SOUTHERN SPAIN AND SOIL COMPONENTS

The adsorption of methomyl (S-methyl N-(methylcarbamoyloxy) thioacetimidate) by 14 soil samples of Southern Spain with different characteristics has been investigated studying the correlation between adsorption and soil properties and the effect of methomyl concentration and soil/solution ratio on adsorption of methomyl by soils. Adsorption has been expressed as the soil-water distribution coefficient Kd. Kd values slightly decreased when soil/solution ratio and methomyl concentration increased. These values indicated that methomyl is adsorbed weakly in soils. Kd values were compared with the soil properties by simple and multiple regression analysis. A highly significant correlation between Kd and soil organic matter and clay content was observed in every experimental condition. Adsorption of methomyl by clay minerals (two montmorillonites, hectorite, illite and kaolinite) and by humic acid has been investigated as well. The higher adsorption coefficients values obtained with humic acid indicated greater affinity of methomyl molecules for the organic fraction of soils. Kd data obtained with clay minerals indicate that methomyl is adsorbed not only on the external surfaces of the clay minerals but also in the interlamellar space of the swelling minerals.