Sorption of Pesticides to Soil and Soil Components.

Sorption processes affect every other aspect of pesticide behavior in the soil, determining movement, biological activity and persistence. Sorption on soil surfaces makes pesticide molecules less available for leaching, volatilization and biodegradation, whereas desorption makes pesticide molecules available for transport and degradation processes. Although most non-ionic pesticides tend to adsorb on the organic fraction of soil, in the case of polar uncharged pesticides, when soil organic carbon content is low and/or soil clay content is high, the contribution of mineral soil colloids to sorption becomes more evident. This is mainly due to the high surface areas associated  to their small particle size and, in the case of smectites, because they have internal surfaces accessible to water and molecules such as pesticides. Moreover, the association between soil components plays an important role in pesticide soil sorption. In this context, the relevance of the different soil colloids and the interassociation between them in sorption of pesticides and the practical use and application derived from this knowledge will be discussed.  We will mainly focus this discussion on the role of soil colloidal components and their interassociations in sorption of polar or ionizable pesticides. The contribution of Bill Koskinen to the subject matter and his collaboration with our research group for 20 years will be present through all the presentation.