Sorption of a hydrophilic pesticide : Effects of soil water content

Transport of pesticide to groundwater is governed in part by sorption of the pesticide to soil particles. Sorption may be dependent on soil moisture conditions, but limited data are available from which to elucidate the effect. Our objective was to determine the effect of soil water content on the sorption coefficient of a hydrophilic pesticide. Sorption of dicamba (3,6‐dichloro‐2‐methoxybenzoic acid) was measured in three soils, each at two initial water contents. At low water contents (∼0.05 kg kg −1 ), sorption coefficients were similar for all three soils, ranging from 0.01 L kg −1 for the loamy sand to 0.07 L kg −1 for the silty clay loam. At higher water contents (0.19–0.24 kg kg −1 ), the sorption coefficient for the loamy sand was unchanged, for the silt loam it was doubled, and for the silty clay loam it was increased almost sixfold. Multiple regression analysis revealed a strong linear relationship between the sorption coefficient and the product of soil water content and organic C content ( r 2 = 0.86). The number of dicamba sorption sites probably increases with soil organic C content, while the accessibility of these sites appears to increase with soil water content. This may be caused by the decreasing hydrophobicity of soil organic matter with increasing water content. The effects of water content on pesticide sorption require further research and may ultimately have implications for the methods used to determine sorption and for managing pesticide application.