Pyrithiobac Sorption on Reference Sorbents and Soils

The paucity of information available on the behavior of the new herbicide pyrithiobac [sodium 2-chloro-6-(4,6-dimethoxypyrimidin-2-ylthio)benzoate], prompted an evaluation of the sorption of pyrithiobac on reference sorbents and cotton-producing soils from central to south Texas. Batch equilibration techniques were used to measure pyrithiobac sorption at concentrations commonly encountered in field soils. Pyrithiobac was strongly sorbed by peat, which indicated that the organic matter fraction will significantly influence sorption. The goethite sorption maximum (0.30 mmol kg^-1 for 0.23 mM pyrithiobac added), which represented the reference sorbent with the least sorption capacity, sorbed 1.7 times more than the soil with the greatest sorption capacity (Houston Black, 0.17 mmol kg^-1 for 0.23 mM pyrithiobac added). Strong correlations among measured soil properties made it difficult to identify soil components controlling pyrithiobac sorption. These results indicate that pyrithiobac was weakly sorbed by four soils due to repulsion of anionic pyrithiobac in predominantly alkaline and smectitic (negatively charged) soils.