Competitive sorption between glyphosate and inorganic phosphate on clay minerals and low organic matter soils

Inorganic phosphate may influence the adsorption of glyphosate to soil surface sites. It has been postulated that glyphosate sorption is dominated by the phosphoric acid moiety, therefore, inorganic phosphate could compete with glyphosate for surface sorption sites. We examine sorption of glyphosate in low organic carbon systems where clay minerals dominate the available adsorption sites using 32P-labeled phosphate and 14C-labeled glyphosate to track sorption. We found glyphosate sorption strongly dependent on phosphate additions. Isotherms were generally of the L type, which is consistent with a limited number of surface sites. Most sorption on whole soils could be accounted for by sorption observed on model clays of the same mineral type as found in the soils.