ADSORPTION AND DESORPTION OF GLYPHOSATE IN SOME EUROPEAN SOILS

The interaction of glyphosate [N-(phosphonomethyl)glycine] with four t ypical European soils is reported. Results of adsorption and desorptio n isotherms show that the interaction of glyphosate with these soils w as mainly related to content of iron and aluminium amorphus hydroxides . Moreover, it was found that the presence of divalent cations in 2:1 clay minerals also contribute to glyphosate adsorption. The S-type for m of the adsorption isotherms revealed the existence of two different binding sites. These were exchangeable cations at low herbicide concen tration and Fe and Al at higher glyphosate concentrations. The K maxim um values of adsorption provided by the linear form of the Langmuir eq uation were found to be more consistent with soil parameters than thos e calculated by the Freundlich equation. The order of desorption from the soils was the reverse of that found for adsorption. Moreover, deso rptionvaried from around 15 to 80 % of the adsorbed herbicide accordin g to the soil characteristics. This indicated that glyphosate adsorpti on on soils is far from being permanent and leaching to lower soil hor izons with limited biological activity may occur.