Effect of phosphate on the adsorption of glyphosate on soils, clay minerals and oxides

The effect of phosphate (ortho-phosphate) on the adsorption of the widely used glyphosate herbicide was evaluated with three typical Danish agricultural soils as well as pure oxides (goethite, FeOOH and gibbsite, Al(OH)(3)) and silicates (illite and montmorillonite), which are considered the most important glyphosate and phosphate adsorbents in soils. Batch experiments where made in order to find out how phosphate affects adsorption of glyphosate and how glyphosate affects adsorbed phosphate. Solution glyphosate was quantified by liquid scintillation counting of C-14-taggered herbicide and the concentration of phosphate by the molybdenum blue method. All experiments showed competition between phosphate and glyphosate for adsorption sites but the various adsorbents exhibited great variation in affinity for glyphosate and phosphate. Thus, gibbsite and, in particular goethite strongly prefer phosphate, whereas the competition on the silicates is more equal. The current studies showed that the competition in soils is almost equal, but still phosphate affects the sorption of glyphosate in soil. The amount of glyphosate and phosphate adsorbed by the various kinds of adsorbents was found to decrease in the order: oxides > silicates > soils. For the soils tested aluminium oxides, and to a lesser extent iron oxides seem the most important components in determining a soil's ability to adsorb phosphate and glyphosate, whereas the clay content and clay type seem of minor or little importance for adsorption of these species.