Changes in soil pH and addition of inorganic phosphate affect glyphosate adsorption in agricultural soil

Glyphosate is the most widely used herbicide in Argentina. The strong sorption of glyphosate to the mineral fraction of the soil can be affected by environmental conditions and agricultural management, such as the application of fertilisers. This work aimed to study the effect of pH and the presence of inorganic phosphate on the affinity of glyphosate for nine different surface soils of Argentina. The effect of pH on glyphosate sorption was investigated by batch experiments with pH adjusted between 3 and 12. The greatest glyphosate adsorption occurred at a certain pH value and then adsorption decreased with increasing or decreasing pH. The effects of pH on adsorption could be described by a model that includes changes in electrical potential. The effect of inorganic phosphate on glyphosate adsorption was studied by batch experiments in the presence of 0.5 or 1 mM phosphate. The results showed a significant competition between phosphate and glyphosate in all soils. The Freundlich glyphosate coefficients decreased by 40%-65% with phosphate in solution, and the amount of glyphosate adsorbed decreased between 1% and 5%, depending on the particular characteristics of the soil. For the glyphosate-phosphate competition, the competition terms were not reciprocal with each other because the competition between ions for adsorption sites also involves electrical effects. Highlights There is a pH at which glyphosate shows a higher affinity for the soil. The presence of phosphate significantly reduces the adsorption of glyphosate to the soil. The effects of pH can be explained by rather small changes in the estimated surface potential.