The batch equilibrium procedure is a common method to examine the effect of phosphate additions on glyphosate sorption. All researchers, except, added phosphate to soil in the laboratory prior to or during the batch equilibrium experiment, and hence exclusively studied the effect of “fresh” phosphate on glyphosate sorption. Phosphate did compete with glyphosate sorption in a wide range of soils, but not in other soils. Some research demonstrates that phosphate is preferentially sorbed by clay minerals, for example because the phosphate molecule (0.25 nm) is smaller than the glyphosate molecule (0.43 nm). The impact of phosphate on reducing glyphosate sorption was stronger for synthesized Fe and Al- oxides than for pure clay-minerals. All batch equilibrium studies utilized analytical-grade glyphosate, while herbicide products applied on agricultural land contain other ingredients that could impact the sorption of the active ingredient by soil. Hence, in evaluations of the impact of phosphate additions on glyphosate sorption, it is desired to include a commercially available glyphosate formulation in the study.
Borggaard (2011) reported that the interaction of phosphate and glyphosate sorption remains unclear, even though this interaction has been studied for four decades. Is the only study in these four decades to have examined the effect of “aged” phosphate on glyphosate sorption. Utilizing soil from field plots that had received various amounts of phosphate and lime fertilizers for more than 60 years, batch equilibrium studies indicated that less glyphosate was sorbed in soils with greater Olsen-P concentrations. To the best of our knowledge, the combined effect of fresh and aged phosphate on glyphosate sorption in soil has not been investigated.
The objective of this study was to examine under a range of pH conditions, the impact of field-aged phosphate and cadmium concentrations on glyphosate sorption by soil, with and without phosphate co-applications in the laboratory. The pH conditions were chosen to include the range of possible glyphosate speciations, as described. In general, studies have reported that glyphosate sorption generally decreases with increasing soil pH.