Interactions among Glyphosate and Phosphate in Soils: Laboratory Retention and Transport Studies

The purpose of this study was to determine the effect of PO4 on the sorption and transport of glyphosate [N-(phosphonomethyl) glycine, GPS] in soils. The results of batch experiments indicated significant competition between PO4 and GPS in two different soils, with PO4 being preferentially sorbed. The 24-h Freundlich partitioning coefficients for GPS sorption were decreased by 50 to 60% with PO4 in solution. High sorptive capacities exhibited by soils in the presence of PO4 suggest the existence of both competitive and ion specific sites in either soil. Miscible displacement transport studies indicated limited effects of competition when GPS was applied in conjunction with or subsequent to pulses of PO4. However, when a PO4 pulse was applied after the application of a GPS pulse, a secondary GPS breakthrough was observed where an additional 4% of the applied herbicide mass was recovered in the effluent solution. This is likely attributed to the PO4-mediated displacement of GPS bound to competitive sites. These results are further emphasized by the distribution of residual herbicide in this column, with enrichment of mass at lower depths in the column and a corresponding decrease in GPS mass closer to the column surface. These results indicate that the timing of inorganic P fertilizers relative to GPS applications has a significant impact on the fate of the herbicide in soils. In particular, these findings suggest that GPS may be more liable to leaching in scenarios in which P fertilizers are applied after the application of GPS-based herbicidal formulations.