Influence of pH and solution composition on the sorption of glyphosate and prochloraz to a sandy loam soil

Standard protocols for batch sorption experiments prescribe the use of 0.01 M CaCl2 as the aqueous solution, but sorption can strongly depend on the solution composition. The present work quantifies the variation in sorption behavior of the herbicide glyphosate and the fungicide prochloraz to a sandy loam soil, arising from differences in pH, ionic strength, ortho-phosphate concentration, and dominant cation in solution (Ca2+, K+, NH4+). Using batch experiments, we measured the amount sorbed to the bulk fraction and clay-sized particles. From the adsorption and desorption isotherms, we estimated the Freundlich parameters, Kƒ and N. Sorption isotherms were mostly non-linear and manifested adsorption-desorption non-singularity. Adsorption Kƒ values were in the range 0.6–78.5 L kg−1 for glyphosate and 31.2–155 for prochloraz. The pH and ortho-phosphate affected the sorption of both glyphosate and prochloraz, whereas ionic strength and dominant cation only affected sorption of glyphosate.