Phosphate and glyphosate sorption in soils following long-term phosphate applications

Phosphate and glyphosate molecules compete for sorption sites in soil. The objective of this study was to quantify the impact of Olsen P concentrations in two contrasting soils on phosphate and glyphosate sorption. Soils were a sandy clay loam soil rich in iron oxides (SCL-Fe2O3) and a clay loam soil rich in calcium carbonates (CL-CaCO3). The phosphate Freundlich sorption coefficient (K-f) ranged from 3 to 68 L-1/n mg(1-1/n) kg(-1) in the SCL-Fe2O3 and from 21 to 76 L-1/n mg(1-1/n) kg(-1) in the CL-CaCO3. Glyphosate sorption coefficient (K-d) ranged from 293 to 1173 L kg(-1) in the SCL-Fe2O3 but only 99 to 141 L kg(-1) in the CL-CaCO3. Glyphosate K-d and phosphate Kf values decreased significantly with increasing Olsen P concentrations in both soils. Glyphosate K-d values were further significantly reduced when phosphate was added to the slurry solutions, but phosphate K-f values were not impacted by the presence of glyphosate in solutions. We conclude that annual phosphate fertilizer applications leave phosphate concentrations in Prairie soils to the extent that soils have a lesser capacity to retain glyphosate and phosphate that are subsequently applied, but glyphosate residues will not influence phosphate sorption.