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

Abstract 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-Fe 2 O 3 ) and a clay loam soil rich in calcium carbonates (CL-CaCO 3 ). The phosphate Freundlich sorption coefficient (K f ) ranged from 3 to 68 L 1/n  mg 1–1/n  kg − 1 in the SCL-Fe 2 O 3 and from 21 to 76 L 1/n  mg 1–1/n  kg − 1 in the CL-CaCO 3 . Glyphosate sorption coefficient (K d ) ranged from 293 to 1173 L kg − 1 in the SCL-Fe 2 O 3 but only 99 to 141 L kg − 1 in the CL-CaCO 3 . Glyphosate K d and phosphate K f 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.