2,4-D sorption in iron oxide-rich soils: role of soil phosphate and exchangeable Al

This study examined herbicide retention in iron oxide-rich variable charge soils (Ultisols) under no cultivation (forest), agriculture (farm), and turf maintenance (golf course) to explore the following hypothesis: inorganic phosphate accumulation from soil fertilization and liming to decrease exchangeable aluminum (Al) content will influence carboxylic acid herbicide sorption onto soils and leaching into groundwater. A suite of soil properties, including mineralogy (particularly soil iron and aluminum oxide content), exchangeable Al content, and soil phosphate content, influenced sorption of the anionic, 2,4-D. In general, 2,4-D sorption was lower in the presence of phosphate, possibly due to competition between phosphate and 2,4-D for surface sites or increase in surface negative charge resulting from phosphate sorption. Additionally, 2,4-D sorption was greater in the presence of exchangeable Al. It appears that 2,4-D may form surface complexes with or be electrostatically attracted to exchangeable aluminum in the soil. Our results suggest that carboxylic acid herbicides may be more easily leached in intensively managed Ultisols subject to continued phosphate fertilization and liming.