Adsorption and desorption of nicosulfuron in soils

Nicosulfuron (2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-N,N-dimethyl-3-pyridinecarboxamide) is a new, low-rate, sulfonylurea herbicide that is being promoted for weed control in corn (Zea mays L.); however, there is a lack of published information on its behavior in soils. In this study nicosulfuron sorption was measured In hatch experiments, at a 1:2 soil/solution ratio, for 10 Iowa and four Brazilian soils. Adsorption isotherms were nonlinear with K-f,K-ads values ranging from 0.21 to 8.78 with the highest values obtained for Iowa soils. Nicosulfuron K-t,K-ads values in Iowa soils were correlated with clay content (r = 0.92), while in Brazilian soils the K-t,K-ads values were correlated with organic C content (r = 0.97). These differences are interpreted in terms of different clay mineralogy of Iowa (expandable 2:1 clay minerals) and Brazilian soils (kaolinite, Al and Fe oxides). Nicosulfuron desorption isotherms, obtained using one-step extraction with 5 x 10(-3) M CaCl2, were nearly single valued with adsorption Isotherms in soils with pH > 5.5, while hysteresis was observed in soils with pH < 5.5. Up to 100% of the nicosulfuron was desorbed from Brazilian soils in one desorption step with a 2:1 acetonitrile/water mixture. Only 50 to 70% of adsorbed nicosulfuron could be desorbed with the same extractant from Iowa soils, possibly due to chemisorption of nicosulfuron on 2:2 clay minerals. This study suggests that nicosulfuron will not be highly mobile in the soils with a 2:1 clay mineralogy indicating that in such soils its potential to leach to groundwater may be lower than that of other sulfonylurea herbicides.