Dissipation of Norflurazon and Other Persistent Herbicides in Soil

Abstract

Norflurazon applied for weed control in cotton, as well as some other herbicides, sometimes persists in soil from one growing season to the next at levels phytotoxic to the following crop. Dissipation modes of norflurazon were characterized by using14C-labeled herbicides for adsorption, mobility, photolysis, and volatility studies to aid in the prediction of conditions influencing herbicide carryover problems. A direct soil-counting technique modified for use in these studies yielded a norflurazon recovery as high as 83% of the applied radioactivity. Relative mobility of the herbicides on soil thin-layer chromatography plates was fluometuron ≥ atrazine > norflurazon. Norflurazon adsorption increased and mobility decreased as soil organic matter and clay content increased. Considerable upward movement of norflurazon and atrazine occurred in subirrigated columns containing herbicide-treated Hebert silt loam. Upward movement in excess of 5 cm occurred in 10 days and 8 weeks for atrazine and norflurazon, respectively. Photolysis and volatilization studies with norflurazon and atrazine revealed low volatilization but significant photolytic losses for norflurazon when herbicide-treated soil-coated slides were exposed to ultraviolet or sunlight. After 98 h atrazine volatilization was greater and photolysis in sunlight less than that observed for norflurazon. Laboratory studies showing upward movement of norflurazon and atrazine, in conjunction with dissipation losses occurring at the soil surface, suggested that losses of norflurazon and atrazine are facilitated by movement in capillary water referred to as the “wick” effect.