Effect of Soil Properties and Surfactant on Primisulfuron Sorption

Soil properties affect the environmental fate of herbicides. Little is known about how soil organic matter and pH affect sorption and fate of the sulfonylurea primisulfuron (2‐[3‐(4,6‐bis(difluoromethoxy)‐pyrimidin‐2‐yl]‐benzoic acid methyl ester). Primisulfuron sorption and desorption by six soils with different organic carbon (OC), iron oxide, and clay contents were investigated using batch reaction at controlled pH. Sorption was measured at a soil/solution ratio of 1:2 with 0.01 M CaCl2 as a background electrolyte. Sorption and desorption isotherms were linear. When pH was adjusted to values between 4 and 6.5, sorption coefficients decreased with increasing pH. At pH 4.5, sorption coefficients increased with increasing OC and ranged from 0.6 for a soil with 0.2% OC to 15.5 for a soil with 48% OC. Surfactant (Triton X‐77, 0.25%, v v−1) caused primisulfuron sorption to increase in a low OC soil but caused decreased sorption in a soil with 1.7% OC. Sorption coefficients were correlated with dithionite‐extractable iron in soils with <1% OC, but not in soils with greater OC. Desorption coefficients at native soil pH were up to five times greater than were sorption coefficients; about 50% of sorbed primisulfuron did not desorb during a single 24‐h batch desorption reaction. Primisulfuron is most likely to leach in soils with high pH or low organic matter contents.