Increasing the pH of the spray water to solubilize the weak acid herbicide nicosulfuron and then decreasing pH below its pK_a so that it converts into a neutral form enhances biological activity under some conditions. The water-dispersible granule formulation of nicosulfuron starts as dispersed particles. Adding 1% wt/wt K₃PO₄ solubilizes nicosulfuron and increases its activity compared to its dispersion without base. The type of buffer and the surfactant HLB or hydrophilic lipophilic balance, a measure of the molecular balance of the hydrophilic and lipophilic groups, altered the activity of nicosulfuron. Adding 1% wt/wt K₃PO₄ increases the pH, and the optimum HLB ranged from 13 to 17 on large crabgrass. Adding 1% wt/wt H₃PO₄ reduces the pH and lowers the optimum HLB range from 10 to 14 on large crabgrass. Adding the acidic buffer converts the solubilized nicosulfuron into its neutral form and increases activity under some surfactant conditions. Thus, neutral nicosulfuron is more active with lipophilic surfactants, while ionic nicosulfuron is more active with hydrophilic surfactants. When tested on other species, low HLB surfactants are the most active at low pH. These results support the concept that the physicochemical properties of the herbicide, adjuvants, and weed species should be matched for optimum activity.

Nomenclature: Common cocklebur, Xanthium strumarium L. #³ XANST; common lambsquarters, Chenopodium album L. # CHEAL; giant foxtail, Setaria faberi Herrm. # SETFA; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; purple nutsedge, Cyperus rotundus L. # CYPRO; sicklepod, Senna obtusifolia (L.) Irwin and Barneby # CASOB; velvetleaf, Abutilon theophrasti Medicus # ABUTH.

Additional index words: Acid, adjuvant, base, basic blend, buffer, formulation, hydrophilic–lipophilic balance, nicosulfuron, nonionic surfactant, phosphoric acid, potassium phosphate, sulfonylurea, water-dispersible granule, weak acid.

Abbreviations: HLB, hydrophilic–lipophilic balance; NIS, nonionic surfactant; NPE, nonylphenol ethoxylate; WDG, water-dispersible granule.