Wettable powders are finely divided (dust-like) solids that are easily suspended in water. When an herbicide is insoluble in water or oil, the formulation chemist may turn to wettable powders. They are formulated by impregnating the active ingredient in or on an inert material such as a clay and adding wetting and dispersing agents.
The wetting agent wets the active ingredient when it is mixed with water. Dispersing agents disperse the finely ground particles when mixed with water. A wettable powder with 50% active ingredient may contain 42% clay, 2% wetting agent, 2% dispersing agent, 4% inert ingredients, and 50% active herbicide. Because wettable powders form suspensions, not solutions, they will settle without continued agitation in the spray tank.
They typically have less foliar activity than liquid formulations. Because they are suspended, finely divided solids, their abrasive action can wear pumps and spray nozzle tips. Frequent calibration is required. To aid dispersion and assure homogeneity, wettable powders should be mixed in a thick slurry before mixing with water in the spray tank. Most of the triazines, phenylureas, uracils, and members of several other herbicide groups have been formulated as wettable powders. A major problem with this formulation is the difficulty of measuring weight of a dry powder in the field.
Wettable powders can be an inhalation hazard to those measuring or mixing them in water. Vigorous agitation in the spray tank is required. They are the most abrasive formulation for nozzle tips and pumps, and frequent nozzle plugging can be a problem. A few decades ago they were common. This is no longer true as most formulations are now liquid.