The importance of water pH on herbicide performance is again being questioned, primarily due to the marketing of additives designed to acidify water. The interest in carrier pH is partially due to studies with glyphosate (Roundup) that have shown reduced activity when hard water is used as the carrier. The decline in activity is due to the binding of glyphosate with calcium ions present in hard water, which inactivates the herbicide. Lowering the pH of the water decreases calcium availability, and improves glyphosate performance. However, the negative effect of calcium on glyphosate activity can be overcome more effectively by adding ammonium sulfate to the spray tank than by lowering the pH of the water. This is the recommended strategy for dealing with water quality problems when applying Roundup.
A second reason promoted for reducing water pH is to prevent alkaline hydrolysis of herbicides. This is not a concern at the pH of groundwater found in Iowa (typically between 6.5 and 8.0). On the other hand, certain herbicides are prone to acid hydrolysis (specifically the sulfonylureas Classic, Accent, and Beacon). Reducing the pH of the carrier below 6.0 can greatly shorten its half-life and diminish herbicide effectiveness.
In summary, water pH is not an issue for herbicide application. Select herbicide additives based on herbicide label recommendations.