Eric Yu, UMN Extension Crops Educator, Ryan Miller, UMN Extension Crops Educator, Debalin Sarangi, UMN Extension Weed Specialist
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| Photo: Liz Stahl |
The role of pH and water quality in herbicide effectiveness
When the pH of the spray solution is too high, certain herbicides may begin to degrade in the tank or form complexes with mineral ions, reducing their absorption and effectiveness. Higher water hardness, especially from calcium and magnesium ions, can also bind to herbicide molecules and limit their activity. Although ammonium sulfate (AMS) based adjuvants can help mitigate some of these issues, their effectiveness depends on the starting quality of the water. If herbicide molecules break down or bind to minerals before they reach the plant, the result may be inconsistent weed control and a higher likelihood of repeat applications.The challenge
Previous studies have shown that spray water quality can significantly affect herbicide efficacy. While hard water generally reduces herbicide performance, the optimal spray solution pH can vary depending on the herbicide. For instance, a series of field and greenhouse studies conducted at Purdue University found that mesotrione (e.g., Callisto) activity was maximized when applied with AMS in a near-neutral pH solution (Devkota et al., 2016). In another set of experiments, dicamba and glyphosate, both weak acid herbicides, were most effective when applied in acidic solutions with no water hardness (Devkota and Johnson, 2020). Similarly, glufosinate (e.g., Liberty) appeared to provide optimal weed control at an acidic pH range of 4.0 to 6.5 (Devkota and Johnson, 2016). But what about more complex herbicide tank mixes that contain multiple active ingredients? Is there a “sweet spot” for pH in those situations?In practice, applicators must deal with multiple interacting factors, including varying herbicide chemistries, resistant weed populations, and fluctuating water sources. There is also limited field research on how pH buffers influence modern herbicide formulations and tank-mixes. Current data is heavily weighted toward AMS for addressing hard water, but there is a need for more comprehensive field-based research that evaluates both hardness and pH, especially as applicators diversify their post-emergence herbicide tank-mixes.
Considerations for This Growing Season
While research continues, here are a few steps growers and applicators can take now:- Test spray water for pH and hardness. Simple test kits or lab services can help identify issues before they impact control.
- Be alert to variability. If certain fields or zones consistently show reduced control, water quality may be contributing.
- Use water conditioners where needed. Products like AMS or pH buffers can improve herbicide availability and uptake.
- Limit tank mixing too far in advance, particularly when water quality is questionable or environmental conditions fluctuate.
For help testing your spray water or choosing the right adjuvants for your program, contact your Local Extension Educator or crop consultant. University of Minnesota Extension will continue to share updates from the Rochester trial as results become available later this season.
Sources
Devkota P and Johnson WG (2020) Efficacy of dicamba and glyphosate as influenced by carrier water pH and hardness. Weed Technol. 34: 101–106. doi: 10.1017/wet.2019.110Devkota P, Spaunhorst DJ, Johnson WG (2016) Influence of Carier Water pH, Hardness, Foliar Fertilizer, and Ammonium Sulfate on Mesotrione Efficacy. Weed Technol. 30:617-628
Devkota P, Johnson WG (2016) Glufosinate Efficacy as Influenced by Carrier Water pH, Hardness, Foliar Fertilizer, and Ammonium Sulfate. Weed Technol. 30(4):848-859. doi:10.1614/WT-D-16-00053.1
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