Grateful for the rain! With the anticipation of many post-herbicide applications occurring this week, just a reminder to make sure you know the specific herbicide traits each field has to avoid incorrect products applied and potential crop damage. The remainder of this column will be about the importance of water quality in pesticide applications, but is a brief overview. Additional resources are mentioned at the end. Water quality including pH, hardness, bicarbonate, and turbidity can make a difference on the efficacy of the pesticide. One always needs to read the pesticide label to determine what pH is best for the spray solution and also any other factors it mentions about water quality for the most effective pesticide application.

For example, do you check the pH of the final pesticide solution before it gets sprayed on the field? As a quick reminder, a pH of 7.0 is neutral. Above that is alkaline (basic) and less than 7.0 is acidic. Many of us have well water that is more alkaline. For example, my well water has a pH of 8.0. In general, herbicides, insecticides, and fungicides perform best in slightly acidic water (if the label doesn’t mention a specific pH assume between a pH of 5.5-6.5). Many herbicides are formulated as weak acids. Reading the pesticide label is important, though, because some pesticides like sulfonyurea herbicides and copper fungicides need slightly alkaline solutions.

Why is the pH important? The pH of the pesticide solution can impact the product performance including rapidly reducing the pesticide half-life. The half-life of a pesticide is the time it takes for that product’s active ingredient to degrade 50% in the environment. For example, a herbicide’s half life can go from just 10 minutes in a pH solution of 9.0 to 16 days at a pH solution of a 5.0. That’s one of the reasons why one should check the pH of the water using a pH meter before a buffering agent and any pesticide is added to the tank. It’s good to test the pH as various products are added to the tank and before the final solution is sprayed onto the field.

Water hardness can also impact pesticide performance by creating a poor mix, not allowing the product to dissolve in water, or reducing the pesticide half-life. Water hardness mostly indicates the amount of calcium and magnesium in the water, but aluminum, iron, sodium, and potassium are also positive ions that can bind to the pesticide potentially forming precipitates. Water hardness should in general be below 342 ppm. Water quality can be tested by sending a water sample to a laboratory.

The following are some corrective measures from North Carolina State University that can be taken to help correct poor water quality in pesticide solutions:

• “Add buffering solution to correct for improper solution pH.
• Add acid or buffering solution to correct for high alkalinity.
• Add ammonium sulfate (AMS) or 30% nitrogen to correct for hard water. (Generally, add 8.5 to 17.5 lbs dry AMS per 100 gallons water or 1.25-2.5% by volume of liquid fertilizer such as 28%N, 32%N, or 10-24-0).
• Mix pesticides in the field and apply within two hours.
• Filter turbid water or water with high organic compounds.
• Find an alternative water source if mitigation is not feasible.”

For those who’ve asked about using citric acid to adjust pH, University of New Hampshire shares, “Citric Acid (granulated): A food-grade acidifier. Example: 2 oz per 100 gal typically lowers pH from ~8.3 to ~5.4 (always test to confirm).” This guide from Purdue University on Water Quality and Pesticide Performance can also be a helpful resource that goes into more detail: https://www.extension.purdue.edu/extmedia/ppp/ppp-86.pdf.