Water quality management is critical to effective pesticide use. In fact, water pH, turbidity, hardness, and temperature can all affect pesticide performance substantially. For example, some herbicides, including glyphosate and glufosinate, will bind readily with Ca- and Fe-containing mineral particles in water and can become quickly deactivated or otherwise unavailable for their intended use. Many fungicides and some insecticides are also sensitive to solution pH, performing best is acid solutions around pH 5. Additionally, when spray water temperatures are too cool (<50°F) or too hot (>100°F), adsorption and/or effectiveness of a pesticide also can be compromised. Pesticides generally perform best when present in an acidic solution that has been blended around 75 degrees Fahrenheit with water that is free of dissolved minerals and turbidity.
Two ways to ensure that water quality does not negatively impact pesticide efficacy are to test and, if necessary, treat your spray water to make sure it is of suitable quality. Conventional treatment options at the farm level fall into two categories, filtration and acidification. Sand filters, alone or in combination with chemical treatments, are used to reduce both particulates and dissolved minerals (e.g. excess iron and calcium salts) which negatively impact water quality. Because most municipal supplies and many on-farm wells have an alkaline pH (i.e. > 7.0), acid solutions are often needed to prevent problems associated with reduced pesticide efficacy. However, addition of the right amount of acid can be tricky as the appropriate types and amounts of acid required to get a water into the proper pH range will depend on several different water quality factors.
Ag Spectrum recommends testing spray water sources to understand its characteristics and prevent pesticide inactivation. Midwest Labs can provide data on pH, hardness, and dissolved particle concentration and guidance on when the water is suitable for use as is, or needs some form of pretreatment. With regard to testing, it is important to note that well water quality can change significantly over time, so be sure to test each year around the times in the spring, summer, and fall that you will draw from the well for spray preparations. In contrast, most municipal water sources vary only slightly during the year so testing just once a year is often enough.
If a water test indicates excessive hardness and/or a high pH, Ag Spectrum recommends treatment of water in the spray tank prior to addition of a pesticide. Insure® is a water softener that can prevent binding and reduced precipitate formation due to excessive hardness. Indicate 5® is used to easily and accurately modify spray water pH to the optimum range. The color of a solution with added Indicate 5 will visibly change from yellow to pink to red as the solution pH approaches the target range of pH needed to ensure pesticide performance. Both products also contain surfactants which can help to aid in absorption and translocation of pesticides within the target crop plant or pest. And, remember to read and follow all directions on the pesticide label.
—Submitted by Dr. Jim Smart and Brian Gardener, Ph.D