In Water Effects On Pesticide Performance, Montana State pesticide education specialist Reeves Petroff reminds us that pesticides may react with water, especially if the water is considered hard. Hard water contains positively charged calcium, magnesium, or iron minerals (known as cations). A greater concentration of these minerals results in very hard water. If a pesticide is negatively charged (classified as an anion), the pesticide and water will bind together and will not separate once applied to the targeted pest. This binding reduces the effectiveness of the pesticide. Herbicides, such as glyphosate, Poast (sethoxydim), Pursuit (imazethapyr), and Liberty (glufosinate) are subject to being bound in the water. 

Taking a simple water test of your primary spray water supply will determine how you manage the water. Water with a hardness of less than 50 ppm is considered soft, 50 to 100 ppm is classified as medium, and 100 to 200 ppm is considered hard. It is not uncommon for water in our region to have greater 200 ppm hardness.

We've had several herbicide and insecticide failures that could not easily be diagnosed but suspect it was a hard water issue. So how do you reduce the hardness of the water? Here are some tips to consider:

  1. Ammonium Sulfate (NH4SO4) — Ammonium sulfate (AMS) has been used successfully to increase herbicide efficacy on a broad spectrum of weed species. AMS helps tie up mineral cations and adjusts the pH so that more of the active herbicide is transported across the leaf surface and into the plant. A general rule-of-thumb for adding AMS is the addition of 2% AMS by weight or 17 pounds of dry AMS per 100 gallons of water for most applications. AMS should be added to the spray carrier solution prior to the herbicide and always, consult the pesticide label for mixing instructions.

  2. Organic Acids — The addition of an organic acid such as food grade citric acid will reduce the solution pH and can help increase the activity of herbicides that are weak acids such as glyphosate. Commercial buffering agents offer a similar service. Buffering agents are added to the water carrier prior to the addition of the herbicide.

  3. Urea Ammonium Nitrate (UAN) — UAN may also reduce water hardness, but it is not as effective as AMS and does not provide the same utility as AMS particularly for glyphosate. Some UAN utilizes a sulfuric acid source to add sulfur to the fertilizer mixture and may enhance the acidification from UAN.

In addition to hard water, the pH of the water also affects some pesticide performance. Determine the pH of your spray water and consider the following guidelines:

  1. pH 3.5-6.0 — Satisfactory for most spraying and short-term (12 to 24 hours) storage of most pesticide mixtures in the spray tank. However, the sulfonylurea herbicides (Accent, Harmony, etc) degrade in low pH solutions.

  2. pH 6.1-7.0 — Adequate for immediate spraying of most pesticides.

  3. pH 7.0 and higher — May need to add a buffer or acidifier depending on the pesticide.

Finally, choose a water source that is free of soil, grit, and organic matter. Soil particles decrease glyphosate and paraquat activity and can cause equipment wear. This type of antagonism cannot be corrected by adding AMS or buffering solutions. 

Always read the pesticide label to determine what products can be included in the pesticide mixture and the proper mixing order. By following some simple rules, the maximum effectiveness of herbicides, insecticides, fungicides and plant growth regulators may be achieved and avoid failures in the field.

For more information on how hard water can affect glyphosate, check out the article, "Why Your Glyphosate May Not Be Working," published in the May 2014 issue of Conservation Tillage Guide.