Water quality can play a significant role in pesticide performance, but it’s a consideration that often flies under our radar until a herbicide application doesn’t work as well as we hoped. If you suspect hard water, adding ammonium sulphate (AMS) could the right call, but first it’s important to know what’s actually in your water. 

Start by testing your water

The first step in managing potential spray water issues is to test your water. While it’s generally good practice to test your water quality, it’s even more important if you suspect that poor water quality is affecting the performance of your herbicides. Agricultural spray water analyses are offered by many accredited agricultural laboratories, such as AGVISE Laboratories, A&L Canada Laboratories, Central Testing Laboratory and Horizon Lab.

Once you have a water quality test, Sprayers 101 has a helpful article on how to make sense of your water quality test results.

Why is hard water a problem?

Hard water contains high levels of positively charged minerals such as calcium (Ca²⁺), magnesium (Mg²⁺) and others. These minerals bind to herbicide molecules, preventing them from being absorbed by the plant and reducing efficacy.

Glyphosate (e.g., Roundup) is commonly associated with hard water antagonism, but hard water can reduce the efficacy of all weak acid herbicides (found in Groups 1, 2, 4, 6, 9, 10, 14, 19 and 27).

How much hardness is too much depends on the herbicide, rate and water volume you’re using. For glyphosate, Bayer suggests a hardness limit of up to 700 ppm, when using higher rates or lower water volumes.

Should you add AMS?

If you have hard water but don’t have access to an alternate water source, adding AMS is an effective strategy.

When added to spray water before your herbicide, the negatively charged sulphate ions in AMS tie up the hard water cations before they can antagonize the herbicide. To calculate how much AMS to use, Sprayers 101 has a helpful calculator, but many agriculture labs that offer spray water tests will include AMS recommendations in your test results, and herbicide labels and the Manitoba Agriculture Guide to Field Crop Protection often include water quality recommendations.

In some cases, reducing water volumes and/or increasing your herbicide rate can help counteract the effects of hard water, but always make sure to stay within label guidelines. 

Other water quality issues to be aware of

Along with hardness, there are a few other water quality factors to keep on your radar:

• Dirty water (turbidity): Particles of soil and organic matter can bind herbicides and reduce performance. Clean water is particularly important for herbicides that are known to strongly bind to soil, such as glyphosate and diquat (e.g., Reglone).
• Bicarbonates: Bicarbonate ions can inhibit herbicides, particularly the Group 1 “dims,” including clethodim (e.g., Select, Centurion) and tralkoxydim (e.g., Achieve) and the Group 4 herbicides MCPA amine and 2,4-D amine.
• pH (acidity or alkalinity): The pH of your spray water can impact pesticide solubility and breakdown. Unless recommended on the product label, it is generally not advised to adjust the pH of your spray solution. For those interested in learning more, Sprayers 101 has a helpful article.

My last piece of advice: always read and follow the directions on your pesticide label. While they can be long and unwieldy, the pesticide label includes key details on how to use your herbicide safely and effectively.

Learn more

• Sprayers 101
  • Water Quality and Spray Application
  • The Real Story Behind pH and Water Hardness
  • How to Interpret a Water Quality Test Result
• Purdue University Extension – The Impact of Water Quality on Pesticide Performance
• North Dakota Weed Control Guide, pages 85 and 132