Hard water hampers weed control

To get the most from the herbicides you’re spraying in your field, check your water quality first

A North Dakota State University extension specialist says the performance of some herbicides can be adversely affected by using hard water to prepare spray mixtures. According to Richard Zollinger, there are a variety of water quality issues that can impact herbicide efficacy, including the presence of clay, silt or organic matter in the water. But a key factor in “antagonizing” herbicides is hard water that includes high levels of mineral ion content.

“Water hardness is based on the amount of calcium and magnesium in the water,” Zollinger explains. Hardness is expressed as the amount of calcium in the water added to the amount of magnesium present as calcium carbonate equivalent. “The presence of the other minerals would be additive to the hard water value,” he says. “Hard water may contain mostly calcium, magnesium, iron, and potassium. Soft water contains sodium, similar to how a water softener works in homes.”

Minerals such as calcium, magnesium potassium, iron and sodium contain cations, or positively charged ions, that bind with herbicides, reducing absorption, which results in less activity from the herbicide.

Zollinger delivered a presentation entitled, “Weed Control Issues: How Water Quality Can Affect Herbicide Efficacy” at Manitoba Ag Days, held at Brandon, Man. in January.

During the presentation, Zollinger showed evidence that much of the water in the Prairie provinces is very hard, showing an up to or greater than 250 milligram per litre (ppm) concentration of hardness as calcium carbonate.

Almost all “weak acid” herbicides are antagonized by minerals in hard water, he explained, including glyphosate (Group 9) and Liberty (Group 10), and other “weak acid” herbicides in various chemical groups. Calcium levels of 150 ppm and sodium levels of 300 ppm in spray water are enough to antagonize herbicides.

According to an article on water quality and weed control prepared by Saskatchewan’s Ministry of Agriculture, hard water can also reduce the activity of 2,4-D Amine. If the water is hard enough, this herbicide’s effectiveness is severely reduced.

Additionally, some Saskatchewan groundwater contains relatively high levels of bicarbonate ions. “Bicarbonate content can be a factor affecting the performance of some herbicides, particularly those in the “dim” group such as Achieve (tralkoxydim), Poast Ultra (sethoxydim) and Centurion/Select (clethodim) as well as 2,4-D Amine,” explains the article.

Managing hard water

There are a few steps you can take to mitigate the impact of hard water on herbicide efficacy. One strategy is to apply the maximum rates of herbicides when hard water is an issue. Also, water volume can be reduced to the minimum required for adequate herbicide coverage. This is effective for glyphosate in particular, due to a higher concentration of glyphosate in spray droplets.

Zollinger recommends applying ammonium sulfate (AMS) fertilizer with glyphosate, as it prevents cations in hard water from binding to glyphosate and minimizing its effectiveness.

AMS application rates should vary based on water hardness. Most glyphosate labels recommend an application rate of 8.5 to 17 pounds per 100 gallons, but as little as four pounds per 100 gallons may be enough to overcome most salt antagonism. Zollinger’s guideline for dry application rates is 8.5 pounds per 100 gallons, or 0.8 kilograms per acre. AMS can also be purchased in liquid form for easier application. One guideline for liquid AMS application rates is 1.6 litres per acre. “This will overcome salts in the water and enhance all weak acid herbicides,” Zollinger says.

AMS is not perfect — application requires lots of product, as well as time to let it dissolve in the spray water. In addition, it slows speed of loading and spraying overall, and impurities, as well as low water volume, may plug nozzles. However, Zollinger is convinced that the benefits outweigh the negatives.

“AMS has been the cheapest and most effective method to overcome hard water antagonism of herbicides,” he says. “There are liquid water conditioners on the market that contain low amounts of AMS and other substances, but they are more expensive than AMS and some of them are less effective.”

Zollinger concluded his presentation with a warning: “Know your water quality.”

Growers who suspect they may have water quality issues should send samples for testing. In Saskatchewan, growers can refer to the ALS Environmental Laboratory’s testing services. In Manitoba, Central Testing Laboratory offers water testing packages.

About the author


Julienne Isaacs

Julienne Isaacs is a Winnipeg-based freelance writer and editor. Contact her at [email protected]

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