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Know pH, hardness of your spray water

The quality of the water going into a spray tank may have more impact on the effectiveness of a herbicide application than most people realize. That’s the message from Fred Whitford, director of Pesticide Programs at Purdue University.

Key Points

Hard water can cut down on the effectiveness of herbicides.

An acid-loving chemical in alkaline water spells disaster.

Follow label directions on water hardness and pH carefully.


There are two parts to water quality — water hardness and water pH level. The pH is the measure of whether something is acidic or basic. Neutral substances read 7.0, while acids read lower and alkaline materials read higher than 7.0.

Mineral water

Hard water contains positively charged ions, Whitford says, including iron, calcium and magnesium. Negatively charged pesticide molecules attach to these particles. The new molecules either can’t enter the target pest, or enter at a much slower rate.

The issue is serious enough, Whitford says, that one manufacturer states: “A water conditioner may increase the performance of this product on annual and perennial weeds, particularly under hard-water conditions.”

How do you know if your water is “hard”? Test kits are available, but if you’re pulling your water from a well in Indiana, it’s likely hard water.

The best advice is to pay careful attention to directions on the label, Whitford says. If the label recommends adding ammonium sulfate, add it. Don’t risk hard water making the application less effective. The more you shave rates of active ingredients, the more important water quality becomes because you can’t afford to lose more killing power.

Why pH matters

The pH level can be even more devastating. Except for sulfonylureas, most herbicides prefer acidic water. Water coming out of wells is likely to be higher than 7.0.

Why does that matter? Because some herbicides break down rather quickly in alkaline water and, once broken down, are no longer effective. That especially comes into play if you let spray solutions sit in the tank overnight, or if you’re rained out and a solution sits for several days. With certain products and alkaline water in these situations, there may be less active ingredient left by the time you head to the field.

The best advice is to check pesticide labels, Whitford says. There’s a wide variation of suggested optimum pH ranges, and specific directions on what to add or not add to the solution. Treat each herbicide by itself, and follow the directions listed for that pesticide. None of this matters unless you know the pH of your water source.

Inexpensive paper test strips for hardness and water pH are available from a number of suppliers. You won’t nail pH or hardness numbers to the level a researcher needs, but it’s adequate for spray solutions.

You may want to get a copy of “The Impact of Water Quality on Pesticide Performance,” Purdue Extension bulletin PPP-86.

Order from: mdc.itap.purdue.edu.

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Check water first: Make sure you compensate for water hardness and pH before you head to the field.

This article published in the May, 2011 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.