Is Your Chemical Really Not Working?

Or is your spray water just lousy?

Published on: Apr 8, 2011

Quality of the water going into the spray tank has more effect on herbicide application effectiveness than most people realize. That's the message Fred Whitford wants to pass along to farmers this spring. He is director of Pesticide Programs at Purdue University.

Whitford calls it the little factor that makes a big difference. There are two parts to water quality that anybody spraying herbicides should be aware of, he notes. They include 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.

Hard water contains positively charged ions, Whitford says, including iron itself, calcium and magnesium molecules. Negatively-charged pesticide molecules can attach to these particles, drawn like a magnet to the positive charge. Once that happens 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 actually states on the label: '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, noted below, but if you're pulling your water from a well in Indiana, likelihood is that it is hard water. Based on the World Health Organization Water Classification system, water is soft if it contains 0-114 parts per million of the positive mineral ions, 'moderately hard' at 114-342, hard at 342-800 and extremely hard at 800 and above. Whitford considers 'hard' at levels around 200.

The best advice is to pay careful attention to directions on the label, Whitford says. If the label recommends adding ammonium sulfate, then add it. Don't risk hard water making the application less effective. The more you shave rates of active ingredients, the more important factoring in water quality becomes.

The pH level can be even more devastating. Except for sulfonylureas that prefer pH between 7.0 and 8.0, most herbicides prefer being in acidic water. That can be problematic since even distilled water is 7.0. Water coming out of wells is likely to be higher than 7.0.

Why does that matter? Because some herbicides break down 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, there may be less active ingredient left in the tank 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. And if you switch from pulling out of a pond or stream to well water, assume it may make a difference. Retest it. Inexpensive paper test strips for hardness and water pH are available from a number of suppliers. Some also carry relatively inexpensive meters. You won't nail pH or hardness numbers to the level a researcher might need, but it's adequate to get you in the ballpark for spray solutions.

You may also want to get a copy of 'The Impact of Water Quality on Pesticide Performance," Purdue Extension bulletin PPP-86. Order from: www.the-educaiton-store.com.