Water Hardness Wreaking Havoc with Pesticide Applications?

Jesse Fulbright, MSU Liberty County Extension

At a time of the season when there are plenty of times when spraying is an ongoing practice, let’s take a couple of minutes to talk about something that might make your pesticide applications less effective: water hardness.

According to an MSU Extension MontGuide titled, “Pesticide Performance and Water Quality,” the term water ‘hardness’ refers to presence of metals with a positive charge of more than 1, such as calcium, magnesium, and iron.  Total hardness is measured in parts per million (ppm) or in grains per gallon and labs typically report results in terms of calcium for simplicity, even though other metals are making up part of the hardness.  One grain (per gallon) equals 17.1 ppm.

These cations, or positively charged elements, can further reduce the effectiveness of weak acid pesticides, especially if the pH of the water is above the ideal range, which is so often true in our area.  The effect happens because of the pesticide dissociating into positively and negatively charged parts and the elements, such as calcium, magnesium and iron in the water, binding with the negatively charged portion of the pesticide.  This results in molecules that either can’t be absorbed by the target pest, enter at a slower rate, or form insoluble salts.  Hardness can range anywhere from 0 to over 800 ppm.  Water with a hardness between 0 and 114 ppm is considered soft, 114-342 ppm, moderately hard, 342-800, hard, and anything above 800 ppm is considered extremely hard.

Consider the following guidelines regarding hard water:

•Always read and follow precautions regarding hardness on the pesticide product label.

•Weak acid pesticides such as clopyralid, 2,4-D amine, glyphosate and dicamba may lose efficacy if hardness exceeds 150 ppm, especially if pH is greater than 7.0.

•2,4-D amine formulations can be totally deactivated if the hardness is greater than 600 ppm.

•Many other herbicides will lose efficacy if hardness is greater than 400 ppm, if iron is present.

Hardness can be reduced with the addition of dry ammonium sulfate at 8.5 to 17.5 lbs. per 100 gallons of water, or liquid fertilizers (such as 28 percent N, 32 percent N, or 10-34-0) at a rate of 1.25 – 2.5 percent per 100 gallons.  It works by reducing the pH and also through sulfate combining with hard water cations.  Performance might be enhanced further by the addition of a non-ionic surfactant.

If you have questions regarding your water hardness, please consider visiting with your local county Extension office about water testing options.