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OSU Extension

College of Food, Agricultural, and Environmental Sciences

June 1, 2015 - 11:10am -- Anonymous

            Do you know your water quality numbers?  A few years ago in our private pesticide applicators winter recertification classes I asked that question.  Typically, only a few in every group raised their hand as an affirmative response.  Water quality of groundwater is measured by several factors such as pH, hardness, alkalinity, turbidity, and temperature.  These measures of water quality are important because they are all factors that can affect the performance of pesticides such as herbicides, fungicides and insecticides.  In this article I’ll briefly review how these water quality factors can have an impact of pesticide performance.  My sources of information are two publications from Purdue Extension; “The Impact of Water Quality on Pesticide Performance” and “The Influence of Spray Water Quality on Herbicide Efficacy”.

            Many of the commonly used herbicides, fungicides and insecticides work best when mixed with water that is slightly acidic, in a pH range of 5.0 to 6.5.  An exception is the sulfonylurea herbicides that work best at water pH values above 7.0.  The pH of the spray solution is important because it determines how long that pesticide molecule stays intact, and it can influence the electrical or chemical charge of the pesticide molecule which in turn can limit the ability of the chemical to penetrate the leaf surface or be absorbed by the plant.  Water pH can influence the solubility of the pesticide.   If the pesticide is not able to completely dissolve there can be issues with blocking nozzles or screens or in how the pesticide is deposited on the leaf surface.

Recently I took a phone call and the caller asked if they could mix up a surplus of a pesticide and let it stand in the tank until the next application 7-10 days later.  That is not a good idea.  Most pesticides are going to lose their effectiveness over that time frame.  Pesticides should generally be mixed and used the same day.  In one of the Purdue publications that I listed at the beginning of this article there is a chart which illustrates pesticide half-life in the sprayer tank solution.   If the pesticide is one which needs a water pH in the 5.0 to 6.5 range, and the sprayer solution pH is instead at 9.0, then the effectiveness can be reduced by 50% in 10 minutes for some herbicides, in 2 minutes for some fungicides and in 24 hours for some insecticides.

Water hardness is determined by the presence and amount of certain minerals in the water including iron, magnesium, calcium and sodium.  Water hardness is usually measured in parts per million (ppm) and values of 115 and above are categorized as moderately hard, hard or extremely hard.  The issue is that these minerals all have a positive charge and they can bind up negatively charged pesticide molecules.  When the pesticide molecules are bound to these minerals, the pesticide can precipitate out of solution, or its effectiveness at entering the plant tissue is much reduced.  A common example of a negatively charged pesticide molecule that is greatly affected by water hardness is glyphosate.  For this reason ammonium sulfate (AMS) is used with glyphosate in hard water conditions because the sulfate ion in AMS binds with the calcium, magnesium, and/or sodium cations.  This reaction frees the glyphosate molecule, which then binds with the ammonium ion and this complex is more effectively absorbed across plant membranes.

A related issue to water hardness is the carbonate and bicarbonate concentration of water.  When these compounds are present in levels greater than 500 ppm they can reduce the efficacy of certain herbicides, most notably some grass herbicides such as Select and Poast and also plant growth regulator herbicides such as 2,4-D.  Once again the addition of AMS to the spray tank solution can be used to improve herbicide effectiveness.

Turbidity is a measure of suspended soil or organic matter in water.  Those suspended particles can bind with negatively charged pesticide molecules and once that happens, the pesticide molecules are not available to be absorbed by a plant.  The other issue is that those particles can clog or block screens and nozzles, affecting the delivery of the pesticide to the target plant.  Finally, water temperature can be a factor.  This is primarily from the standpoint of mixing and solubility.  Some pesticides do not dissolve completely or mix thoroughly in cold water.

Of course pesticide manufacturers are aware of the importance of water quality and how it affects their product.  They want their product to work and be effective.  So, as always, read the label.  It often contains statement about water quality that can help you determine if a water conditioner product or another product such as AMS needs to be added to the sprayer tank. 

When you consider the cost of the pesticide products and your sprayer equipment a water quality test and the information it provides may offer you a good return on your investment.  For more information about water quality and its impact on pesticide effectiveness, contact the Wayne County Extension office at 330-264-8722.