How did you do in high school chemistry class?  Maybe you had organic chemistry in college, or maybe you have a graduate degree in chemical engineering. Many farmers are challenged periodically when applying for their pesticide applicator license, and some find it easy as coffee shop talk.  Nevertheless the season is upon us for putting the sprayer through its paces and covering several hundred acres per day with an efficient pre or post emergent herbicide.  But what goes in the spray tank in addition to the pesticide?

Like many foods, water is the main ingredient, and in a spray tank the quality of the water can be just as important as the nursery water you use to mix baby formula powder.  Some of you may be thinking that drawing water out of the nearby waterways may not be such a good idea, and you should have it tested.  Others may be thinking that hauling water from the local municipality’s source of less expensive untreated water may not be such a good idea and you should have it tested.

Purdue water quality specialists say both of you are correct.  And because water is more than 95% of the spray volume and can vary widely in many chemical characteristics, it is important to know how your herbicide will perform, and take precautions if a problem can be foreseen.  Their recent factsheet on the impact of spray water makes the point that, “The properties of water used for carrier in spray solutions can greatly influence the performance of herbicides including glyphosate, Ignite, Clarity, 2,4-D, Sharpen, Pursuit, Poast, Accent, and many other herbicides. Therefore, defining the role of water quality on herbicide efficacy is very important. Unlike pure water, water quality of groundwater is variable between sources.”

You may have a great well at home and it has supplied the farmstead and livestock operation for many years without fail.  It has filled hundreds of sprayer tanks, just like the one on the other farmstead where the sprayer is filled most of the time.  They are only a couple miles apart, how could they be different?  Just like green and red iron, the ground water from different sources can vary in pH, hardness, alkalinity, turbidity, and temperature, and the presence of dissolved cations like calcium, magnesium, iron, aluminum, zinc, manganese, sodium, potassium, cesium, and lithium can influence herbicide efficacy, say the Purdue chemists.

pH is one of the most important factors because the degree of acidity or alkalinity of the water could have a major impact on the efficacy of the herbicide.  Herbicides such as glyphosate, 2,4-D dicamba, and others have a weak acid and will remain neutral in water less than 7.0 ph, which is acidic.  But they become negatively charged ions in alkaline water that is more than 7.0 pH.  The result of the later is the inability to get through the cuticle on weed leaves.  On the other hand, herbicides that are basic will react similar in more acidic water below 7.0 pH and will be less effective as the spray becomes more acidic.  Those include ALS herbicides such as Accent.

The Purdue weed specialists report that extreme pH levels, such as below 5.0 or above 8.0 not only affect the performance of the herbicide but also impact its residual in the soil.  And the last thing you want to do is pay a lot of money for an herbicide and have it last only half as long as it should.  Extreme pH levels will also reduce the solubility of the herbicide and leave residues in your tank that will clog screens and nozzles.  Such residue can also react with the next herbicide you use and neutralize it.  If you need to use water to fill your sprayer that has an adverse pH,  there are pH adjusters that allow you to create the optimum chemistry for your herbicide.  But before you use a buffer, consider whether the herbicide already has one.

Sometimes rural water sources are very soft and others are very hard, with a lot of mineral content.  The hardness level of the water and the metals in the water can also have a dramatic effect on your spray efficacy, such as reducing their ability to be absorbed into the plant tissue.  To the rescue is ammonium sulfate or AMS which has ions that bind with the cations of the other metals, and increases the absorption of the herbicide into the weed tissue.

Grass herbicides such as SELECTMAX, Poast, and growth regulators like 2,4-D can give a poor performance if your spray water has a high level of carbonates and bicarbonates.  A tank mix with diammonium sulfate could help overcome the problems but it needs to have the right amount.  Check the source page for information on computing the amount of AMS to use.

If you are pumping spray water from a drainage ditch, there may be a turbidity problem with mud and cloudiness.  The presence of organic particles such as soil can bind to the herbicide molecules and neutralize them.  Such particulates can also clog screens and nozzles. 

The bottom line for spray water is a wide variety exists, and many can innocently be acceptable for nearly all uses, except as a carrier for your herbicide spray.  Spray water from ground wells can vary widely in chemical makeup, particularly in pH, turbidity and hardness.  Depending upon the needs of the herbicide, the well water used for spraying may have to be modified.