Remedying spray water quality pays big dividends

Spray water quality attributes such as suspended particles, dissolved minerals and acidity influence the efficacy of agrochemicals sprayed on the farm

Spray water with suspended sediment or clay particles and organic matter is cloudy and brown. Some of these particles can bind to pesticides and reduce their effectiveness. Dirt particles also block nozzles on sprayer. Debris and sediment is generally found in water obtained from ponds, lakes, rivers or ditches. Some of the debris could be removed by using appropriate filters, but it is best to avoid waters that are visibly muddy or murky for spraying.

Hardness of water is caused by dissolved minerals. Technically water is considered “hard” if it contains greater than 342 ppm calcium and magnesium. These minerals are capable of binding to the active ingredient of the pesticide and reducing their effectiveness on the target pest. Depending on the label instructions, hard water can be treated with an adjuvant. An example would be the addition of ammonium sulfate to glyphosate spray.

Water pH is another property that influences pesticide activity. It reflects the acid level of the spray water. As a general rule, most herbicides (with a few exceptions), insecticides, and fungicides perform best in slightly acidic to neutral water ranging in pH 4.0-7.0. Depending on the label, high pH water can be corrected by adding buffers and acidifiers to spray water prior to adding pesticides. The popular belief that “if it is safe to drink then it is good for anything else” is not accurate. Municipalities disinfect water and this process elevates the pH to around 7.8-8.5.

Even when water characteristics are good, most pesticides breakdown in water with time by a process called hydrolysis. The number of days it takes for half the active ingredient to break down in water is referred to as the half-life of the pesticide. After reaching this point, the product will cease to have the desired efficacy. Half-life varies from one chemical to another, but the pH of water has a dramatic effect on hydrolysis. Because of this reason most products mixed in alkaline water should be sprayed immediately.

Liquid fertilizers are sometimes combined with pesticides, but certain combinations may not be compatible. Depending on the pesticide label, the addition of a compatibility agent may be needed or the label may simply specify “Do not use with any liquid fertilizer solution less than pH 3.”

Do-it yourself test kits for measuring hardness and pH of water and spray adjuvants are readily available from your local fertilizer dealer. An inexpensive pH meter may be another simple and efficient option, if you want to avoid carrying test kits and tanks are being filled from multiple water sources. Checking the quality of spray water on your farm is well worth the time considering the returns on improved pest control.

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