Not that long ago, creeks, rivers, and ponds were an acceptable source of spray water. This practice seems unthinkable today, given our understanding that products like glyphosate are rendered inactive by clay particles and other impurities in the water. Through research it has been demonstrated that most pesticide chemistries are impacted, often negatively, by the various dissolved minerals and pH of the water used as the carrier.
Weed resistance, rising input costs, the need for effective cover crop kills, increased use of companion products such as foliar fertilizers, along with an increase in spray solution modifying adjuvants reaching the market have increased the need to quantify the quality of water used for pesticide dilution. Currently it is more common to analyze spray water quality after something has gone wrong rather than proactively testing to identify potential problems.
The stability of pesticides in the spray tank is often directly tied to the pH and the presence of dissolved minerals in the spray water. Depending on the pesticide chemical formulation, the active ingredient can be rendered inactive by either reacting with hydroxyl groups at high pH or with additional hydrogen ions at low pH. These chemical alterations of the active ingredient can also drive chemical reactions with the dissolved solids in the water rendering the pesticide inactive.
Herbicide products like dicamba and 2-4,D amine can be unstable at pH’s above 7.0. Insecticides and fungicides are even more sensitive to spray water pH. For example, some can be stable in the spray tank for days to months at a water pH of around 5, while at a pH of 9.0 are stable for only minutes. Many of your brand name pesticides that are pH sensitive are buffered in the formulation; however, this is not the case for all generics. Adjuvant manufactures have been addressing this need with a wide array of spray water modifiers to buffer pH concerns and tie up dissolved minerals before they impact the pesticide performance.
Analysis of your spray water will greatly improve the success in identifying the right adjuvant and using the product at the correct rate. The use of ammonium sulfate (AMS) with glyphosate applications is a good example of this. When spraying glyphosate, the label rate for AMS is 8.5 to 17 pounds per 100 gallon of spray water. By testing your spray water, you can pinpoint the rate needed for the application, possibly saving on the cost of excessive AMS while still ensuring adequate product to protect the efficacy of the glyphosate. If you are interested in seeing where your spray water stands, please contact the lab for sampling kits and for more information.