Tools of the Trade: Water Quality 3

This article was written by Minnesota Soybean Director of Research David Kee and can be reached at dkee@mnsoybean.com or 507-388-1635.

Pay attention to your water when applying herbicides. Testing your pH levels prior to tank mixing is now more important than ever before. Understanding your water’s inital pH will impact the performance of your pesticide solution, but could also be impacting the volatility of certain pesticides as well.

The labels for Xtendimax with VaporGuard^TM and Engenia^TM have advisory warnings about managing dicamba volatility by maintaining a final spray solution pH above 5. Apparently, the volatility of any dicamba product increases with decreased pH, especially if the pH is below 5. To prevent dicamba volatility loss, addition of certain acidifying agents, such as ammonium sulfate, are not allowed.

What isn’t discussed as part of the label, is the impact of initial water chemistry on final solution pH. Mueller and Steckel, weed scientists from the University of Tennessee, recently published the results of several studies regarding impact of different herbicides and adjuvants on final solution pH.

In part of the project, the authors procured water from 12 different sources across Tennessee and Kentucky. Initial water sample pH ranged from 4.2 to 8.3. Water samples then received either N, N-Bis-(3- aminopropyl) methylamine salt of dicamba (BAPMA) or diglycolamine with VaporGrip® dicamba followed by potassium salt of glyphosate. Solution pH was measured at each phase of chemical additions.

Figure 1, below, was developed by modifying two figures found in the original paper (with permission from the author). The addition of dicamba, in either form, modified the solution pH. The addition of the two dicamba formulations increased solution pH of the more acidic water and decreased pH of the more alkaline water. Both dicamba solutions were still acidic. The BAPMA solution was less variable with a pH range of 6.2-6.8, and an average solution pH of 6.6. The diglycolamine with VaporGrip solution was more variable with a pH range 5.3 to 6.9, and an average pH of 6.

The addition of potassium salt of glyphosate dropped pH of all solutions. With the BAPMA solution, it dropped the pH to a range of 4.6 to 5.6. With the diglycolamine with VaporGrip solution, it dropped the solution pH to a range of 4.7 to 5.3. Multiple samples had a final spray solution pH below the EPA advisory threshold of 5.

Referring to the BAPMA with potassium salt of glyphosate addition, the authors state “Interestingly, a group of water sources near an initial pH of 7.0 had about half of the final pH readings >5.0 and about half <5.0. Water chemistry is affected by several factors, and the cations present, other dissolved materials, dissolved oxygen, as well as small particulates may have affected our pH measurements.” The chemistry of the water source may have a profound impact on a final spray solution behavior and performance.

Multiple studies have proven glyphosate has a greater efficacy if the application solution is acidic. Many glyphosate products are formulated to optimize the efficacy of glyphosate by decreasing (acidifying) spray water pH. One Bayer/Monsanto publication states: “The pH for XtendiMax and approved glyphosate products within a tank-mix solution is expected to be within the range of 4.8-4.9.”

This study demonstrates pesticide company expectations are not always correct. The modification of solution pH via adjuvants, herbicide formulation, etc. is a routine practice with companies, farmers and agronomy consultants. The chemistry of your water will impact how a solution is modified to optimize the pesticide.

Simply put: test your water! Your initial pH will have a significant impact on your final solution’s performance.