“…water treated with reverse osmosis results in a more effective spray. And ultimately, more wholesome crops.” – Culligan Water Website
There has been a lot of discussion in farming circles about water quality and pesticide efficacy in recent grower’s meetings I have attended. I have been somewhat alarmed about the potential for attendees to misunderstand aspects of this topic. So, let’s take a moment and clear the waters a bit about water.
Water is not just H20 molecules as you might have been led to believe in a high school science class. As it comes from the tap, out of the well or river, water is actually H20 and dissolved solids that can include cations (+ charged), anions (- charged), clay-sized minerals and organic matter (with +/- charges & nonionic binding sites). The amount and type of different dissolved solids ultimately determines a specific water source’s properties, like hardness (a measure of the amount of dissolved calcium, magnesium and iron) and pH (a measure of acidity or alkalinity). A pesticide’s chemical structure and formulation determines how best it can be carried and applied in a liquid spray application. Spoiler alert: not all herbicides are formulated the same way and do not all require the same water properties – read the label.
Let’s use glyphosate (Roundup brand products like RT3, others) as an example. Glyphosate is a salt formulated herbicide with a negative charge (also called a weak-acid). Glyphosate effectivity is severely limited when mixed with “hard” water that has high amounts of calcium, magnesium and/or iron dissolved salts. The cations in the hard water associate with the glyphosate ion and reduce its ability to enter the plant. Fortunately, we’ve long known that there are two work-arounds for these situations. We either chose a “softer” water source and/or adjust the pH to a more acidic level that reduces the potential for the glyphosate ion to form those effectiveness-killing associations. An example of this is adding ammonium sulfate (AMS) to solutions, which adds acidity (lowers pH) and binds hard water minerals to the sulfate ions, softening the water. The Sprayer Mix Calculator can be used along with water test results to determine how much AMS is needed to neutralize the cations from a particular water source. Pesticide labels spell out which additives or adjuvants need to be added to optimize herbicide efficacy on a case-by-case basis depending on the herbicide formulation.
Some folks have reasonably asked the question, “What if I use a ‘purer’ water source? Won’t I improve my herbicide application by starting with water that has low hardness or dissolved solids?” The short answer is yes if a formulation, like the weak-acid herbicide glyphosate, is being used. What concerns me is the assertion I keep hearing that by using “purified” water (water stripped of some components, typically organics and some dissolved inorganics) one could cut their herbicide rates and realize similar or improved results when compared to a “regular” herbicide application.
I understand the desire to lower input costs and appreciate arguments about lowering the overall use of pesticides in the environment, however, there are a few issues here that need to be addressed. First, herbicides are tested, and plant tolerances set, by evaluating products under “real world” conditions, which means using common sources of available water adjusted using appropriate adjuvants. Pesticide labels do not contain special rates for de-ionized, “purified”, or reverse-osmosis (RO) sourced water. Until and unless these labels are revised, a producer who cuts application rates-below labeled recommendations is effectively flying blind, conducting a field scale research experiment.
Second, agricultural research is limited in this sphere. Claims like the one at the head of this post are not founded in established science, at least not at this point. Sure, water filtered through a RO system and used to apply a glyphosate formulation should perform better than hard, dirty ditch water, but is it more effective or economical than a comparatively clean “regular” water source? How do we even measure plant wholesomeness? Dr. Misha Manuchehri, Texas Tech University, tested herbicide efficacy using RO water to control volunteer wheat and palmer amaranth for two years in west Texas. While adding AMS increased efficacy in 5 out of 6 trials, including RO water did not significantly increase efficacy over locally sourced water with one exception (at a low rate of glyphosate without AMS).
Ultimately, aqueous chemistry, the science of water, is complicated and there have long been people ready to make money off our societal confusion. The bottled water industry has made untold millions from our often-unfounded fear of tap water. Water purification as a term relates to the intended use of the water and not some inherent property. The benefits to some herbicide’s efficacy of using water with low dissolved organic and inorganic solids corrected to an optimal pH for the pesticide formulation is well established – see the glyphosate example. However, more research is needed, potentially followed by some label adjustments. Some Soil and Water Conservation Districts in our region are experimenting with RO systems on an on-farm trial basis – so there should be some, limited, information about the practically applied efficacy and costs/benefits of these systems available soon. In the meantime, please keep in mind that there is no “magic” water being kept hidden by nefarious interests. There is just water, a variable substance with measurable components that, if tested and understood, will behave predictably.