Water often comprises 95 percent or more of the spray solution. It should be no surprise, therefore, that the chemistry of water added to the spray tank greatly impacts herbicide effectiveness.
Water is a simple molecule composed of two hydrogen (H) atoms attached to one oxygen (O) atom. Water is one of nature’s most remarkable substances. It is capable of dissolving or suspending minerals and organic matter, can freeze and return to its normal state once it thaws and is an essential part of all living organisms.
Before any foliar-applied herbicide can perform the desired biological function, it must be transferred from the leaf surface into the plant tissue. The above-ground portions of plants are covered by a continuous non-cellular, non-living membrane called the cuticle. The cuticle is the first barrier any herbicide must overcome to be effective.
Cuticles are extremely diverse and vary greatly between different species of plants. Surfactants added to the spray tank modify the spreading, wetting, retention and penetration of the spray solution. The type of surfactant added to the spray tank can enhance the performance of the herbicide and almost always reduces spray runoff from treated plant leaves.
When making herbicide applications with weak acid herbicides such as glyphosate, farmers need to be concerned about hard water. Hard water contains high levels of calcium (Ca), magnesium (Mg), sodium (Na) and iron (Fe). These positively charged ions attach to negatively charged herbicide molecules, often rendering the herbicide ineffective.
Herbicides with amine formulations, which include glyphosate, 2,4-D amine, MCPA amine and dicamba can be adversely affected by hard water. The herbicide formulation, combined with the elements of hard water, can lead to it being less absorbed by the weeds. Hard water can also plug spray nozzles and cause buildup in spray units.
Adding ammonium sulfate (AMS) to the spray tank overcomes the adverse effects of hard water. The ammonium cation preferentially attaches to the glyphosate or amine molecule and thus prevents Ca, Mg, Fe or Na from doing so. When ammonium is attached, the molecule binds readily to the EPSP synthase enzyme and the herbicide functions normally.
Common lambsquarters, a weed found almost everywhere, is often difficult to control with glyphosate herbicide. This plant species contains high levels of Ca on the leaves, often giving the plant a granular or mealy appearance.
Just like hard water in a spray tank, high Ca levels on plant surfaces can reduce herbicide effectiveness. AMS in the spray tank overcomes the negative influence of hard water and alleviates biologically induced herbicide failures observed in calcium-rich plant tissues.
Sometimes the question is asked, “What about filling the tank with tap water from the house if it’s hooked to a water softener?” Using soft water may be the solution to the problem; however, most people do not have a tap on the outside of the house through which soft water runs. It would be necessary to run a hose through the house to a soft-water source.
Although it would be effective, it would be a serious drain on the water-softening ability of the system since filling a spray tank would require hundreds of gallons of soft water.
Another drawback to this approach is the mess and hassle associated with running a hose through the kitchen, laundry room, or other clean area of the home. In reality, it presents too many obstacles to be practical. Likely the simplest, easiest, cheapest and certainly cleanest way to manage hard water when spraying glyphosate is to add AMS to the spray tank.
There are economic and agronomic benefits of using surfactants and AMS products when spraying herbicides. This article is a reminder to growers to read the label and follow recommendations regarding the addition of surfactants and AMS when spraying weeds.
Because of the hard water interaction with glyphosate, and thus the decrease in weed-killing activity, it is recommended that whenever a glyphosate product is applied, AMS should be added to the spray tank.
For additional reading and more detail regarding glyphosate performance, see "Understanding Gyphosate To Increase Performance." FG
Earl Creech is an extension agronomist, and Clark Israelsen is an extension educator – both with Utah State University extension.
