Hard water is classified by high concentrations of positively charged atoms (cations)1. Hard water can be problematic because these cations can bind to herbicides causing a decrease in efficacy of the product2. Most research looking at herbicide antagonism with hard water has found Ca2+, Mg2+, Mn2+, Na+ and Fe3+ to be the most problematic. The pH of the water solution can also exaggerate the impact of hard water on herbicide activity.
|Mineral Parts per million (ppm) in water||World Health Organization water classification|
Note: Some hard water test result will be in "trains", which is the ppm divided by 17.
Most post-emergence herbicides are weak acids, meaning they have a pKa value less than 71. If the pH of the water is greater than the pKa of the herbicide, the product has a greater chance to dissociate1, thereby reducing the efficacy of the product1,3. Addition of ammonium sulfate (AMS) can reduce the interaction between hard water and herbicides, which helps the product to penetrate the plant cell membrane1.
Soft water can be high in bicarbonates (HC0-) or carbonate (C02-), which can also interfere with some herbicides, similar to the hard water ions. When testing water for alkalinity, levels should be below 300 ppm1.
Turbid water describes a water source that has suspended particles which could include soil, organic matter, algae, salt or contamination from runoff. Herbicides have the potential to bind to these particles in the water source, ‘tying up’ the active ingredient and decreasing the efficacy of the product. An example of an active ingredient that binds tightly to organic content is Glyphosate, therefore it is very important to have clean water when applying a Glyphosate product.
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1Hall, Linda, Beckie, Hugh and Wolf, Thomas. 1999. How Herbicides Work: The biology of application. Alberta Agriculture and Rural Development. Information Management. 7000-113 Street, Edmonton, Alberta.
2Patton, Aaron J., Weisenberger, Daniel V. and Johnson, William G. 2016. Divalent Cations in Spray water Influence 2,4-D efficacy on Dandelion (Taraxacum officinale) and Broadleaf Plantain (Plantago major). Weed Technology 30:431-440.
3Zollinger, R.K., Nalewaja, J.D., Peterson, D.E. and Young, B.G. 2010. Effect of hard water and ammonium sulfate on weak acid herbicide activity. J Am Soc Test Mater Int 7(6):1-10.