The measure of water hardness is the level of calcium, magnesium, or bicarbonate ions that can be found in the water. Just a minute of chemistry: Calcium (Ca⁺⁺) and Magnesium (Mg⁺⁺) are cations, meaning that they have a positive charge. These cations can cause antagonism with many herbicides including glyphosate, non-ester forms of growth regulators (2,4-D and other phenoxy materials), ACCase inhibitors (fenoxaprop, sethoxydim), ALS inhibitors (SU herbicides), HPPD inhibitors (mesotrione), and glufosinate (Finale, Cheetah Pro).
Cations can also complex with glyphosate forming salts that are not readily absorbed by leaves. Most post-emergent herbicides are leaf absorbed so you can see how this can become a very big problem. Drought conditions will make control more difficult particularly on those tough to control weeds. Calcium levels above 150 ppm and sodium bicarbonate levels above 300 ppm will impede control in all situations.
Adding this aspect into our previous pH conversation can yield poor performance in herbicides. One of the reasons is that many herbicides are weak acids (glyphosate, 2,4-D, dicamba and many others), and in the presence of dissolved cations like calcium, magnesium, iron, sodium, potassium, and others become negatively charged at mix water pH over 7.
Plant cuticles and cell membranes can create a barrier to the uptake of negatively charged materials leading to little or no movement of the active ingredient into the plant. Just remember to consult the label. Sulfonylureas (metsulfuron, rimsulfuron etc.) and triazines (atrazine, simazine) tend to be more affected by pH lower than 7.
Water sources with suspended organic materials will add another layer to herbicide failure potential. Molecules like glyphosate and diquat have a great affinity for organic matter and are easily adsorbed by clay particles rendering them less than effective. What’s more is even water temperature can put you at a disadvantage. Some materials are less soluble in cold water.
All of these factors should lead one to consider analysis of their tank mix water sources. It is also a topic of conversation when there are complaints of products not performing the way that they should.
The most cost-efficient way to remedy the hard water situation is the addition of ammonium sulfate. The sulfate portion will bind with the cations conditioning the water. Again, one should not take action without the proper testing first.