By Jim Isleib, Michigan State University Extension
Should I be adding ammonium sulfate into my glyphosate spray mix? The answer is yes, particularly if the water you are using is '"hard." It is surprising how many producers don't bother to add the ammonium sulfate.
Hard water in glyphosate spray mix result in "hard water antagonism," which ties up the glyphosate and does not allow it to be absorbed as easily into plants. The magnesium and calcium contained in hard water combines with glyphosate to form glyphosate-magnesium and glyphosate-calcium compounds which result in poor uptake and poor weed control.
Adding surfactants to glyphosate products improves the spreading of the spray over the plant and results in greater absorption, but does not address the hard water problem. So, a product like RoundUp Ultra that contains surfactant will be enhanced by addition of ammonium sulfate if mixed with hard water.
MSU recommends adding 17 pounds of ammonium sulfate to the spray water before the glyphosate is added. Use only a "spray grade" ammonium sulfate product. This does two things. First, the sulfate ions tie up calcium and magnesium in the mix water. Second, some of the glyphosate binds with ammonium to form a compound that improves the effectiveness of the glyphosate on some species of weeds, especially velvetleaf. Other liquid water conditioners are currently being sold to replace ammonium sulfate.
All water conditioners are not equal in their effectiveness, and AMS is the standard for performance. Proprietary water conditioning products typically contain a number of ingredients, including AMS, surfactants, and anti-foaming agents. They usually cost more than straight, spray grade AMS, but can provide handling convenience. Not all water conditioners are as effective as AMS at protecting glyphosate performance when hard water is used as the carrier.
Another way to increase the effectiveness of glyphosate when mixed in hard water is to reduce the amount of water used. Less water equals fewer calcium and magnesium ions to tie up the glyphosate.
Including ammonium sulfate may add a little work and expense, but getting the best performance from your glyphosate application is worth it.