Test your sprayer water

Water quality factors will reduce herbicide performance. Hard water ions, for example, tie up glyphosate molecules and make them ineffective. Ammonium sulphate added to the sprayer tank will counteract hard water cations and improve glyphosate performance. 

Hard water 

Calcium and magnesium are the most common cations in hard water. Sodium, potassium and iron are others. Cations have a positive charge and they bond with negatively-charged herbicide molecules, making the herbicide molecules useless for weed control. The more cations in the water, the more herbicide molecules lost. 

Glyphosate is particularly sensitive to hard water. Hard water cations can also affect Group 10 glufosinate and certain Group 1, Group 2, Group 4 and Group 27 herbicides. 

Calcium, for example, has a plus-two charge and glyphosate has a negative-one charge. Thus, every calcium cation in hard water can bond with two glyphosate molecules. This creates a useless new molecule called calcium glyphosate.  

Water with hardness over 300 ppm (parts per million) is the general threshold for action. 

Hard water treatment options 

1. Test water from a few local sources to find one with lower cations. 
2. Use the higher label rates of herbicide. Glyphosate at the low rate of 180 grams of active ingredient per acre could see a big drop in efficacy if sprayer water has high cation content. The high rate of 360 grams per acre can provide decent control, even if cations tie up half the glyphosate molecules. 
3. Ammonium sulphate added to water before the herbicide (this mixing order is critical) will bind with the hard water cations. It dissolves in water to form ammonium and sulphate. The negatively-charged sulphate ions bond with the cations so they can’t interact with the herbicide molecules. The positively-charged ammonium ions bond with glyphosate to create ammonium-glyphosate, an effective herbicide. Agriculture labs that test sprayer water will recommend how much ammonium sulphate to add to counteract cation content