Water is a universal solvent that serves as the primary carrier for pesticide applications. The quality of the water used as a carrier can have a large influence on the performance of herbicides such as glyphosate. Dissolved cations such as calcium, magnesium, zinc, iron, and manganese form complexes with glyphosate that reduce its efficacy.

Ammonium sulfate (AMS) conditions water by reacting with the dissolved cations to form insoluble sulfates that will not react with glyphosate. Spray grade AMS should be added to the spray tank and thoroughly mixed before adding glyphosate.

Here is a handy calculator that uses data from a standard water quality test to determine the amount of AMS to add to your spray tank, in pounds of AMS per 100 gallons of water. The calculator uses an equation developed at North Dakota State University (Nalewaja and Matysiak, 1993) to determine the required amount of AMS needed to neutralize the effects of cations in the water on glyphosate activity.

Adding more AMS than called for to neutralize the effects of cations may improve glyphosate activity by providing extra N that helps weak acid herbicides like glyphosate pass through cell membranes. The addition of 8.5 to 17 pounds of AMS per 100 gallons of water is generally recommended to improve glyphosate activity. Liquid forms of AMS are equally effective if used at equivalent rates.

Give the calculator a try and see what you think. Questions may be directed to Wayne Thompson wayne.thompson@wsu.edu(509-524-2691) or to Drew Lyon drew.lyon@wsu.edu (509-335-2961).