These crystals were not present when AMS was not included in the spray solution. The authors speculated that the sulfate ion in AMS combines with calcium to form calcium sulfate crystals, therefore preventing the formation of the calcium-glyphosate complexes. In addition, the positively charged ammonium ions in AMS may compete with the calcium and magnesium for the charged sites on the glyphosate molecule, therefore reducing the amount of glyphosate tied up by the antagonistic salts. The Roundup Ultra label recommends 8.5 to 17 lbs per 100 gallons, whereas the Touchdown label states 4.25 to 17 lbs per 100 gallons. The specific amount required is determined by the hardness of the water used as the carrier.
Water hardness is an indicator of salt concentration.
Calcium in water can antagonize glyphosate at concentrations as low as 150 ppm. In order to use this equation it is necessary to have the water tested in order to determine the amount of salts present. The 17 lb rate of AMS is probably higher than needed for most water sources found in Iowa. NDSU states that 8.5 lbs of AMS is sufficient for most water sources, and this recommendation should hold up in Iowa also. However, the only way to know for sure is to have the water tested.