Ammonium sulfate is negatively charged and can bind to hard water ions if added to the spray tank before the glyphosate. This allows the glyphosate to work as intended and may even increase efficacy of weed control as the herbicide may be absorbed more readily by weeds.
Adding ammonium sulfate to soft water is not helpful. Test your water to determine the level of hardness. If your water is above 120 ppm it is at a level that could benefit from including ammonium sulfate in glyphosate mixes. In general, add 8.5 pounds of ammonium sulfate per 100 gallons of water (1.4 ounces per gallon; 4 TBSP per gallon). (Cynthia Domenghini)