On the one hand, the sulfate ions in ammonium sulfate react with hard water cations, which would otherwise tie up the herbicide’s active ingredient. On the other hand, the ammonium ions react with the herbicide’s active ingredient, forming a very soluble compound that makes it easier for the herbicide to enter the leaf and be assimilated. So, ammonium sulfate can prove particularly helpful whenever hard water is used or stress conditions are prevalent.