Glyphosate is a weak acid herbicide and can bind with calcium in the spray carrier. Diammonium sulfate is commonly used as an adjuvant with glyphosate to enhance phytotoxicity and overcome antagonistic effect of these cations. Most postemergence herbicides are also weak acid herbicides. Data is limited for other weak acid herbicides and the effect of diammonium sulfate in enhancing herbicide activity and overcoming antagonism. Field studies were conducted with aminopyralid, tembotrione, dicamba plus diflufenzopyr, and glufosinate to determine if (1) these weak acid herbicides are enhanced by ammonium, (2) if they are antagonized by calcium and magnesium in the spray solution, (3) if diammonium sulfate overcomes salt antagonism, and (4) if a previously published equation for the amount of ammonium sulfate required to overcome salt antagonism of glyphosate based on cation concentration in spray water correctly predicts to other weak acid herbicides. The activity of the four weak acid herbicides increased with the addition of ammonium to the spray solution, all were antagonized by calcium and magnesium, and diammonium sulfate overcame the antagonism. The previously published equation to calculate the amount the diammonium sulfate needed to overcome 500 and 1000 ppm hardness was accurate and can be used for these herbicides and possibly other weak acid herbicides.