The solubility of Fe(III), Al(III), Mg(II), Ca(II), and S in plant phosphoric acid was determined as a function of mole ratio of ammonia to phosphate (MR) and at temperatures of 25, 60, and 90 degrees C. For each metal ion, there is a unique MR characterizing the onset of a sharp decline in solubility and another characterizing the transition from sharp decline to a more gradual change. At any given temperature, these characteristic MR values generally increase with the following order: Ca(II) = Mg(II) > Al(III) > Fe(III). The temperature effects on the solubility behavior of these metal ions and S and the most likely cause(s) of such behavior were presented and discussed.