Magnesium is part of the chlorophyll in all green plants and essential for photosynthesis. It also helps activate many plant enzymes needed for growth.
Soil minerals, organic material, fertilizers, and dolomitic limestone are sources of magnesium for plants.
PPS-Pro includes it as one of the Macro ingredients in it's dosing program. I have used it in my Macro solution for about 6 months now. BTW, Magnesium sulfate is Epsom Salts, about $1 a pound at your local pharmacy.