Potash and magnesium are definitely good, not sure about sulfur though. I don't think any of the pot nutrient companies use sulfur in anything. Kind of high doses too, might want to take it easy if you do use it. I'd recommend just using a Cal-Mag type of supplement with Dyna Gro Pro Tekt (if you can find or order it). Pro Tekt has the potash as well as silicon.
Oh yeah by the way, that alone won't be enough if that is what you were thinking. You'll need something that has nitrogen and phosphorus.