it will make the BUDS bigger, but they wont necessarily be as potent without the X/X/potash, and they wont get the light needed to support the buds without the nitrogen/x/x.
if you can find a high nitro/potash mixer that would be perfect, but without it may be a strugle........... alls i do to jack the potassium up is add molasses to 15/20/15 to make 15/35~/15......
Edit: i just remembered something, thats probably 58% potassium.... i wouldn't use it, it might burn the shit out of your rooting systems.