That MG Organic still does have nutes in it, which is why your plants actually don't look as bad as they would if you were using a truly nuteless soil. It's entirely possible too that you are seeing symptoms of a problem that you had while the plant was still root bound. You should start looking for a nice 'grow' fertilizer for your next step, or if you are ready to flower (it looks it from here), go for a 'bloom' fertilizer.
Do you have a hydro or specialty gardening shop nearby?