"Dirt" is no good!<---soil is what you want.
You say, "Ive heard that Miracle Grow is totally bad for your plants but I bought a bag of Potting mix..." "...but you bought a bag of potting mix?"<----is it MG? IMHO MG is the worst of the worsed(except 'fill-dirt?'), and I know there are people who do like it, do have success with it as well.... Most of MG nutes are chemicals, for one bad thing, the other is that it's "time released.." You dont want anything 'time released' because how do you know when it's releasing what?? One day your plant looks to be lacking something, are you safe to add to it?<----or is the 'timed release' shit going to kick in?
just my opinion. I never skimp on soil or nutes.....you very well may get out of it what you put into it......
good Luck!
growinman