All I could find on the subject was this. Technically though, dirt is soil. It just depends on whether or not it has nutrients. It's all just a medium
Most dirt from outside is well ... dirty it mostly has bugs in it and stuff. I have not tried it but have always read that its a bad thing. Good luck
I'd use the organic MG, there are a few grows around here doing well in it. I wouldn't use the non organic MG though my whitewidow took forever in it. I'd use any bought in soil rather than risk dirt from outside