I haven't had any experience using MG, but I've also heard that it is not the best to use for MJ. Does it have any N-P-K ratios on the bag? Any word on added ferts? If it does, I would suggest putting it in different soil. No ferts are to be used until 3-4 weeks into vegg.

I buy my soil at wal-mart. It's like $10 for a big bag, has added vermiculite, and is a basic potting soil. I've had good results using it.

If there is no word on the bag about added ferts, or added food, then don't worry about it, just leave them.