I'd say its your soil. I'm using the same stuff actually, I was in a hurry and rushed out to get soil before checking into too much. It looks like most people have issues with MG, but I'm not sure what the actual nutrient issue is. It looks like a nitrogen deficiency, but this is my first grow so I couldn't really say. I just figured out how to post pictures, so hopefully that will be some help. I have mine under 4 23-watt cfl's, the humidity has been (guessing) around 60-75, and the temp between 70-90. And there are some fungus gnats
A little info about the plants:
The big one was planted in some peat starter soil a little over a month and a half ago. I transplanted it into straight MG Organic choice about a 3 weeks ago. It might look weird cause I'm LSTing it The interesting leaf burn is from about 2 weeks ago when I had to stash the plants in the dark for 2 days. It didn't seem to effect the small ones too much though. The new growth looks Great, but the lower leaves are yellowing and dying.
The little ones were planted at the same time (3 weeks ago) that I transplanted the big one. They are growing slower than the big one, and the stalks are thin (though sturdy). They look pale green, and some leaves have wavy edges.
Is this at all what yours look like? I'd think it would be similar at least, hopefully someone can help both of us!