unoit loves to handle this question You should find your answers from other posts (I tried, but couldn't find the ones i was looking for) and make this sticky.
Let me answer this question for the umpteenth million time. MG added potting soil is developed for the enhanced growth of heavily hybridized summer bloomimng flowering plants. That is, all the brightly colored flowers for sale at the Wal Mart Nursery section starting around March 15. it is used in outdoor pots by plants that were developed to be heavy continuous bloomers that require a bunch of nutrients. It was not developed for indoor growth of MJ seedlings in stealth conditions. In most situations, but not all, if you use MG-brand potting soil the indoor heat, humidity, and water will accelerate the time release fertilizer added to the potting soil. The soil then drops in pH becoming more acidic and heavily fertilized. Thus you may wind up killing your seedling by indirect overfertilization. Now this does not mean this happens in every situation, but it has happens most of the time. Following the advice handed down from the great Australian grandmaster gardneer, K.I.S.S., which stands for keep it simple stupid. All you really need is the plain, cheap potting soil for sale at Wal Mart, not the expensive stuff. Once your seedling has matured and survived to the later veg stage, you can add MG or MG-type nutrients then.