I've seen a number of people say MC isn't a good soil to use for this type of plant. One reason is that its PH is low - acidic, because of the high amount of mulch used in it. Something around 5.7 ... I think, I forget. Neutral soils are better - closer to 7 PH is better.
Another reason is that a lot of the MC soils have slow release nutes that could create 'hot spots' in your soil and create all sorts of troubles during the grow.
I'm just a noob at this, but thats what I have read in the past ... if I understood it correctly.