I don't think Miracle Grow soil is designed with growing consumables in mind. If you want a soil that will last most of the grow, without any additives, then I'd suggest Happy Frog soil by Foxfarm, sold at your local hydroponic grow store. If they don't have it, Roots should be okay too. Towards the end of the grow you might have to add a little fert kicker just to feed them while they finish. This is actually a good thing because you don't want any fert to be in the plants in the last week or so of flowering. The fert in the soil should deplete, then maybe you add a little if they don't look so good, but don't give fert in the last week or so of finishing, just water.