im using miracle gro soil with time release capsules, as well as their food, and its been working great. first grow, and i bet organic would yield more etc. etc., but to say you cant grow with MG is false
anyway, if your soil already has nutes, dont feed it for 2.5 weeks. then start w/ 1/4 the recommended dose, and then keep upping the concentration each time you fertilize until you get a slight nute burn. obviously, this isnt recommended, but it will give you a good idea of what your plant can handle. every new grower needs to make this mistake, so why not minimize it and learn from it?