When planting trees or roses it is important to mix the native soil in with the potting soil so that the roots don't hit the native soil and can't penetrate the clay or whatever. Also it isn't a shock to the plant to hit the native dirt. You see a lot of arborvitae die because of this. When i grow cannabis outdoors I usually mix the soil and native together at least 24 inches down.
I called miracle grow they told me its to save the nutes in their soil, that are normaly washed away when watering. the nutes soak into the native soil and are thus saved for the plant .
I would not use thier soil for MJ. I have used thier line of ferts on an outside grow once and it worked okay,,,but using the soil which already has nutes mixed in the soil means everytime you water you are feeding the plant and that will be an ovet kill of nutes. Even then the only reason i used thier ferts is because its all i had at the time.
kurtis told me this about mirical grow and kurtis would know because he is the best grower in lake county. I know this is so because kurtis told me so. kurtis said there is a federal standard minimum as to what the soil must contain. most companys like mirical grow comply by adding the minimum. other companies that specialize go beyond the minimum standard. he says mg is low standard soil