I've never actually used Miracle Gro, so don't know for sure - but it sounds like those round balls you've identified must be the slow release nutrients. If you can remove them then I'm sure it will help. I'm not sure if the vermiculite will have nutrients bound to it too - but I feel like it probably won't. One thing I do know for certain is that vermiculite does retain moisture, which is not especially desirable. Another volcanic growing medium is perlite, which doesn't retain moisture, but does improve aeration and help with drainage. If you can source some perlite, sift out the vermiculite and the nutrient balls from the Miracle gro and replace it with the perlite you will have a vastly improved growing medium. Might just be easier to do away with the Miracle Gro and source some soil designed for growing cannabis - all hydro shops will sell it.
This article might be of use in explaining how and why Miracel Gro is bad for growing cannabis.
by Nebula Haze Miracle Gro Plant Food is Not Suited to Growing Marijuana!
www.growweedeasy.com