I would use topsoil still instead of the Miracle Grow stuff. There will still be plenty of organics available this way at first. Most people don't know this, but that Miracle Grow potting mix really should be mixed 50/50 with dirt when using it for it's intended pupose. Otherwise you end up with plenty of carbon and not a lot of minerals. Make sure to screen it with something like chicken wire to get the big chunks of wood and rocks. If anything they become unsightly if you move things around a lot in your tank.
Make sure to start with lots of plants right off the bat if you choose not to mineralize the soil. Also, plan your layout so you aren't moving things around much for the first few months.
Once they plants develop healthy roots systems they will mineralize the soil in the tank. So in the long run either way you end up with mineralized soil in your tank. I like to prepare it this way as I tend to run things high-tech and it poses less initial problems.
In the D.C., Maryland, or NOVA area? Come check out The Greater Washington Aquatic Plants Association! www.gwapa.org