I have used a lot of Miracle-Gro potting soil with mixed results. It usually contains a large percentage of peat moss which has a tendency to retain water. Mixing sandy soil with it is exactly the correct thing to do to help with drainage. Earthworms are a great bonus. I move any I find into my raised beds. The nutrients in the Miracle-Gro should provide all the food necessary for the first year. You want the roots to grow outward and seek more nutrients.
