After running into an issue with expensive vermiculite ($25 per 3.5 CF), I decided to experiment with my own soil mixes to bypass buying vermiculite and peat moss (for Mel's mix.) I'm starting with 4 - 4x4's and a different mix in each one:

1) Sta-Green Garden Soil for Flowers and Vegetables (this looks very similar to Miracle-Gro's product except I see evidence of Vermiculite in it.) (6 CF of it, $29)

2) Miracle-Gro Garden Soil for Flowers and Vegetables (6 CF of it, $33)

3) Miracle-Gro Garden Soil for Flowers and Vegetables + Kellogg Garden Compost Soil "for bigger vegetables and better flowers" 50/50 of each. (6 CF total, $17)

4) Sta-Green Garden soil (from 1) and Miracle-Gro (from 2) 50/50 mixture. (6 CF total, $31)

I guess I will go through the season and see how it goes. Does anyone have any advice for me? If you know one or more of the above mixes will fail, I'd appreciate if you will let me know so I change it now. I may go out and buy that bag of vermiculite if I get enough advice that I should do that and split it between the 4 squares.