filler for raised beds

---

 Hi all! I am an AmeriCorps Volunteer helping city residents in an impoverished neighborhood to start a raised bed community garden. We have three 4ftx6ftx18in. beds, one 4ftx6ftx24in., and one 6ftx6ftx18in bed built. We are getting soil delivered next week to fill the beds. My question is, do we need to fill the bottom 6-12 inches with soil or can some other material be used as filler. such as a lower quality dirt, stones, rocks, etc. I have been researching raised beds on the web and have talked with several horticulturists.  One horticulturist actually told me that those styrofoam peanuts used in packing work well?!!??!!

We plan to grow collard greens, tomatoes, cucumbers, turnips, mustard greens, onions, and carrots.As well as some perennials, and annuals.

thanks