I'm building several new raised garden beds for a kitchen garden on my patio, and am looking for affordable sources for filler materials. I would ideally like to use something comparable to "Mel's Mix" from the square foot gardening technique - that's equal parts composted manure, peat moss and fine vermiculite - though I'm open to other suggestions. Because the beds will be on top of a concrete slab, I think the vermiculite will be important for water retention, and whatever I put in those beds is all my plants will have to grow on. I'm prepared to put money into this, since I figure after the initial investment I'll be able to maintain fertility in the soil by regularly adding home-made compost, but the 1-3 cubic foot bags I've found so far are prohibitively expensive.

I live in northern Virginia, and my problem is that the local garden supply stores don't seem to carry any of these products in bulk. I'm estimating I'll need close to 7 cubic yards of bed material. I'm looking for suggestions for inexpensive sources for any suitable bed filling materials. I'd be willing to go pick up aged manure from a farm if it isn't more than an hour or so away. I've also heard that some swimming pool companies carry vermiculite in bulk, and thought I'd try posting here before blindly calling my way through the phone book.

Thanks for any help & suggestions!

-Leah