I agree with Dean_W that contacting a landscaping company would be a good way to acquire this large an amount of soil (56 square feet, helpfully calculated by jusconjill .) You want topsoil and some compost, if possible. If you can go pick up the soil you can avoid delivery charges--but lacking a truck, you might find it easier just to have the company deliver.
That's 40 cubic feet to fill them to the top. There are 3 cubic ft bags of soil at most garden centers and if you ask for the damaged bags you can usually get them for half off. May not get all you want at once but saving a buck here and there starts to add up!
I got the new "Square Foot Gardening" book by Mel Bartholomew for Christmas and plan to try it. He gives a fairly inexpensive formula for box soil. Not sure if I would be allowed to include it here but it is only 3 ingredients, including compost as one. Oh well, the other 2 are peat moss and vermiculite. He makes it all sound easy. It is an entertaining and informational book so I would recommend it to you. I am just a regular person, not affiliated with Mel or the book! Will report back on my success as time goes on.
Hi Kapr - I'm starting my first box/raised garden this year too. How did you construct your boxes? Someone recommended using 2x12 cedar planks, but we are having a hard time finding them. Any thoughts?
Cedar can be hard to find and expensive, it depends on what part of the country you live in. You can use regular UNTREATED 2x4 or 4x4. You don't wanted treated lumber for a veggie bed. To extend the life of the lumber you can line your bed with porous landscape fabric or plastic, just have to put holes in it for drainage. Please see my article http://davesgarden.com/guides/articles/view/618/ this has some examples and a better explanation. :)
GreenerBeaner, raised beds we've made this year are 2 high 1" x 6" x 6' cedar or redwood fence planks. Deck screws attach the planks to 2" x 4" x 16" treated lumber outside the planks at the corners and every 3' along the sides. A 2 x 2 untreated is screwed to opposing 2 x 4s across the top. Seems pretty sturdy. Final touch is 6 mil plastic (water barrier) along the inside and under the edges. Some beds have an underlying cardboard barrier, some not.
Quite economical. Cedar planks are $2.39 each, redwood $2.89, finished cost under $1.50 per linear foot of side.
Compost, peat moss and vermiculite are the three fastest-breaking-down soil amendments I can think of, other than grass clippings.
I hope his new book advises adding something that lasts longer, as those degrade into powder and CO2 over 2-4 years! But I guess he has a "new system", and I don't know how the new one is supposed to work, long-term.
I'm a newly-converted fan of clean pine bark mulch (shreded and screened) , expecting it to last 4-7 years, and grit, crushed rock or even coarse sand (minerals are forever).
I'm also a long-time fan of "real soil", even if it was clayey or sandy to start with.