I think you should reconsider the potting soil option. It will be very expensive to fill a bed that big with potting soil. You are probably better just getting a load of topsoil and buying some other material to amend it. I have seen several threads on beds. Try a search and you will probably find all your answers. Others will probably chime in here as well.
Bruce

Call around to your local nurseries and see who carries bulk potting mixes/soils/ composts in your area. A 4'x8' bed is close to a cubic yard, which I can get for $29 for a raised bed mix and $19.99 for a yard of compost at my local nursery. Buying by the bag will cost you a fortune.

Nurseries usually have a couple of different mixes to choose from and you can always further ammend with specific things if need be.

This post was edited by VedaBeeps on Tue, Jul 22, 14 at 19:45

Check out the link for raised bed ideas.

Here is a link that might be useful: Square Foot Gardening/Raised Bed

I know it's different everywhere, but the local dump here in Orlando gives away free compost. All you have to do is bring your truck or containers and shovel it yourself. Maybe check your area?

Jay

As a first time grower this year, I made a raised bed 2' x 20', and 2 - 2 x 8's tall. It took just about 2 yards of topsoil. I did not know to add other things to the mix until after it was too late. I have many plants in it, I believe 28 to be exact. I was unaware of how big and bushy these things would get. Anyways, here is a couple of pictures of mine in case you wanted to see.

Current picture taken today.

I agree with Bruce. Although you can use potting soil but it would be costly.

Lets do a math: 15 plants, each needing 3 sq-ft. That is 45 sq-ft of raised bed. I would make it about 1 ft deep. That requires 45 cu-ft of flller. Bagged potting soils are sold in average at $5 -7 per cu-ft. some cost more. That would cost you about $200 ++. You can buy 2 cubic yards of 50/50 topsoil/compost mix at $ 40/cu-yd , a total cost of $80.

In the past two season I have built about 120 sq-ft of raised beds. I have done exactly what i just said. I also amend with bagged manures and composts plus some of my own.

Stoney,
I have 7 raised beds and you want to go the bulk method not bagged. Like someone else said most dumps/landfill locations sell in bulk. I purchased 1.5 yards of compost OMRI approved as well for 27 bucks here in the east bay. My raised bed mix is compost,vermiculite and garden soil based. I also mix in a bunch of different organic ferts and azomite. I dont follow the mels mix to a t with the 3 base components but my plants grow fine. I add more vermiculite to keep it light and fluffy lol.

Square Foot Gardening was a thing I swore by 25 years ago ,and till this day I highly recommend it to anyone growing many crops in the garden. A 4/ 8 raised bed would be perfect for that number of plants with enough room left over to throw some marigolds around the edge too. Every time i built a raised bed though I dug a hole inside doubling the depth therefor using more fill.
Mel Rules Loved his show on PBS do they still show it?