one cubic yard will cover:
40 square feet about 8 inches thick
80 square feet about 4 inches thick
160 square feet about 2 inches thick

so your 600 square foot bed would need:
about 15 yards for 8" thick
7 1/2 yards at 4"
4 yards would do a little over 2" thick

hope this helps

I don't think you want to till compost in. I think you want to mix it into the top layer of topsoil and then cover that with a mulch.

I'd recommend two yards worked into the top three inches of soil.

Lloyd

I agree with Lloyd. One inch of compost is good enough. Use a potato fork to dig it in to the depth of the tines.

To compute how many cubic yards of any product you would need you multiply your Width (in feet) times Length (in feet) times Depth (in inches) to get the cubic feet involved and then divide that be 27 (the number of cubic feet in one cubic yard) to find out how many yards to order.
So, 15 x 40 equals 600 square feet. If you want 1 inch of compost on that bed you multiply 600 times 1/12 (the Depth converted to feet) and find 50 cubic feet which is divided by 27 to get 1.85 cubic feet of compost needed.

((snicker))

Ya, you could do all that math, but laziness breeds efficiency...

Lloyd

Here is a link that might be useful: Calculating quantity of compost you need

>>and find 50 cubic feet which is divided by 27 to get 1.85 cubic feet of compost needed.

kimmsr, I think you meant 1.85 cu yards, not feet.

One can skip a step too, by making the initial calculation using depth in feet rather than inches, so you get cubic feet of compost as your first result, then divide by 27 to get cubic yards.

Or just use Lazy Lloyd's online calculator. :-]

Thanks, everyone, this is terrific information. The online calculator works great. I guess the only issue now is how many inches I should use. The ground looks pretty hard, so I'm inclined to err on the high side.

Organic material that is buried too deeply into soil does not get enough air to decompose properly. The result is that it releases its nutrients more slowly. The biologically active zone is rather thin, on the order of two inches, the top two inches of the topsoil.

Contact of compost to soil at the soil's surface creates a very highly active region where the food soil web is complete and functioning properly. You can add more compost later in the season, but in this situation more compost is not better.

To amend clay soil, I use a product that is sold locally and is made from humic shale ore. Do a google search on 'humic shale ore'. It's like the end product of compost.

Just to be contrarian, I find that digging/forking compost and other amendments quite deep into the soil helps greatly. And for hard or depleted soil, I think that 2 or even 3 inches of compost would be helpful. Fork down as far as you can, amend the top 15 inches, and you'll get great, healthy plants. In future years, you can tone down the application to, say, an inch a year, and use a broadfork to keep the soil well-aerated.

Here is a link that might be useful: Another broadfork design

There are other considerations. Amending soil is an ongoing thing and doing it just once in a big way probably won't get much greater yields this season than just adding an inch or two. If you can get a better deal (less $$ per yard) by buying more, then that might be a factor and working it in deeper will benefit long term.

I'm still going to go with a couple of yards now with the intention to add shredded leaves and grass clippings again after the season is over. This is predicated on the idea that you have 'seasons' down there, maybe you don't, I've never been that far south.

Lloyd

P.S. I'm not a gardener so take all this with a grain of salt.

At our house we had poor clay soil - yours may be different, - but over the years our process has been something like this:

Initially, we dug in compost fairly deeply (6-8 inches or more) into the soil. Recognizing that a lot of the 'action' is at the surface, we tend to not dig deep anymore.

So first the initial breakup, then plant, then use a lot of mulches. In the perennial beds and around trees and shrubs we used free shredded yard waste mulch. In the veg. garden, grass/leaves/sawdust or partly finished compost, or sometimes lasagne style stacks. Over the years this is tilled in by the worms. We don't do much deep digging now, even the tomatoes just get a small hole with a scoop of compost mixed in. The clay develops a porous structure from wormholes, freeze/thaw, etc. that is destroyed by digging.

Of course if you don't have clay you may not care about any of this. I just had too much coffee. :-]