Personally, I like beds that I can wander through without compressing the soil. Thus I like narrow beds (3-4 feet wide). It makes weeding, mulching, planting and harvesting much easier and more fun.

Maybe a much larger bed in a much larger yard could have foot oaths wandering through it, or at least some kind of stepping stones so that you can get at everything without compressing the soil.

Also, personally, no matter how good the soil that you buy, it can always be improved by being mixed with up to 50% good compost (or even bad compost). Two or three more yards of compost, mixed in as you spread the soil, will get you off to an even better start. But the compost will be digested within a year or two, and then the soil will subside and give you more room for deeper mulch.

= = = = = =

7 cubic yards x 27 cubic feet per cubic yard = 189 cubic feet

If you use the purchased soil 12" deep over the whole bed, you could have a bed that was 189 square feet (47' x 4' or 10' x 19') ... rounding off a little.

Or, say you kill or till the grass and then till some of the new soil into the top 4-6", to create a gradual transition from new soil to old soil. Lay down cardboard or 6 layers of newspaper to discourage grass from re-sprouting.

Then you can expect the plants' roots to take advantage of whatever new soil you give them, then push through the cardboard and keep growing deeper. In that case, you could spread your new topsoil thinner and still have a deeper root zone (after a year or two). Continued addition of compost and mulch will continue to improve the subsoil.

7 cubic yards, spread 6" deep, would give you 378 square feet (almost 100' x 4' or 19' x 20').

Let's see ... with 7 cubic yards and soil "X" inches deep, the size of the bed you can make, in square feet, is:

189 X 12 / X.

(For 18" depth, divide 12 by 18 and get 0.67.
Multiply that by 189 square feet to get 126 square feet.)

= = = =
For 10 cubic yards of soil plus compost:

10 x 27 = 270 cubic feet of new soil
With 10 cubic yards and soil "X" inches deep,
the size of the bed you can make, in square feet, is:

270 * 12 / X

I do plan to have a path or two going through the bed

The nice thing about good drainage is that every inch deeper you drive the water level is an extra inch in your root zone.

Since you'll probably keep the bed mulched and organic, and/or fertilized, it hardly matters how bad your deep subsoil is at first. Organics and nutrients will leach down into that subsoil, attract soil life, and eventually create good soil.

As long as water drains fast enough to not drown roots, and enough water remains reachable between rains to keep the shallower soil moist.

REALLY good suggestion about mixing in the compost, Rick!

Jennifer, don't forget that when you get the soil, it will be all fluffed up from being loaded into the delivery truck, and then unloaded at your house again. So if you want your bed to be a foot deep, better pile the soil 18in. or so to allow for settling. You'd be disappointed in the fall when you're ready to plant and find your soil is only 6in. deep.

We used to buy compost at the landfill, and they called a 'yard' of compost one scoop from their front end loader. That amount filled the bed of my son's small Toyota pickup pretty full. That's my 'visual' of a yard of soil if that helps at all.

I think killing the grass with Roundup, then tilling it in and putting the new soil on top is going to work fine for your bed.

You may also consider using some Preen on the newly tilled bed if you do not layer it with cardboard. This will keep the weeds down until your plants have a chance to fill in and keep them smothered. Otherwise the weeds will take over before your plants have time to establish.

I roughly outlined how I want the bed. My husband eyballed the measurements. He is confident that seven yards will do the trick. My gut is still telling me that is not quite enough. But we'll go with that. I can always get more if needed.

I hope August is not going to be steamy hot. I have A LOT of rocks to move!

I'm late to the party, but I have added 7 raised beds to my yard since moving here in 2007, and I stopped de-turfing it a LONG time ago. Layered cardboard kills the grass/weeds (wet the cardboard/newspaper as you lay it out), and acts as a weed barrier. I built a raised bed a couple years ago with almost no dirt. It's 6 landscape timbers high, and I filled it with brush and yard waste (this would take too long for you, probably), then covered them all with cardboard/newspapers and about 3-6 inches of soil. The yard waste will decompose into fantastic soil, and even though there's minimal soil in the bed, my dayliliies are beautiful, and have long healthy roots. I use Preen as a pre-emergent, and that bed has very few weeds in it.

I collect all my office paper/newspapers/boxes, etc, and store them until I need a weed barrier or a new raised bed. Then I put them to use.

jvdubb said:I plan to make a new slightly raised bed in my large area of lawn. I am not exactly sure how deep it will be or how big. I am ordering 7 yards of planting mix from the nursery and I will use rocks to for the border. I may add bags of topsoil and compost if I think the seven yards is not enough. Originally when I had a landscaping company give me a quote to make it they insisted the lawn had to be removed there.

Can anyone tell me why? Wouldn't the dirt on top of it just kill it? Could I at least cut a corner and just use something like Roundup and kill the grass?

The key issue here, I think, is the plan for a slightly raised bed; that's most of where the problem of grass re-emergence starts. If you begin with, say, 10" of lift and it quickly settles to 5 or 6", then yes, grass/weeds can find their way to the surface. Raised beds in lawn areas that are quickly covered to a significantly deeper depth (and stay that way) can kill the grass beneath. The raised beds I've been making here were designed to be deep enough to match the tops of large tree stumps (about 20" high maybe), and well into year two these beds are still grass-free.
If I have cardboard available I'll still use it under any sloping edges, but otherwise, I haven't found it to be absolutely necessary.

Hopefully your ordered mix has a lot of compost included in it, since you probably can't make a pile of yard debris for the bottom layer. It'll be a viable bed so much quicker that way.

It's exciting to build a new bed, isn't it? Can't wait to see it, Jennifer!