Depth of soil in bed

[oneziphefty]

First off my father is very big into gardening and and got me into planting one of my own for the first time this year. I have very poor soil in my yard and was going to plant a raised bed with 1/2 yard of screened loam and 1/2 yard organic screened compost. The area we decided on was a 18'x6' raised bed with soaker hoses along the areas I will be planting. I planned on planting heirlooms, cherry tomatoes, zucchini, cucumbers, green beans, basil, and thyme. My question is how deep should the beds depth be for the roots to grow. I'm not sure if I should dig out the soil underneath the bed or make it a higher raised bed allowing more soil depth.

[wolfwalkerpa]

This is how I made one of my raised beds. On top of the soil that was to be a raised bed I layered cardboard to kill the weeds and grass then I built up the bed with 4x4 inch timbers to a high of 16 inches . A good size is 4 foot wide so the center can be reached from either side of the bed. Into the bed box I added soil and compost mixed as you said 50-50 .

[TZ -OH6]

Deep rooted plants such as tomatoes and corn will send roots down 4-5 feet, shallow roted vegetables are 1-2 ft. If you have pretty much any clay in your soil the combination of nasturalwater percolation and construction compaction frim building the house will creat a hardpan clay layer starting at about 8 inches down. that layer can be any where from a few inches to a food deep befor you hit sub soil that drains again. Any plants above the clay layer are pretty much stuck with whatever soil moisture those top few inches can hold. The deeper you can dig the base soil the better even if you are using raised beds. Even breaking through the hardpan layer in a few places will let root penetrate.

[GolfnGardener]

From all that I've read and all that I've experienced, soil is the key to good gardening. And, regardless of what you have, you can always make it better. If "poor soil" means lots of clay and/or rocks, then remove the bigger rocks and improve the tilth of the soil by adding compost and some sand. If the soil is already too sandy, add lots of compost or topsoil so that the soil will hold more moisture.
I'd recommend tilling the soil on which you're going to place your raised beds. Remove bigger rocks and as much of the grass roots as you can. A tilled depth of 12" should be sufficient; but, deeper would even be better. You can kill the grass by covering it with plastic for several weeks, if you have the luxury of waiting that long. Otherwise, strip off the sod or pull out as many roots as possible.
I've used a little Mantis tiller for more than 20 years in my two raised beds. It really digs well and helps mix in all of the compost and sand that I add. The Mantis tiller also has a number of clever attachments that come in handy for other lawn chores, like edging and dethatching the lawn. They have a cute little animated site that shows all of the attachments at powertogrowityourself.com.
You can also rent a small tiller at most rental shops; you'll definitely get better soil - especially in a new garden - if you add compost, screened top soil, or sand (if necessary) and till it all in. Another great soil additive is pulverized, or finely chopped, leaves. I save mine in big plastic bags each fall and add them to my compost pile and to my garden. Finely chopped leaves make an excellent vegetable garden mulch, and they add a lot of organic material to the soil as they break down.
Good luck with your new garden. And, don't be discouraged if you have some little disappointments; we all do.