For tomatoes, I think you should ensure to have at least a foot of depth. Most vegetables will be fine with that much, although some longer carrots as well as vegetables that need blanching (celery, leek, etc.) and deeply planted vegetables (potatoes) may need a bit more.

If you removed tomatoes from your list above, you could already go for a much shallower depth of six inches (lettuce and beans would be fine there).

What else would you like to plant in that plot? Also, what surface would your plot be installed on? Would that be a raised bed garden? It would be useful if you could post this info as many of the folks on this forum could give you valuable tips on how to get it right the first time.

If you are installing raised beds, it would be a really good idea to lay the raised beds on landscaping fabric (go for the professional grade if you can afford it). This will help choke weeds underneath and your raised beds will be easier to separate from the underlying soil if you decide to move them (raised beds are often reconfigured for the second season after realizing they are not quite in the right spot). It is also an efficient barrier against some critters like slugs.

My raised beds are installed on top of a bad case of japanese knotweed. Let me tell you that the landscaping fabric is really the one thing that made it possible at all to grow anything in that spot, and it is actually helping big time to get rid of the knotweed.

The vegetable bed is at the top of a hill which has been leveled a bit, and will be edged on the downward side with 4x4s. So technically, it will be a raised bed, sort of, but not a movable one as you described. It is still a work in progress. DH prefers the baby carrots, not so much the longer ones. Potatoes are on the wanted list as well as tomatoes. Blanching is probably not, but you never know. Thanks for any advice. Still waiting for the mud to subside after a ton of rain. Must dry some more before tilling anyway.
Bluebars

Don't forget that over time your amendments will work their way deeper into the soil courtesy of worms and other soil critters.

My tiller's tines will go - max - 13" deep and I try to let them, at least one time. You absolutely need to get below the freeze line - the depth the soil freezes to in winter. Normally, that's about 6-9 inches, depending on the weather, ground cover (-20 degrees but a 12" snow blanket means the freeze level will not be as deep as -15 with no cover!).

Mike

4 people and 4 different answers, so I'll add a 5th since you are starting new.

You may learn something from an experiment I'm doing.

I made compost bins out of 10 feet of hog fence which makes them 3 feet tall and 3 feet in diameter. (One of them is just 2 feet tall). I filled them to the top with grassy/weed type of mostly green stuff that I cut with the lawnmower. (This takes a LOT of cutting.)

Then I let them settle a few days and filled them to the top again.

After that, I added 5 cubic feet of mediocre soil which is about 8 inches.

One would think the pile would get real hot, or anaerobic, but it seems to have done neither. The temperature fluctuates from about 75 to 85. There is no smell as the composting material has dried out.

All the plants are doing well. Interestingly, I seldom have to water.

Here are pictures. (It should cycle each 3 seconds.)

Here is a link that might be useful: pictures

BlueBars, here is yet another perspective: don't go much deeper than your top soil.

I know, "plow deep 'cause there's another acre below what you are tilling" - or something like that. But, subsoil is an unkind addition to your growing mix if it is there in very significant amounts. Over time, you can continue on down, inch by inch.

Your soil may be very different than mine. It is easy for me to see, anywhere I've gardened hereabouts, that 8" of topsoil is all I've got to start with. I'm down to 11" in my permanent beds, these days. But, I'm not getting there with the tiller. That's the length of the tines on the spading fork.

My large veggie garden is tilled each year by a guy with a tractor. There's no way he is ever going to get anywhere close to 11". So, it depends, at least partly, on the equipment.

Steve

Have to agree with digit here and say that subsoil is not the kind of soil you want to be working into the upper layers. If your soil is anything like ours, there is a rock hard layer of clay that resides about 8" down and extends about 10" down from there. I dug down into a bed last year and brought some up to the surface. I'm still raking out the rock hard clods of clay off the surface!

Leave the soil improvement to the experts: worms, organic matter and roots. I would prefer to use vegetative means to work organic matter into the subsoil for sure. That and the occaisional broadforking works wonders.

Good luck!
Michael

I think that going down a few inches into the subsoil, provided you break it up and mix it well with compost, gives plants more root room and does no harm. I went down a total of 18" in my first vegetable garden (by double digging  moving aside the top 10", the depth of my fork and spade, and going down a further 10") and got really good results. There's a classic study of vegetable roots that showed that tomato roots go down (and sideways) more than 3'. As long as the resulting mix is friable and fertile, I think forking and amending to 2' would be optimum.

My main gardening bed has about 10" of good topsoil and that is as deep as I prepare the bed. My tiller reaches down to near this level. All of the plants I've tried have grown well.

My subsoil is extremely rocky and is nearly all clay. It is hard to dig deeper than 10" because of this.

I would just let them till to the depth the tiller goes and you should not have any problems. If you want to dig deeper, I would not let the subsoil mix into the topsoil.

I let the plant roots break through the hard subsoil.

The more recent studies of vegetable roots show much the same thing as that 1927 information on soilandhealth.org. Tomatoes have a maximum root depth not limited by soil depth (my emphasis) of 6 feet, according to Washington State University.

In my soil, the caliche layer is not found until you are down 24" or so, however, getting anywhere close to that "calcrete" and bringing that material into what will be the root zone of most garden plants isn't a good thing to do.

Organic material is almost non-existant beyond just a few inches from the surface and raising the percentage to 10%, say, at any greater depths is almost unthinkable over the first few years of gardening here.

Steve

I'm also a devotee of the concepts behind the double-digging method. The important aspects, IMHO, are tilling as deep as you can when you originally make new beds. Then you establish regular beds and paths, and don't walk on the beds if you possibly can. Amend heavily in the fall, and then as Michael said, leave soil improvement to the experts. You can work the amendments into the soil with a rake or a garden fork a couple inches if you want to, but you don't have to.

Lots of folks till every year, and that works fine. But I'm worried my "micro herd" of worms, soil bacteria and fungi, etc. will be too disrupted. Besides, I don't know anyone with a tiller, so all my bed prep is done by hand. (Thankfully, I have the help of an agreeable hubby and sometimes of a strong teenaged daughter.)

This year, I was able to plant some things in my oldest beds with my hands only if I wanted to. Not tomatoes that I plant deep, but peppers, lettuce, etc. As of this year, the tilth in those beds is amazing. I've been working those beds for about 5-6 years.

Good luck with your new garden! I'm curious if it's windy on the top of the hill?

So much info, and thanks for it! Hopefully the garden will be done today. DH had a bunch of guys over and they dug out ALL the clay (claycrete LOL) and even put in a little French drain! In the gardens, we will have good new topsoil and organic additives for vegetable beds. They will cover it with straw mulch and be ready for planting. Hopefully, there is still time for peas, lettuce, spinach and maybe a cover crop for the rest until spring.
Our back hill was so steep it was dangerous to mow. We cut into the hill with a wall about 4' tall and spread the extra dirt to make the rest more level (and safer). So now we have 2 new gardens 4-5 feet wide at the top and bottom of the wall. WOO HOO! I'm psyched to get planting.
BlueBars