Are these boxes going to be on top of something that isn't soil?

I have 12" boxes that sit directly on top of soil and I grow all kinds of root vegetables in mine: carrots, turnips, beets, rutebaga, parsnip, daikon...but then the plants have the ability to grow past the bottom of the box.

However, even with the tap root, I'd say that my turnips and parsnips probably didn't grow much deeper than the depth of my box and if they did it was just the very tip of the tap root. The daikons on the other hand...I had some well over a foot long.

Root vegetables like loose soil: sandy loam. If the soil is too hard they will get deformed as they try and grow down.

To answer your question, no they wouldn't be sitting directly on soil (soil on the ground). The boxes would have a bottom; the depth, as stated above, would be 24" deeper, if necessary.

Would having the square foot box having a bottom create issues with growing rutabagas and parsnips? Would it just be better for it to be bottomless, or does it not matter either way?

Regarding the loose soil, I make my own soil mixture:

I would use either this....

NOTE: This soil mixture is what I've used since I started container gardening, and it has worked beautifully. However, never grown root veggies in it; is it too 'compact' for growing them in?

2-3 cu ft pine bark fines
5 gallons peat
5 gallons perlite
5 gallons sand (just normal run of the mill sand)
2 cups dolomitic (garden) lime (or gypsum in some cases)
2 cups CRF (if preferred)
1/2 cup micro-nutrient powder (or other source of the minors - provided in some fertilizers)

The other recipe that I'm debating using, which I've never had experience using is Mel's Mix (the recipe that the author of the book square foot gardening, Mel Barthlomew)

It doesn't give specific measurements, it just says

1/3 part EACH vermiculite, compost, and peat moss

I have grown rutabags and they form mostly on the surface so 8-10" is enough depth. I am growing parsnips in the section where (due to a hill) the raised bed is 10". My parsnips have leaves that are about 18" tall. I used Mel's mix for the most part.
Keski

I have Mel's book but one thing puzzles me, how come he never mentions Turnips in any of his planting guides? Does anyone know the proper spacing required. Presumably 9 should be about right. Any suggestions

4-9 per square foot is about right. Depends on their size. I had some turnips that were the size of a softball last year so 4 per sq foot was probably better than 9 per sq foot.