Besser Block Retaining Wall How Deep Does A Vegetable Patch Need To Be?

How deep does a vegetable patch need to be? - besser block retaining wall

I think to be a garden bed for non-use of newspaper laid on the grass, and facilitate the use of large blocks of better (about 30 / 1 m high) that the edge of the garden and the placement of compost and soil in the middle.
My children want to grow corn and carrots, it will be 1 foot of soil is not deep enough? Otherwise, should the depth of the earth?