Soil Depth and Square Foot Gardening
Mel Bartholomew in his book "All New Square Foot Gardening'" responded to the question of whether or not you could grow vegetables in only six inches of soil. Mel replied, "I've been doing it for the last ten years in my display and home garden, and it really works."
Mel puts a lot of weight in providing the perfect soil to grow in and he has given us the recipe, known as "Mel's Mix." Mel's Mix is a blend of three components and are 1/3 each by volume of: blended compost (a mix of several different composts from different sources), peat moss, and vermiculite. This blend of components allows the plant to uptake the needed nutrients, provide a good soil structure to allow drainage, and root development, and finally it remains light and airy to allow roots to breath and for water retention and drainage.
In the picture below I have tested and feel that six inches is plenty of soil to grow in as this small container that measure 12 inches by 12 inches and 3 1/2 inches deep has a soil depth of 2 3/4 inches of Mel's Mix and is supporting a bell pepper, chives, simpson lettuce and a cabbage. The plants are healthy and are thriving. I will admit that I pay close attention to watering and plant health and have added a little bit (Tablespoon) of chicken manure twice this growing season so far, but you can't argue soil depth.
I will keep you posted as the summer continues.
"Keep Those FIngers Dirty!" Brian