With SFG, you only need to manufacture about 6″ of Mel's Mix for planting; if you want to plant stuff that grows underground, you need to raise the level of the ground to make a deeper area of Mix… who wants carrots that grow in only 6″ of soil?!?
The solution is a box, to make it possible to have 12″ of Mel's Mix! See?
We've planted carrots, onions and potatoes here… it'll be interesting to see how they turn out.
As an aside for our boxes next year, we want to make a single large box with dividers for each square; the overlap of the double width is throwing things off a little. Live and learn, and remember better for next year!