Yes, six inches is enough for just about all plants if that six inches is Mel’s Mix.  (Mel’s Mix is made up of 1/3 parts of coarse vermiculite, peat moss and compost.)  It’s so important to use a good compost made up of several different ingredients, none of which take up more than 10-20% of the volume.  This will produce a very rich material filled with nutrients and trace elements that will eliminate the need for fertilizer.

I know it is hard to believe that 6 inches is really deep enough, but it is!  With Mel's Mix, everything is there that the plants need and so they don't have to go deeper to seek water and nutrients.  My garden boxes have been placed right on top of my old garden soil and I'm always amazed when I pull the plants up in the fall that the roots have stayed in the 6 inches when they had the opportunity to go deeper.  As long as compost is used that is made up of several different ingredients, all the nutrients are right there in those 6 inches.  And, of course, when replanting a square, simply add a trowel full of compost to replenish those nutrients.  If you want to grow something that does take deeper soil like 12-inch carrots, you can also do that easily with the SFG high-rise method.  You can read all about that on our website under "Mel's Column" on "High-Rise Gardening".