How much soil mix for lettuce

I've got some lettuce started to grow in a season-extending greenhouse. How deep does the soil need to be to get good root growth and hence good sized heads? I'm planning on using some old window boxes, with drainage holes drilled in them.
The good news; I dug into the old part of the back-to-back compost bins, and there's nothing there but about a foot deep layer of beautiful black humus.
Gary Woods AKA K2AHC- PGP key on request, or at home.earthlink.net/~garygarlic Zone 5/6 in upstate New York, 1420' elevation. NY WO G
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Hi there Gary i think what you are saying is that you are going to try growing the lettuce into the window boxs??? which is no problem at all and you should get good lettuce although you might not get amazing size not because the roots wont develop i'm thinking more about the cramping conditions above the soil I don't know how big the boxes are but i i would consider a cut and come again lettuce which will do better in a cramped enviroment and you can just keep using the leaves more often when you need them instead of waiting for ages until the lettuce have developed into big heads. I hope this helps :-) Need more information try and visit www.gardeningcareblog.com
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My leaf lettuce does great in flower boxes 8" deep.
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