Just looked at the roots of 6 vegetable containers that I used different organic soil mixes in.

It's quite interesting, I've been using branches, stumps, leaves and compost in different arrangements to see

what grows best.

http://lowcostvegetablegarden.blogspot.com/2012/09/eggplant-stump-branch-pot-comparison.html

http://lowcostvegetablegarden.blogspot.com/2012/09/pepper-containers.html

Here is a link that might be useful: Organic containers evaluation