Plain dirt with coarse mulch on top.

There is an extensive thread that discusses this in the Container Gardening forum here. The mix that is recommended is called Al's mix, named after a gentleman on that forum that has helped several people over a long period of time with their container gardening. I put the link below for you to at least read the first post in that thread that includes the recipe for the container soil.

~Angela

Here is a link that might be useful: Al's Mix / the Container Soil Thread

You'll find that plain 'dirt' is never an appropriate medium to use for container-grown plants. Even the best garden soil turns into something quite awful when put into the confines of a pot.

Your ideal medium will be extremely porous and loaded with conifer bark fines.