apple tree bonsai

About a year ago my children and I planted apple seeds from the apple we ate. No idea what kind. One has grown great, the other is stunted and the roots are showing on the surface. I think it will be a great bonsai. I need to know what soil mix would be best. Thanks to any and all.

Well I had a few questions and thought it best to consolidate into one thread. The first question is it too early to put my juniper bonsai in for wintering? Also, I've decided the egress window would be a great place for wintering, do you think it should be fine in this egress window? My...