There are a lot of things that Bonsai artists do without really knowing why. One example is the use of bonsai soil mix over a standard nursery soil mix. The reasons are many but for the most part it is to help stimulate more feeder roots over the long leggy roots that nursery soil promotes. In the following video I show examples of roots as they have grown in each of the medium and tell why bonsai mix works so much better.
If you need a good bonsai mixture, I sell some on Orlando Bonsai.