Maybe a picture would help!
The picture shows a tree that had been in an Anderson flat. A year ago, it was "half bare rooted". Soil was picked out of exactly one half of the rootball. Gently washed with the hose, too. Then replaced into the Anderson Flat, and filled with Boon Mix.
A year later, this is what we see. Japanese Black Pine roots and michoryzzae in the white half, and potting soil (with very little root growth) in the dark half. The dark half has the type of soil we often see commercial nurserys use, pine bark, some sand, etc.
This year, the old soil was removed, that is the other side was "half bare-rooted", and planted in a bonsai pot.
This picture was taken at the Winter Intensive at Boon's in January. And that's my tree. It lives at Boon's.