Natural soil occurs in "horizons", but this layering occurs because of
a combination of weathering, microbial activity, and most especially,
the action of water on a large horizontal surface. Phenomena such as
"perching" have meaning when you are talking about a surface area of
acres; it has NO meaning in a bonsai pot with a surface area of square
inches interrupted in each direction by the SIDE OF A POT. There is no
need to worry about soil structure in a bonsai pot as long as the soil
is well-draining and the pot has good drainage holes.

Nina.