Regarded as one of the best potting media for bonsai, Japanese masters use akadama for their highest quality bonsai, both to promote root growth and provide a medium for optimal water and nutrient retention. Akadama is a large-grained, neutral pH (not acidic or basic) clay.  It is dug out of old cryptomeria forests of Japan and flash-fired to remove any organic matter (sterilized). 

Our current recommendation for planting Kusamono is to add 1/5 akadama to your local nursery's potting soil. 

A remarkable light, soft, volcanic, pumice-like, material from Japan. A very acidic material with a typically pH 5.5. The soft airy structure means roots actually penetrate the particles. This granular clay is most popular for ericaceous (acid-loving) plants such as azalea, gardenia, and camellia. It is also the best choice for growing terrestrial orchids, particularly the White Egret Orchid. This particular material comes from the Kanuma area of Japan which is basically the center for growing azalea bonsai.  It is low-fired and acidic, unlike Akadama.

Pumice is a porous, volcanic rock used as a component of bonsai soil. Pumice varies in color and grade. Most have some ability to absorb water and yet retain a pore space that promotes good root growth. It is one of the few US native product suitable for replacement for Akadama, due to the similar hydration and ion exchange properties, however it is to hard to allow for direct root penetration.