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.
Akadama is entirely unique and can't be fairly compared to any other mineral, so let's deal with that first. Akadama is a naturally occurring clay-like (but not clay) mineral found only in one region in Japan. It is surface mined, dried, graded and packaged. No baking or firing is involved. When wetted, it does not form a slurry like clay but forms a gritty paste. Akadama is without any doubt the absolute best growing medium for Japanese maples and many other broadleaved species, and is arguably one of the best ingredients to incorporate in smaller proportions for moist other species.
The big advantage of Akadama is that roots can grow through the parcels as well as between them, which vastly increases the amount of space in the pot that is available to the roots. Once roots penetrate Akadama particles they become fine and highly ramified, forming a dense pad of functioning feeding tips. When you foist use Akadama the result is really quite astonishing compared to most other media. Although Akadama has a relatively low cec (around 18-20 meq/100g) the fact that the roots are so prolifically generated means that this is not a defect.
ome growers complain that Akadama readily breaks down into much smaller particles and therefor impedes drainage, but they are not watching closely enough. Certainly it does break down, but this only means that the pore spaces become smaller, not that they disappear altogether. Since Akadama is not a clay, but a kind of cohesive sandy structure, it still drains efficiently after several years in use just as sand would.