According to
http://www.bonsaikc.com/bonsai_soil.htm, Boon suggests the following mix:
1 part lava rock
1 part pumice
1 part akadama
1/2 cup horticultural charcoal (per 5 gallons mix)
1/2 cup decomposed granite (per 5 gallons mix)
My questions:
What is the purpose of the charcoal? I've never seen it recommended in other mixes, but is is cheap and easy to got, so I just wonder what it does. Is if for the cation exchange capacity?
What is the purpose of the decomposed granite? It seems too little for "structural" reasons, but I would think it would be pretty inert.
Would a calcinated clay product (Turface and such) be a reasonable substitute for the akadama?