What is a good generic bonsai soil mix?
What ratio of organic / inorganic material should be included?
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As I said in my answer to your earlier question about fertilizer, I'm not an expert in bonsai, but I have had a Chinese Elm and a Weeping Fig for many years; both are very healthy and, for most of that time, they have been growing in the following soil mix:
Clearly, soil needs vary a good deal from one species to another (conifers, for instance, require considerably more grit, as much as 70% for some species), and also depend on whether the tree is deciduous or evergreen.
There is some useful information about bonsai care and soil mixes, by the Royal Horticultural Society, here.
Soil mixes for bonsai are specific to the species of tree being considered as well as the location in which it will be grown.
I actually make a different mixture for each of my trees (well over 100) tho almost all of them are 100% inorganic. Usually I use 60% Akadama 40% crushed red lava with varying amounts depending on the tree. Some conifers get a little a small addition of composted Fir bark as well.
Depending on your area and your access to specialized soil components like akadama you may want to try using a different option like "oil-dri" or some other form of Diatomatious earth. There are even people using unscented kitty litter as an option. Basicly any hard clay particle will do.
A commonly quoted mixture in the US is by Boon Manakitivipart.
1 part lava rock
1 part pumice (which is lighter and holds less water than lava)
1 part akadama (which will break down in about 2 years)
1/2 cup horticultural charcoal (per 5 gallon mix)
1/2 cup decomposed granite (per 5 gallon mix)