There are many soil mixes available at garden centers and tree nurseries …
DON'T use them .

These soils have small particles and retain too much water to use for bonsai trees . Useable bonsai soil must have :
            1. Grit 2-7 millimeters (mm) in diameter . This is usually decomposed granite or high                                                         
                     fired clay called hadite (hay-dite) . Crushed  marble or quartz 2-7 mm works too .
            2. Coarse organics 2-7 mm . Ground up pine bark or orchid bark works well .
            3. Peat moss or other fine organic material .
I mix these components  with a ratio of   5 parts grit , 3 parts coarse organics , 2 parts peat  ,  plus I add 1/2 part pearlite .

Most bonsai retailers have their own soil mix . They are invariably a variation of this mixture . Remember , the goal is to water your trees once each day . You may have to adjust the mixture if you don't live in NE Ohio .

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            The size of the particles in the soil is of paramount importance . Particle size is directly proportional to the size of the pot and the tree . Smaller trees in smaller pots need smaller particles . Many people enjoy the extra challenge of growing Mame bonsai (less than 6 inches tall) . Soil particles in these small pots should be 1-2 mm . Trees growing in very shallow pots 1-2 inches deep will require smaller particles for greater water retention . Larger trees 12 to 35 inches tall grow well in the above mentioned soil mix .
             Some species of trees require different soil mixes . Azaleas need a soil with a lower pH value and require more moisture . I use more peat and ground bark in their soil . I also use Miracid fertilizer on Azaleas . Junipers grow best in a higher pH soil . I add more grit to the soil . I also use crushed charcoal in the bottom of the pot to neutralize the acidic affects of fertilizer . Larches grow best if some medium grit sandblasting sand is added . Playground sand is too fine . I don't understand why larches like a sandy soil , but the results are clearly noticeable . Succulents like Crassula (Jade Plant) will grow in sand + peat or regular bonsai soil .

               Please note that in Japan clay known as Akadama is used  . It is mined in Japan and has particles 2-7 mm and is mixed with coarse (2-7 mm) sand . I have been using  this clay with various species and have had very favorable results .

Akadama retailers :Stone Lantern Publishing  , Satsuki Bonsai  (email  satsukibonsai@juno.com)