
See Soil Mix.
Different tree types require different types of growing medium. Also different climates will also influence the soil mix with mixtures in wetter regions required to be freer draining than in drier areas.
Main components of any mix would be loam, sphagnum moss, peat, granite (or grit) and could also include any one of several growing mediums available as imports from Japan and the Far East (e.g. Akadama or Pumice).
Bonsai growers are notorious for having their own recipe for the "best" mix which is developed through trial and error and the bonsai grapevine. The truth is plants do not need "soil" to grow well, modern hydroponics grow plants in nutrient rich water. Subtrates range from peat through to crushed granite lumps and really depends on your regime of fertilising and watering your bonsai.
There is an excellent article on the Kaizen Bonsai web site which you will find liked from the Web Link above.
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