Bonsai Soil Demystified

Bonsai Soil Demystified

The term "Bonsai Soil" first of all is extremely misleading.
There is no "gardening" soil involved at all for one thing. In fact a lot of experience bonsai practitioners use different terms as alternative to the medium in which they pot their bonsai trees.

Alternate Terms for bonsai soil:

• Bonsai mix
• Bonsai soil mix
• Bonsai substrate

Basic attributes of a bonsai "soil" mix:

• Water retainage, to keep the roots moist. 
• Good drainage, to remove excess water and allow air [oxygen ] to reach the roots.
• Compact, but with enough space to promote shorter roots.

Basic components of a bonsai "soil" mix:

• Inorganic medium : Usually contain little to no organic matter, volcanic rock, fired clay, pebbles, gravel, perlite, cat litter, akadama.
• Organic medium  : Dead plant matter such as coco peat, leaf-litter, bark, charcoal

The organic medium will slowly decompose over time and release nutrients into the pot. Its not always sufficient for the plant, so organic fertilizer can be used at intervals. 

The inorganic medium should ideally not decompose at all, but depending on the medium, some will decompose over 1~2 years while other may take 50 years or more.

Akadama for instance is a clay like granular medium naturally occuring in Japan, this inorganic medium is widely suggested to beginners on website "selling" bonsai. Akadama is good for a lot of trees that require repotting every year or every 2~3 years, but not for Pines and Junipers which might need repotting after 5~10 years. By then [ 5~10 years later ] the akadama would have becomed a paste like substance that would be toxic to the pine or juniper.

Some YouTube videos on Bonsai "soil" mix worth viewing :

OrlandoBonsaiTV : How to Bonsai - Making Soil

One of the best video for beginners.

OrlandoBonsaiTV : How to Bonsai soil - why use bonsai soil

Walter Pall - about modern substrates used for bonsai - Part I

Walter Pall - about modern substrates used for bonsai - Part II

Side note - Using only Sphagnum moss as bonsai soil mix :
There is a specific channel on YouTube that suggest using only sphagnum moss as potting medium.
Sphagnum moss is extensively used in making air layers [ "marcottage" in French ] because it has a huge water retention capability.
The aim of water retention in a bonsail soil mix is to keep the roots moist, not sluggish in water.
If you are in an extremely dry context, you might want to give sphagnum moss a try, but in a region with daily precipitations, I would strongly warn against using sphagnum moss.
Another issue with sphagnum moss is that it gets "hard" and compact when it dries off, which "might" not be all that healthy for your roots.

Further litterature:

Bonsai4me - AN INTRODUCTION TO BONSAI SOILS
Bonsai4me - Cat Litter as Bonsai Soil (Diatomaceous Earth or 'Diatomite')

Walter Pall - baked loam as main ingredient for bonsai substrate
[ MUST READ ] Walter Pall - Feeding, Substrate and Watering - English