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

Clerodendrum Smithianum [ Light Bulb Clerodendrum ] Bonsai 001

This Clerodendrum Smithianum [ Light Bulb Clerodendrum ] Bonsai was root bound.
After watering, the water would not drain at all, and would create a "pond" inside the pot.
I had to repot it, and ended up choosing a new front [ after cleaning up the roots ]

Back in June 2011 [ Date of aquisition ]
Foilage and branch structure was random.

Same tree in March 2012.

July 2012 - The old front. 

July 2012 - The old side, I did not know it then,
but this would become the new front.

The back - It could well be used as a front,
but because of the long root, it does not fit well
when viewed from the side.

The culprit - The tree was so root bound that the roots had
closed the drainage hole

The exposed root system - you can see the
feeder roots [white roots] tangled at the base.

Root system - after untangling the root ball.
Not much root pruning has been done. The
rotting sections of roots were removed.

Root system - after untangling the root ball.
Not much root pruning has been done. The
rotting sections of roots were removed.

Root pruning - Cleaning up the unwanted roots.
This long root is growing in the wrong direction.

Root pruning - after removal of the diagonal root.

Root pruning - There are some interesting thick root structures
below those feeder roots.

Root pruning - The exposed roots, after removing the feeder
roots. 

Bonsai Soil Mix : 40% perlite, 40% gravel, 20% pine bark.

Bonsai Soil Mix : 40% perlite, 40% gravel, 20% pine bark.
Close-up.

Repotting - Adding a drainage layer. You dont see it well here,
but the middle section is making a small hill shape.

Repotting - Choosing the new front. There are still several
root section that were left intact. 

Repotting - The excess roots are trimmed.

The new front - You can see the newly exposed roots.

Side view

Side view

Back view

Close-up of the "new" front - The exposed roots improves
adds to the nebari and overall view of the tree.
You might now see such structures in temperate areas, but
it is something more natural to a tropical tree.

My Attempts at wiring

Today i've given a try at fully wiring two trees, first time for me. I used to wire one or two twigs, but never a full "tree".
I am aware that there is still lots of room for improvement, but I felt it was time to give it a go.

I started with a banyan - I forgot to take shots from the start, so its only half of the progression.

Removed the leaves

Wired and pulled the branches down

Wiring details 1

Wiring details 2


The second one I worked on is a Privet [ Ligustrum ].
This one I root-pruned then wired.

Full root system

Pruned root system

Nebari - well once it thickens a bit

Potted

Removed the leaves to get a better idea of the branch structure

Wiring in progress

Wiring completed

Wiring details 1

Wiring details 2

Wiring details 3

Wiring details 4


Well that's it.
Critiques are most welcomed.

Xavier