Bonsai Soils and Ingredients by Ken Goebel
(printed in MBS Newsletter April 2007, revised July 2012, November 2013)

As a physical chemist, a major project I had was the study of the surface and physical chemistry of midwest soils in relation to performance of carpet
finishes.  With my recent hobby of growing bonsai trees, soil has once again become a fascination for me, but now as a bonsai growing medium.  This
article was my inspiration to place some order into the chaos of bonsai soils, and also to share my practical knowledge from various readings and
personal experiences.

First the basic ingredients, what they are and what they're good for:

Clays

These clays are usually available in two forms, low-fired which is essentially sun-dried or flash-baked at low temps (about 400-800 degrees F), and
high-fired which is kiln-baked at high temps (about 1400-2200 degrees F). The softer, low-fired clays retain moisture and nutrients, and yet maintain
drainage.  But they tend to compact or degrade over time, and eventually drainage is reduced.  The high-fired product is a stable, gritty substance much
like lava rock. The particles are hard and porous, yet somewhat rounded, retaining some moisture and nutrients (but mostly air) and providing excellent
drainage.

Kanuma -  a soft, granular clay that is used primarily for ericaceous (acid-loving)  plants such as azalea, gardenia, and camellia. This particular material
comes from the Kanuma area of Japan which is basically the center for growing azalea bonsai.  It is low-fired and acidic, unlike Akadama.

Akadama - regarded as one of the best potting media for bonsai, Japanese masters use akadama for their highest quality bonsai, both to promote root
growth and provide a medium for optimal water and nutrient retention. Akadama is a large-grained, neutral pH (not acidic or basic) clay.  It is dug out of old
cryptomeria forests of Japan and flash-fired to remove any organic matter (sterilized). Hard akadama is high-fired in kilns and is basically like ground-up
brick. It takes quite a bit longer to break down.

Turface - Turface looks similar to akadama but is a very porous calcined clay. It has been high-fired until it becomes hard and will not decompose (turn
into mud) with prolonged exposure to water (degrades only 3.5% over twenty years). Each particle is full of tiny holes which absorb water and release it
back to the soil slowly. Its pH is essentially neutral. It resists compaction, thereby leaving more room for air in the soil, so that plant roots can breathe and
provides passageways for better water drainage.  Turface has a high capacity to adsorb nutrient ions from water, and release them as needed by the
plant.

Rocks

Haydite - the rock equivalent of Turface, but without the ion exchange capacity or water holding capacity. Basically it is river rock found along river beds.  
Dense, inert, pH neutral and readily available in most areas, the aggregate fines are used in soil conditioning.  Only recently has this material's value as a
soil additive been discovered. Grey-brown in color, it is really expanded shale, heated to over 2000 degrees F which causes the porous rock to become
even more porous. Like Turface it is full of tiny holes which absorb water and release it back to the soil. Haydite releases water more readily than does
Turface and is less inclined to accumulate salts from watering. Depending upon where it comes from, the expanded rock may be slightly pH acidic.

Pumice - a porous, volcanic rock used as a component of bonsai soil.  Pumice varies in color and grade. Most have some ability to absorb water and yet
retain a pore space that promotes good root growth.  It may be considered as a replacement for Akadama, due to the similar hydration and ion exchange
properties.

Lava - another expanded volcanic rock, it is red or black in color.  Used as a finishing layer on the soil surface or as a drainage layer in the bottom of the
pot, it also makes an excellent aggregate component for any soil mix. Like expanded shale and pumice, lava rock is naturally full of tiny holes which absorb
water and then release it back to the plant slowly. Lava rock is an acceptable substitute for Haydite, but care is necessary since lava rock (depending on
its source) could leach toxic mineral contaminants into the soil.

Crushed Granite (grit) - This material is an excellent aeration additive to bonsai potting soil.  Unlike clay or expanded rock, it is nonporous, dense and
solid. Its size is about that of coarse beach sand.  It resists soil compaction, absorbs no water, is completely inert, not acidic or basic, and has sharp edges
on each particle which cause fine feeder roots to split into a more tortuous root network. Some types of grit are composed of crushed sea shells, which
should be avoided since they are highly pH basic and would be disastrous in a bonsai soil.

River Sand - As a suitable substitute for crushed granite, the various grades of coarse river sand (1/16-1/8 inch) can vary in color from black to brown to
white, depending upon the source. Asaake and Kawasuna sand come from the rivers of Japan. They are quite heavy and dense with sharp edges.  Fuji
sand is black volcanic glass from Mount Fuji.

Inorganics

Zeolite - a "new" soil amendment found in nature, it is a material that holds water and fertilizer and slowly returns it to the medium as the medium dries out.
It is just now becoming recognized as an excellent additive to bonsai soil mixes. An inorganic material that has a micro-porous structure, it is basically
hydrated alumino-silicate minerals with an "open" structure and ion-exchange capacity much like Turface.  It may be considered as an alternative
amendment to Turface.

Organics

The organic components are simply a medium for dispersing nutrients and moisture.  In addition, they will retain more moisture than the non-organic
components described above, and will also absorb and make available more fertilizer necessary for plant growth. My favorite organics are coco peat, pine
bark, peat moss and sphagnum moss, good for holding moisture.  Peat moss and sphagnum moss also help to acidify the soil mix, especially for pines and
azaleas. Some bonsai growers use oak leaf mulch, black garden or nursery dirt, old compost, or even decomposed sawdust. One should avoid using
anything that might have too much nutrient such as cow manure or fresh compost.  For trees that prefer alkaline soils, the addition of a small amount of
horticultural lime is useful.

Micro Plus - This product contains mycorrhizal fungi which grow on (or sometimes inside) roots and assists the roots in water and nutrient absorption,
increases winter hardiness, and antibody production, as well as minimizing effects of drought and decreased transplant shock.  Mycorrhizae are especially
beneficial for the plant partner in nutrient poor soils. Plants grown in sterile soils such as bonsai soils may perform less without the addition of mycorrhizal
fungi to colonise the plant roots and aid in the uptake of mineral nutrients.  It is best used as a small dusting of micorrhizal powder on the plant roots
during transplant.

These soil ingredients can be found on the internet, for example, Dallas Bonsai at
www.dallasbonsai.com or Stone Lantern at www.stonelantern.com,
as already prepared, ready-to-go bonsai soils or as individual materials if one wishes to make the soil from scratch.

Now to the art (and sometimes science) of combining these ingredients into bonsai soils.

Bonsai Soils

Because the growing space in a bonsai container is limited, it is important that soil placed in it should provide excellent aeration and drainage. The health
and well-being of the tree depends on this. Everyone has a slightly different idea of what should go into a bonsai soil, but the basic ingredients generally
remain the same.  I'll suggest some media for an effective potting mix so that readers can construct a working soil tailored to their own individual needs
and growing conditions.

Any usable soil mixture must always meet the two basic requirements: First, the mixture must drain water fairly quickly. Regardless of the components, if
the final mixture does not have good drainage and aeration, it will lead to root rot. Second, the soil should be essentially pH neutral, that is, neither
strongly acidic or basic. A pH value somewhere in the 5 to 7.5 range seems best. (There are all kinds of pH testing kits available on the market to check
the water pH in the soil.)

Inert aggregate is the largest soil component and will comprise about 60 to 80 percent of the total soil volume, the rest being organic. What aggregate to
use is a matter of personal choice. The aggregate portion of the mix may be composed of just a single component or a blend of several components. The
aggregates used should have a uniform particle size, ie. sifted or screened, and essentially neutral pH.  Screening also removes the silt that accumulates
in soil container bags. It's a good idea to wear some kind of mask or filter (a dust and pollen mask is good) to avoid inhaling the silt when screening the
soils. Getting the silt removed is key to the success of any soil mix, as it can compact brutally and really mess up soil drainage.

Some Basic Soil Compositions - These are not the only mixtures or combinations possible. Bonsai enthusiasts may wish to amend the ingredients or alter
the ratios to create a soil mixture which will meet your bonsai tree's specific needs.  Generally the best combinations of components will be about 60-80
percent inert aggregate material and 20-40 percent organic material.

Shohin or small bonsai are better suited to fine soils with 1/16-3/16 inch particles, while outdoor and larger bonsai (greater than about 2 feet) will benefit
from the coarser-grained soils, greater than 1/4 inch in size.  The most common bonsai soil grain size is in the range 1/8-1/4 inch.

I have defined three general categories of soil mixes as a guideline.  These are based on water drainage and growing needs of the bonsai tree.  The
component parts are by volume:

Growing Soil
5 parts aggregate clay/rock
1 part granite
4 parts organic
A slower draining soil for trees that are still in a growing stage of development.

Training Soil
6 parts aggregate clay/rock
1 part granite
3 parts organic
A moderate draining soil for trees that are in a training stage of development.

Maintenance Soil
7 parts aggregate clay/rock
1 part granite
2 parts organic
A faster draining soil for trees in their maintenance stage of development.

Some more specific soil mixes are listed below:

Tropical Soil Mix

1 part Kanuma
3 parts Turface or zeolite
1 part Haydite or lava
2 parts pumice
1 part granite
2 parts pine bark or coco peat
This provides a nice moisture retaining soil with plenty of drainage.

Acidic Soil Mix

3 parts Kanuma
2 parts Turface or zeolite
1 part Haydite or lava
1 part pumice
1 part granite
2 parts milled sphagnum moss or peat moss
This provides the acidity needed for azalea and other acid-loving plants.

Conifer Soil Mix

1 part Kanuma
2 parts Turface or zeolite
2 parts Haydite or lava
2 parts pumice
1 part granite
2 parts milled sphagnum moss or peat moss
A rocky soil for conifers.

Deciduous Soil Mix

2 parts Turface or zeolite
2 parts Haydite or lava
2 parts pumice
1 part granite
3 parts pine bark or coco peat
For maple, elm, hornbeam, and other deciduous trees.

General Purpose Soil Mix

3 parts Turface or zeolite
3 parts Haydite or lava
3 parts pumice
1 part pine bark or coco peat
This can be used for trees whose needs don't match the other specific mixes.

The component parts of these specific soils can be adjusted to fit the three general categories of soil mixes mentioned above.

As a rule-of-thumb, adding 10% (1 part in 10) peat moss or sphagnum moss can drop the soil pH about 0.5 units.  So, starting with a neutral (pH 7) soil,
adding 10% peat or sphag moss drops the soil pH to about 6.5; 20% peat or sphag moss, soil pH about 6; 30% peat or sphag moss, soil pH about 5.5.
Since peat moss shows a tendency to break down over time, peat-based soils may need to be replaced every 1-2 years to avoid drainage problems. And,
if the peat moss is allowed to dry out it becomes difficult to rehydrate, which is not the case for sphagnum moss.  The peat moss can also be replaced with
Kanuma which has a pH rating of 4.5-5.5, similar to peat moss.

My observation has been that soils containing both Turface and Haydite together show better transplant recovery and improved plant performance.

For slightly basic soils (pH 7-8), horticultural vermiculite can be added as an ingredient.  This component contains associated carbonates which results in
an alkaline medium in contact with water.

I have no personal experience with zeolite, but believe it would be a good inorganic substitute for Turface.


My sources of Information:
Labels on commercial bonsai soil mixes
"Introduction to Bonsai" by Thomas Zane
Article "Guidelines For Creating Bonsai Soil"  by Randy Clark, Charlotte, NC
Article "Akadama (or Expensive Japanese Dirt) in a Nutshell" by Terry Bishop
Dallas Bonsai website
Stone Lantern Bonsai website
Various internet bonsai sites