Guidelines For Creating Bonsai Soil.
By Randy Clark Charlotte, NC
The preparation of a workable potting mixture in
which to grow bonsai is certainly not the most exciting
or interesting aspect of the art, but it is just as certainly
one of the most critical. Because the growing space in a
bonsai container is limited, it is important that soil placed
into it should perform perfectly. The health and well-being
of the tree are dependant on it.
eliminated, but also fine dust, which would plug up air
holes between soil particles and inhibit proper drainage.
Screening and mixing soil is mostly common sense and
not rocket science. This is an important point to remember,
because it is possible to get so involved in the creation of
the world’s most perfect soil mixture that one can lose site
of the principal objective... perfect drainage and aeration.
The right recipe for bonsai soil is like the right recipe
for spaghetti sauce. Everyone has a slightly different idea
of what should go into it, but the basic ingredients generally remain the same. Bonsai people will argue for hours
about which ingredients will work the best. The actual
fact is that most thoughtfully prepared mixtures perform
fine so long as they provide for excellent aeration and
drainage. The objective here is not to describe an exact
mixture for making bonsai soil, but rather to discuss the
principals and elements necessary for an effective potting
mix so that readers can construct a workable medium
tailored to their own individual needs and growing conditions.
The actual components and the amount of each
component used in any soil mixture can vary from region
to region and garden to garden. Exactly what should be
included in the final preparation is an individual matter.
Components included in the mixture will be determined by
several factors. First, what types of materials are readily and economically available in the grower’s immediate
area. Second, what are the demands made by the local
growing conditions, i.e., do you live in a desert or a rain
forest? Third, How large is the container into which the
tree is to be planted, i.e., shallow pots will dry out more
quickly than deeper pots. Finally, what is the moisture and
pH preference (acid or base) of the particular variety of
bonsai being planted into the soil mix.
Bonsaists spend a great deal of time sifting various
soil components through a series of sieves. The objective is to make all components approximately the same
size. In the process not only are larger unusable chunks
Any usable soil mixture must always meet two basic
requirements if it is to have any hope of success. First,
the mixture must drain water quickly. This is generally
A properly prepared soil mix for use with
bonsai will contain ingredients of a uniform
size. This is done to provide for perfect
drainage of water from the container and to
allow vital aeration of the root system.
gradually trim away heavier roots, thereby
making more space for the growth of fine
feeder roots which are better able to nourish
the plant. It is the development of these fine,
hair like, feeder roots that a good bonsai
mixture is designed to encourage.
referred to as “perfect drainage.” Second, it should be essentially pH neutral... that is, neither wildly acidic or basic.
A pH value somewhere in the 6.5 to 7.5 range seems
best. There are all kinds of pH testing kits available on the
market. It is a good idea to get one and use it to test soils.
Local agriculture extension agents also offer soil testing
services for a modest fee.
“That Gravel You
Plant Your Trees In”
The appearance of a correctly prepared bonsai
potting mix is so radically different from the heavy black
dirt the public usually purchases for their general potting
needs that newcomers often describe bonsai potting mixes
as “that gravel you plant your trees in.” They are not far
wrong in this assessment, but what they fail to realize is
that there are some very specific reasons for preparing the
soils in the manner we do and some very specific advantages to be gained by doing so.
Why do bonsaists insist on a very loose well draining
soil mixture? The answer is simple. When god created
little green apple trees... and all other plants for that matter... his plan was to grow them in the ground. It was man
who devised the idea of putting them in containers. Horticulturally speaking, our bonsai are being asked to grow
roots in what is essentially an unnatural environment... the
small confines of a bonsai pot. Even though the container
may suit our artistic vision for a bonsai masterpiece, it is
an alien environment insofar as the tree’s root system is
concerned.
When a loose, well draining soil mixture is used, it
creates an environment into which the tree can easily grow
new roots. Vigorous root growth translates into vigorous
top growth and overall plant health. Because the growing
space in a pot is limited, bonsai practitioners attempt to
This concept is fairly simple to understand. As proof, consider the kind of potting
mixture which plant propagators use for
the rooting of cuttings. Normally it is coarse
sand or perlite. Both of these substances
have a uniform particle size, drain water exceptionally well and have no fine dust which
would inhibit air movement through the soil (aeration).
These are exactly the same characteristics on which
a workable bonsai mix is based. The actual ingredients
from which you assemble your soil mixture are certainly
important, but regardless of the components,... if the final
mixture does not have good drainage and aeration, it is
wrong.
There Is No Soil in Bonsai Soil
The term “soil” is really somewhat of a misnomer.
The components normally used to create a good potting
mixture are, in fact, soilless. They are designed to provide
an ideal environment for root growth. Unfortunately, in
creating this “perfect” environment we also create a few
problems for ourselves.
Obviously the soil mix described thus far will dry
out much quicker than the sticky black potting soil most
people are familiar with. Most of the components used are
aggregate (rock of one kind or another). This means there
are virtually no nutrients in a properly prepared bonsai potting mix and that the container will hold onto only enough
moisture for its immediate needs. Both of these aspects
are unfortunate, but necessary if any soil mixture is to be
successful.
Watering and fertilizing then become critical elements
for the development of healthy bonsai. Since the mix used
will hold neither nutrient or moisture, a failure to establish
regular, effective programs for both will quickly cause trees
to weaken and die. Likewise, an effective fertilization and
watering regime will cause a tree planted in a good bonsai
mix to respond with a growth and vigor that could not be
achieved using any other kind of potting preparation. Water and fertilization are subjects for another article. Suffice
to say that both must be done on a very regular basis and
never, ever, neglected.
A bonsai potting
mix for use with
smaller, shohin
and mame class
plants. This
particular recipe
contains pine bark
chips, fired clay
particles and river
rock which have
been screened to
eliminate dust and
produce particles
of 1/8 to 1/16 inch
in size.
Bonsai Soil Composition
A good general bonsai mixture should be composed
of about 75 per cent inert aggregate and 25 per cent organic materials. What aggregate and what organic material
becomes a matter of personal choice and often considerable debate between bonsai people. The best suggestion
is to use materials that are available in the local area. This
will allow the grower to keep costs down and hassle to a
minimum.
it... crushed lava rock. The inert ingredient(s) can vary
greatly.... but generally fall into three categories: hardened
clays, expanded aggregates and non-porous aggregates.
Hardened Clays
Akadama
Akadama and Kanuma are particles of pelletized
clay which the Japanese use extensively for potting their
Inorganically Speaking
Aggregate is the largest and most critical component
and will comprise an average of 65 to 85 percent of the total soil mass. Aggregate is the best term to describe these
substances, although, rock, gravel and drainage material
will also work. The aggregate portion of the mix may be
composed of just a single component or a combination of
components. The only requirement is that the aggregates
used have a uniform particle size and a neutral pH value.
It is not necessary to ship such materials across the
country or around the world in order to obtain a workable
potting mix. In all probability, they will be easily available
locally.
As one travels and meets bonsai growers in other
locations they quickly discover that soil components
change by region based upon what is available in a
particular location. People in Florida use a lot of sand in
their soil mixtures. That’s because they have got a lot of it.
Colorado bonsaists can obtain all the decomposed granite
they want from the sides of their mountains. In Hawaii the
primary inert component in a soil mixture is... you guessed
All components of a good bonsai mix must be screened
through several grades of mesh to eliminate both fine dust
and large particles .
Two examples
of hardened clay
products. At left:
Turface and on
right: Japanese
Akadama.
bonsai. They are virtually unobtainable in the west except
from a retail bonsai dealer. There are several different
grades and sizes of these clays.. Some have been fired
until they are rock hard like the turface described below.
Other grades are simply small pellets of clay that will break
down when they become wet. In either case the particles
will absorb water and nutrient and release it back to the
plant gradually. Purists in the art of bonsai will tell you that
growing trees in kanuma and akadama is the only way to
go and that if you are not using this product, you are not
really doing right by your trees.
While certain types of trees like to have a little clay
included in the soil mixture (notably wisteria and azalea),
The rush to include akadama and kanuma in western potting mixtures is somewhat of a fad. Its popularity is based
on the belief among many western practitioners that if it
comes from Japan it has got to be better than anything
available in the west, i.e., Japanese bonsai are very
beautiful. Therefore Japanese soil ingredients must be the
answer to growing beautiful trees.
The inclusion of minor amounts of Japanese clays in
a working bonsai mixture may improve and will certainly
not impair the mixture’s usefulness, but its overall benefit is
a matter of debate. In any case, the importation of pelletized clay from half way around the world as the primary
ingredient for a soil mix will quickly prove to be a very
costly business. If you have a lot of trees, you may want
to consider less expensive domestic materials.
Turface
Turface looks a lot like akadama, but is not. This
product is used for the aeration of grass on golf courses
and baseball diamonds. Turface is usually only available
in fifty pound bags, from turf supply and lawn maintenance
companies and generally not available at local garden
stores. Essentially it is clay that has been heated in a fire
until it becomes hard and will not break down and turn into
mud with prolonged exposure to water. If you were to take
a terra cotta pot and crush it up, you would have essentially the same thing. Each particle is full of tiny holes which
absorb water and release it back to the plant slowly. Its pH
is relatively neutral. Proper sifting of a 50 pound bag will
net you about 25 pounds of usable material for bonsai. You
can use the rest to aerate your garden.
Two examples of non-porous
aggregate. River rock and
fragmented granite (Poultry grit).
Three examples of
expanded (porous)
aggregrate. Left: Haydite,
Center: Lava Rock, Right:
Permatil
Lava rock (left) is an excellent example of porous aggregate which contains holes that absorb and release moisture to the
plant. River rock (right) is a non-porous aggregate and does not absorb moisture.
Although “turface” is the term generally used to describe fired clay, it is, in fact, a brand name. Other manufactures market similar products under the names such
as Soilmaster or Terragreen. There may be others. Note,
however, certain brands of cat litter and oil absorbent products on the market contain fired clay as their prime ingredient. You should be extremely cautious about using such
products because they often contain chemical additives
which would be detrimental when used in a soil mix.
Expanded Aggregates
Haydite
Haydite is another brand name and is the rock equivalent of turface. Similar products may be found marketed
under names such as permatil and staylite. They come
in different colors (brown or gray usually) depending upon
where it was made and what kind of stone was used. Historically, expanded rock is the primary ingredient used in
the manufacture of concrete blocks to make them lighter.
Only in recent years has this product’s value as a soil
amendment been discovered. Haydite, which is brown in
color is expanded shale. Permatil is grey and made from
slate. The term “expanded” means it has been heated
to over 2000 degrees which causes these two
types of porous rock to become even more porous. Like the turface it is full of tiny holes which
absorb water and release it back to the plant.
Some research even indicates haydite releases
water more readily than does the turface and is
less inclined to accumulate salts from watering.
Depending upon where it comes from, expanded
rock may be slightly pH acidic. This can be easily corrected by adding a little horticultural lime to
the soil mix.
Commercial bonsai dealers often offer a variety
of pre-screened and pre-mixed soils and soil
components for use in transplanting bonsai.
Lava Rock
Yet another expanded stone product is lava rock.
This is usually available in garden centers and is red in
color. Bonsai people are fond of using lava rock as a
finishing dressing on the surface of the pot, but it can also
make an excellent primary aggregate component for any
mix. Like expanded shale and slate, lava rock is full of
tiny holes which absorb water and then release it back to
the plant slowly. Unlike expanded shale and slate, it was
the volcano gods who did the expanding and not man.
Non-Porous Aggregates
Poultry Grit
We’ve used this substance for years and swear by
it. You can purchase it by the bag from your local feed
or farm supply store already separated into the correct
grades. Poultry grit is composed of fragments of crushed
granite which farmers feed to their chickens to help them
grind up corn. (As it turns out, chickens have no teeth).
The particle size in any given bag will vary depending upon
whether it is intended for baby chicks or full grown turkeys.
Unlike clay or expanded rock, crushed granite is dense
Organically Speaking
There are some bonsai growers who believe a good bonsai soil mix need not contain any
organic ingredients whatsoever. They maintain
that drainage and aeration are the two single most
important aspects of a good bonsai mix and that
you, as the grower, have the responsibility for supplying all the nutrients and moisture your bonsai
may require.
Pine bark is a good organic component to include in a
bonsai soil mix. In this case the bark has been passed
through three sets of screens to yield particles suitable for
standard potting needs (on the right) and shohin potting
and solid. It absorbs no water, is completely inert, neutral
in pH and has sharp edges on each particle which cause
fine feeder roots to split and divide when they hit them. A
word of caution. Make sure the chicken grit you are buying
is granite. We have found certain brands that are composed totally of crushed sea shells, which might be fine for
the chickens, but would be highly pH basic and disastrous
in a bonsai soil. We have also seen chicken grit manufactured from pink granite with white flecks. It works fine mechanically, but like perlite, is not aesthetically pleasing on
the surface of a pot. If you can’t find poultry grit, check with
your local stone quarry. You may be able to buy the same
product, but will have to sift it through several screens to
obtain the necessary particle sizes.
Construction Gravel
This is the substance most often included in a good
bonsai soil mix. It is basically river rock and is the ingredient usually mixed with cement to make concrete. You can
find it “down by the riverside;” or at your local construction site; or at the local concrete manufacturer; or in bags
from building supply centers. (If you purchase it by the bag
make sure it is all rock and not a rock/cement mix which
would only compound drainage problems the first time
you watered.) Seriously,... river rock, a.k.a. - construction
gravel, is one of the best products you can include in your
soil mixture. It is non absorbent, dense, inert, pH neutral
and readily available in most areas. Don’t get it confused
with the children’s play sand and the blasting sand also
available in home centers.
This is absolutely true, but it sounds a
little too much like hydroponics for most people.
Although there is some merit to the argument,
the preference among most bonsai growers is
to include something in the finished soil mixture
which, at least, looks a little like “dirt.” The organic
component in an average soil mix is usually about
20 to 30 percent of the total volume. This organic
will decompose gradually and in so doing, release nutrient for the tree’s use. In addition, it will retain a
bit more moisture than the inert components and will also
absorb more fertilizer.
The exact nature of the organic component used
is largely up to the grower. One of the most popular is
pine bark because it is inexpensive and easily obtainable
in fifty pound bags from local garden centers. One bag
yields about 25 pounds of usable material after it has been
passed through three sets of screens. Some people use
oak leaf mulch, some garden soil, old compost, or even
decomposed sawdust. The list can get quite long. Remember that the organic component is simply a vehicle for
dispersing nutrient and moisture and that it should be pH
neutral. You should also avoid using anything that might be
too “hot” (too much nutrient) for tender young roots, such
as cow manure or fresh compost.
Amendments such as activated charcoal, horticultural lime
and peat moss may be used to adjust pH or other qualities of
a soil mix.
Adding Something Extra
Tailor Mix To Your Needs
All components in a bonsai soil MUST be screened.
You will need a variety of screens ranging from one half
inch mesh through one sixteenth. You can purchase ready
made bonsai sieve sets from local bonsai suppliers or
make your own by visiting the hardware cloth section of
the local building supply store. For a normal sized bonsai
container (about 11” wide by 1 or 2 “ deep) the components described above should be screened to provide
particles which are about a quarter inch or less in diameter. Mixes for smaller bonsai (shohin and mame class
plants) should be screened to produce particles which are
between one eighth and one sixteenth inch. In both cases,
fine dust particles should be removed.
A little common sense is the best approach when
deciding what soil mixture is right for you. A recipe which
is composed of three quarters aggregate and one quarter
organic materials will produce a mix which is good for all
evergreens and most types of deciduous bonsai. However, quite often varying the components and ratios in a
mix to accommodate particular varieties of
plants can be a good idea. Likewise, if the
plant prefers a damp soil mixture (larches and
bald cypress) more organic material, which
holds more water, may be called for. If the tree
prefers a dry soil, (pine, ficus, juniper) more
aggregate may be advisable.
Take a look at the growing conditions in
your back yard. If you have a shaded location
you may have problems with pots staying continually wet. In such a case it might be wise
to increase the aggregate content of your soil
mix and thus cause it to dry out more quickly.
On the other hand, if your yard is exceptionally sunny, you might want to add more organic
ingredients which will cause the soil to retain
more moisture.
Above and beyond the materials already discussed,
there are numerous other additions which some growers
may or may not choose to include in their mix. Activated
Charcoal is one addition because of its ability to purify and
neutralize any impurities which may get into the soil. For
growers who are working with acid loving plants such as
azaleas (pH of 7.5 or higher) the inclusion of peat moss
in a potting mixture is a good idea. Some azalea growers
actually include the peat moss as the primary organic ingredient in the base mix. For plants that like alkaline soils,
the addition of one or two handfuls of horticultural lime is
a useful addition. Many growers like to add a couple of
scoops of bone meal (a mild, organic fertilizer) to a base
mix. As noted in the beginning, the right mixture for a
working bonsai soil is largely a matter of personal preference, but remember when adding ingredients not to impair
efficient drainage.
A Couple Of Tips
Moss - Decorative ground covers such a moss can
add a great deal to a bonsai’s appearance. They can also
be dangerous. Dry moss will actually shed water away
from the plant. If you use moss on your bonsai make sure
that the moss does not cover the entire surface of the
pot and that you are always able to inspect the moisture
condition of the soil. There is a Japanese rule which says
moss may be permitted to touch only three sides of the
container. If followed it means that you will always be able
to inspect the condition of your soil easily.
Old Soils - Try to insure that as much of the old soil
as possible is removed when transplanting. Incompatible
soil mixtures can cause problems in maintaining adequate
moisture levels and fool you into thinking the soil is completely saturated when it is not. The success of a good soil
mix can be severely impaired if it is not uniform throughout
the pot. If the old soil mass holds onto more moisture than
the new soil mix it may remain too wet and a condition of
root rot will set in. Likewise, if the center of the root ball is
hard pack clay it may resist absorption of water and roots
will not grow. Such problems are usually only encountered
with plants collected in the wild. Trees that have been
grown in a proper bonsai soil mixture for a number of years
normally “release” most of their old soil particles upon
transplant making the job of soil replacement much easier.
This is another advantage of growing trees in a properly
prepared bonsai mixture.
Standard Bonsai Mix - Components should be
passed through two sets of screens to produce particles of
about 1/4 to 1/8 inch in diameter. Eliminate dust.
Haydite - 1 part
Granite or River Rock - 1 part
Turface - 1 part
Pine Bark - 1 part
Shohin & Mame Bonsai Mix - Components should
be passed through three sets of screens to produce particles of 1/8 inch to 1/16 inch. Eliminate dust.
Haydite - 2 parts
Turface - 1 part
Soil Composition Protocol
What follows is the recipe for the soil mixture used in
a hot sunny garden in North Carolina. These ingredients
in the ratios indicated make an excellent general purpose
potting mixture. They are, by no means, the only mixture
or combinations possible. You may wish to amend these
ingredients or alter the ratios to suit your own specific
growing conditions. Remember that the best general combinations of components will be 75 percent inert aggregate
material and 25 percent organic material. Consider the
climate and growing conditions in your own back yard and
create a soil mixture which will fit you and your bonsai’s
specific needs.
NOTE: This article and images it
contains were written and photographed
by Randy Clark, resident bonsai artist at
the Bonsai Learning Center in Charlotte,
NC. They are intended for private use
only. Reproduction of this material for
commercial purposes is strictly prohibited
without the written consent of the author.
© 2000 Bonsai Learning Center, All Rights Reserved
Pine Bark - 1 part