Perhaps the first confusing element of bonsai that is encountered by beginners is that of soil. Reading about or hearing cursory references to soil "mixtures" and strange component names like akadama and Haydite don't help matters. What is a soil "mix" anyway? Well, this article will try to clear up matters about bonsai soil for you so that you can:
1. know what is a good soil component and what is not so good
2. recognize the various, popular bonsai soil components.
3. understand the purpose of each of them
4. know where to find them
5. know how to mix soils for different situations and species of bonsai
The first thing to know about bonsai soil is that it is not "dirt." In general, dirt is made up of very fine particles of sand, clay and organic matter that has a texture similar to salt or flour. Dirt can get very compact and when dry, will turn into a solid mass.
Bonsai soil is very "large-grained" by comparison to dirt. It usually looks like what most beginners would classify as "gravel." Regardless of the kind of soil component used, the grain size will be far larger than you might expect.
A basic overview of soil sizes includes:
- Small bonsai or surface soil: 1/8" diameter
- medium bonsai or mid-level in the pot: 1/8" to 1/4" diameter
- large bonsai or bottom of pot: 1/8" - 3/4" diameter
The reason that bonsai soil is so large is that life in a bonsai pot is nothing like life in the ground. In the bonsai process, we have to offer on a very small scale what nature offers to a large tree on a grand scale. This necessitates accommodations that may seem odd to the beginner.
First, one of the more important needs of a tree's roots is oxygen. While we often may think that the roots are there merely to absorb water and growth stimulators (fertilizer), they have needs, just as the leaves do, that must be met. In the small environment of the bonsai pot, this means that there must be an easy way for a lot of oxygen to reach the roots.
Second, given the small confines of the bonsai pot and in spite of what beginners may deem to be a good soaking, water will simply not get to all parts of the soil (and therefore the roots) unless the soil is open and granular. This kind of soil structure helps to insure that the moisture from a watering will saturate the entire contents of the pot's soil.
To keep things open and granular in the pot, the various soil components for bonsai must be sifted. This is done for 2 reasons:
1. to remove the fines (the small, sand-like and
2. to sort the soil grains into differently sized
particles for different sized bonsai and levels in
The Soil Components
So what makes a good bonsai soil component? Below, several good components will be introduced and shown so that you can become acquainted with them.
Akadama (Japanese red clay)
Haydite (expanded shale)
Crushed red brick
Blasting sand (natural aquarium gravel)
Sifted pine bark
Soil Component Purposes and Benefits
Akadama is the probably the best all-around base component for a good bonsai soil. It is a volcanic clay that is taken from the ground in Japan. It has the benefits of being completely inert and inorganic. It has good water absorption traits and provides a very "earthy" environment for roots. Akadama will, over time, break down into a fine "dirt" which necessitates a soil change on a regular basis.
Akadama can be found at better bonsai retailers and virtually nowhere else. It is usually sold in 40 quart bags and the grain size varies - small, medium and large.
A grit. Haydite is expanded shale; shale that has undergone a firing process that sort of "pops" it. It is a grit component in bonsai soil (like a gravel) and serves to open up the soil mixture, somewhat diminishing the moisture retentive properties of the soil. Haydite usually comes in a mixed-size bag and can be sifted into 3 or 4 sizes for different uses in the soil.
Haydite can be found at some bonsai retailers and at some aggregate supply companies.
A grit. Pumice, like Haydite, is an excellent bonsai soil component (especially akadama pumice mix ) that serves to open up the mix and reduce the moisture retention properties of the soil. Some enthusiasts don't like to use pumice because of its white color (unsightly in the pot), but this can be amended by using a top dressing of uniform colored soil when showing a tree.
Pumice of a quality used in bonsai is usually only available from bonsai retailers.
Crushed Red Brick
A grit. Red brick chips are a good, inexpensive form of grit for bonsai soil. Like Haydite and pumice, it does absorb water, but helps to open up the soil mix. It can be found in a couple of sizes, but the small size is best for bonsai, with particles of 1/8" to 1/2" in diameter - be sure to sift it into at least 2 sizes.
Crushed red brick can be found at large home/garden centers and is quite affordable - them most economical (at retail) of those listed here.
A grit. Blasting sand is just a large grained sand that can be used in bonsai soil and is good largely due to its wide availability. It is often too small-grained to be used in medium sized and large sized bonsai, so its uses are limited.
Blasting sand is available from some pet shops (sold as aquarium sand) and aggregate supply companies.
Sifted Pine Bark
This is an organic soil component and its reason for inclusion in bonsai soil is its moisture retention qualities. With most bonsai soil being very open and gritty, it tends to dry out quite quickly so many enthusiasts use sifted pine bark to counter this to some degree. Some enthusiasts use pine bark in their soil simply because they feel the need to make it more "natural," since dirt in nature has organic components. Do not use the very fine, dusty pine bark, but rather the small "chips."
Sifted pine bark is available from some bonsai retailers and from some garden retailers.
Some not to use:
Never use play sand, potting soil or plain old "dirt" from the ground in your bonsai pots.
Bonsai Soil Mixtures
Now, how to put all of this together? Here are some basic guidelines:
1. Different soils for different species
This is probably the most important factor in your mixtures. While erring in mixtures does not mean sure death to the tree, providing the best mixture for a specific tree helps it to stay healthy and happy. The list below is a very basic primer and in no way exhaustive or definitive.
Here are some basic mixtures for some popular bonsai species:
Maples: straight akadama
Elms: 60% akadama - 40% grit (either pumice, Haydite, crushed brick or blasting sand). Variation a bit with either % presents little or no problem
Junipers: 50% akadama - 50% grit. Again, variation one way or the other is not usually problematic.
Pines: anywhere between elms and junipers
Beeches: straight akadama or akadama with a bit of grit and/or pine bark.
Larches: 70% akadama - 30% grit and pine bark
2. Different soils for different climates
This is an area of great misunderstanding for many bonsai enthusiasts. There is little need for you to try and match your bonsai soil to your geographic location or climate as this is largely a watering issue and you will be the one who supplies the water. The "logic" is that those living in a dry climate need to pump their soil full of organic components to hold moisture and those living in wet climates need to grow trees in pure grit (or some variation on this theme). The result is often that the trees themselves are not suited to the supplied soil mix regardless of how it manages moisture.
Do not concern yourself with this element of soil mixture planning and base your soil mixtures on your tree species and your watering practices. If you cannot meet the watering needs for a particular species, don't grow that species.
As a basic practice, to increase the water retention properties of a soil, add more akadama or organic component to the soil, and reverse this practice to allow the soil to dry/drain more freely.
Layering is the practice of using different grades of soil (grain sizes) at different levels in the pot. This involves using the largest grain sizes at the lowest level, a medium grain size in the mid-level and a smaller grain size near the top. This practice has a specific aim, but it is not what most people believe. It seems "logical" that the large soil at the bottom of the pot serves as a "drainage" layer, and it does to a certain extent, but the main purpose of the soil size layering is to keep moisture in the mid-level of the pot - where the roots are.
The point at which soil grain sizes change has the effect of tending to keep moisture perched above the larger grain size. Now, while this effect is rather subtle in a bonsai pot, it is beneficial in helping to keep moisture at a level that makes it accessible to the roots that need the moisture.
So, when preparing a pot into which you will plant a bonsai tree, first put in the largest grains of soil components, next some median sized grains, place your tree (and tie it in the pot), add more of the medium sized soil and finish with the smallest sized soil near the top. While this is not a comprehensive description of the process, it is the basic theory.
This article has just brushed the surface of bonsai soil concerns, but it should serve to get you started thinking about bonsai soil the right way, understand the various popular components and understand a bit how to use them. Know that there are plenty of other decent bonsai soil components and more minutiae about bonsai soil theory, but you will pick that up as you go.