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C O L I N   L E W I S   B o n s a i   A r t
B O N S A I   S O I L S   P A R T   2

M I N E R A L   C O M P O N E N T S

Why do we want mineral components in our bonsai soils? The first and most obvious reason is to guarantee good drainage. The second is to maintain an open, oxygen-rich structure; the third is to add 'body' and stability. Whether or not any particular mineral has a high cec (cationic exchange capacity - the ability to adsorb nutrients for later release) is only relevant if you are not using organic matter, all of which has at least an adequate cec. On the other hand, if you feed regularly enough, your soil only needs a relatively low cec.

With the exception of grit, all the common mineral components of bonsai soils absorb and retain moisture, drain well and have some degree of cec.


Akadama is entirely unique and can't be fairly compared to any other mineral, so let's deal with that first.  Akadama is a naturally occurring clay-like (but not clay) mineral found only in one region in Japan. It is surface mined, dried, graded and packaged. No baking or firing is involved. When wetted, it does not form a slurry like clay but forms a gritty paste. Akadama is without any doubt the absolute best growing medium for Japanese maples and many other broadleaved species, and is arguably one of the best ingredients to incorporate in smaller proportions for moist other species.

The big advantage of Akadama is that roots can grow through the parcels as well as between them, which vastly increases the amount of space in the pot that is available to the roots.  Once roots penetrate Akadama particles they become fine and highly ramified, forming a dense pad of functioning feeding tips. When you foist use Akadama the result is really quite astonishing compared to most other media.  Although Akadama has a relatively low cec (around 18-20 meq/100g) the fact that the roots are so prolifically generated means that this is not a defect.


Some growers complain that Akadama readily breaks down into much smaller particles and therefor impedes drainage, but they are not watching closely enough.  Certainly it does break down, but this only means that the pore spaces become smaller, not that they disappear altogether. Since Akadama is not a clay, but a kind of cohesive sandy structure, it still drains efficiently after several years in use just as sand would.

The big draw back?  Cost: at the time of writing even lower grade Akadama is priced at over $50 per bag (compared with $18-$20 a few years ago).  I can understand why it has become more expensive in recent years, but I cannot comprehend why it is almost three times as expensive in the USA than in Europe.

The lava most common used for bonsai soils is either the red or black lava from Colorado. Lava is solidified molten igneous rock that has flowed from volcanic eruptions. It is essentially foamed glass with sharp edges and partially interconnected pores. Depending on its source, most lava contains some heavy metals and potentially some minerals useful to plants. The cec of lava is highly variable, but generally hovers between 15 and 40 meq/100g.

Red and black lava

Roots cannot grow into or through lava particles, but they can and do penetrate the pores to a certain extent. Lava can hold a considerable amount of water (up to two and a half times its weight) but it takes several hours of soaking for the particles to become fully wetted due to the microscopic channels between pores, where channels exist.  

Lava must always be sifted and washed before use. Failure to do this will result in an absorbent layer of fines and sludge at the base of the container that will prevent adequate drainage and can cause major water logging problems.

As a  soil ingredient for bonsai lava performs well. I have seen very strong root regeneration on newly collected trees planted in almost pure lava. Another major advantage lava has over Akadama is that it can be salvaged: after use it can be dried, sifted and re-used time after time.


Like lava, pumice is a product of volcanic eruption, but there the similarity ends. Rather than solidified molten rock flow, pumice is the non-crystalline solid form of molten rock that is blasted out of a volcano at extremely high pressure and temperature.  It is this rapid decompression that happens as it leaves the volcano that gives pumice its softer texture and more interconnected pore structure.


With a cec that hovers around 75 meq/100g (depending on source) it requires less frequent fertilizing that most other soil components when used alone. Pumice can hold large quantities of water - up to four times its own weight. This, coupled with the soft and easily powdered surface means that when used alone or with other absorbent materials, careful watering is necessary to avoid water logging.

Although soft, roots are unable to penetrate pumice - even the surface pores since they are far too small. However, since the surface of each particle is soft, roots do seem to enjoy their company very much, and ramify well. After a while, the roots seem to break down the surfaces of the particles to form a sort of micro environment of small particles and dust that keeps them snug and fine.

Pumice drains well but tends to drain less well after a couple of years in use. However, drainage is never impeded to the extent that it becomes a problem.

TURFACE Also marketed as Terragreen or Biosorb among others, Turface is calcined montmorillonite clay. Calcining is the process of heating a substance to a temperature high enough to bring about a 'phase transition' - a change in physical state (like baking a cake!). If clay is fired to the highest possible temperature, the result is so hard and non-absorbent that it is used for super-sharp knifes. Reduce the temperature and we have stoneware; reduce it further and we have oven-proof earthenware, then terra-cotta (as in flower pots). As the firing temperature decreases, the absorbency rate increases. Turface is heated just enough to stabilize the particles - to prevent them forming a slurry when saturated, but not  high enough to reduce its absorbency rate.


Turface has a moderately high cec of 33 - 35 meq/100g and can absorb vast amounts of water. Pour water onto dry surface and you can hear it soaking and see steam arise as it does so!  This incredibly high absorbency can be both a blessing and a nightmare, depending on the quantity used. although when used undiluted it seems to be a good medium for newly dug trees, as soon as the inter-particle spaces are full of roots, it readily becomes waterlogged. When sold as a soil amendment (its original purpose) the manufacturers of  Turface recommend a rate of no more than 15% by volume, and I would suggest that rate should also apply to use in bonsai soils.

Some suggest that after a while the moderately high cec can cause Turface to adsorb concentrations of nutrients so high that it can cause reverse osmosis resulting in "root burn".  In theory this is possible, but you would have to be consistently feeding very heavily for some time for this to happen.

Turface substitutes such as oil dry and kitty litter are less stable and not really suitable as a soil ingredient.
HAYDITE Haydite is heat-expanded shale, slate or low-grade clay with a relatively low cec that varies between 6 and 15 meq/100g depending on source and raw material. It is essentially a type of semi-absorbent ceramic with non-interconnected spherical pores. For the internal pores to become full of water, many hours of soaking is required, to give the water time to penetrate through the ceramic walls of the pores.


Although commonly used by bonsai enthusiasts (one of whom wrote a bonsai page for the manufacturer's website) I have formed the opinion through using it myself for a while, and from seeing the results of others who use it around the country, that Hayd
ite is most definitely not a good ingredient for bonsai soils. Due to its poor absorbency rate, Haydite is little use as a moisture reservoir; the fact that it has some absorbency negates its usefulness as a drainage aid.  In practice, it is little more than a cheap filler.  

The manufacturer recommends using as a soil amendment at a rate of no more than 15% by volume.
PERLITE Once very popular as a potting soil ingredient in commercial horticulture, perlite seems to have fallen out of favor and has been replaced by cheaper, nastier stuff like pine bark. Perlite is essentially a form of natural glass. When heated the raw material will expand up to 20 times its original volume. It is extremely light in weight, can hold vast quantities of water and has a cec of virtually zero. It is very soft and crumbles very easily - so easily, in fact, that roots can split away fines that can wash to the base of the pot and form a drainage impeding pan.


Root growth in pure perlite is excellent and it is extremely hospitable to mycorrhizal fungi, but its lightness makes it unstable as a single ingredient soil. If using perlite as an ingredient, be careful to ensure that either the other ingredients are virtually non-absorbent, or perlite forms no more than 20% of the total mix.

VERMICULITE Vermiculite is hydrated magnesium aluminum silicate that has been heated to extremely high temperatures which causes it to exfoliate, or expand, into elongated particles, up to 20 times its original volume.  Like perlite, it is extremely absorbent and light in weight, and can absorb many times its own weight in water - and has fallen out of favor as an ingredient in commercial potting mixes. Unlike perlite, however, it has a very high cec at over 85 meq/100g. The plate-like shape of the particles, and their fragility, can cause compaction if used alone.  Hard to find and not worth the effort.

GRIT Grit, gravel, sand
Although this should be the most straight forward to discuss ingredient, there is considerable confusion. What I call grit some call sand and others call gravel. I imagine somewhere there is an official, USDA, Supreme Court approved definition of grit, for this discussion we will use mine!

For this purpose, grit comprises grains of stone between 1.5mm and 3mm. Anything smaller is sand, anything larger is gravel. Grit is used primarily to reduce water retention in the soil in general, to aid rapid drainage after watering, and to maintain an open soil structure. It also adds weight to the soil making it a more stable anchorage for roots.

It really doesn't matter from what material your grit is derived - crushed granite (ie: chicken grit) sifted paving sand, pool filter sand, aquarium gravel - they are all inert. The one major type of grit to avoid is anything that has been gathered or mined from or near the shoreline. Although non-porous, sand and grit can become covered in salt which bonds to the surface and can take a long time to re-dissolve. If using chicken grit, make sure it doesn't contain crushed oyster shell, which is also high in salt content and very alkaline.


Here many myths abound….  Some would have you believe that sharp-edged grit is best because when a root hits the sharp edge it it forced to split in two. Preposterous! For one thing that is not what causes root division, for another, the chance of a root tip colliding precisely head-on with the edge of a grain of grit are millions to one. Others suggest that round grit is best because of the "ball-bearing" effect: the particles cannot nestle in against each other and reduce oxygen space. While this may be true, in practice there are so many other ingredients in the mix, and so many roots weaving between particles, that the ball-bearing effect is not an issue.

Having defined the size range of grit above, that is the size range I recommend. Anything smaller than 1.5mm can either fill valuable pore space in the mix, or wash to the bottom of the pot creating a drainage impeding pan. Particles larger than 3mm add no additional drainage properties and create pore spaces large enough to be occupied by other ingredients. Large particles of grit are nothing more than chunks of rock that serve no purpose other than to take up valuable space that could otherwise be occupied by roots.

What ingredients you choose and how you proportion them is, of course, entirely up to you. Hopefully the information above, coupled with the article on organic ingredients and the upcoming piece on particle size and recipes, will help you make decisions that will improve your success rate and your enjoyment of the trees  you grow.