BY VERN SMITH :: Now that it is getting closer to Bonsai repotting season here in the San Francisco Bay Area, I thought I would risk the onslaught of slings and arrows and offer my two cents worth regarding Bonsai soil, or more accurately, substrate.

owner and instructor WEST HORRIZON BONSAI SCHOOL

owner and instructor

First off, let me clarify a few things about my approach to Bonsai. While I respect the vast amount of experience of many practitioners of Bonsai, there is such diversity of opinions regarding all aspects that it forces a prudent Bonsai enthusiast to ask questions. And certain terms immediately send up red flags for me. When I hear terms like “that’s the way we have always done it” or “that is the traditional way to do it”, or “because that’s how they do it in (insert a place here)”, I don’t walk – I run in the other direction and dive into my own research. I want to understand what I need to do, but I also have to understand why we do the things we do. I need to have the facts to back up the theory. Facts based on valid studies backed up by good science, not unscientific anecdotal hearsay. I am too skeptical and too pragmatic to be concerned with unsubstantiated claims – I want results based on proven horticultural practice, which, in turn, is based on scientific methods of verification. I owe it to my trees.



Let’s think about what we do in our gardens and what we know about gardening, and all the theories related to why we do what we do. Some are based on science. But many are based on tradition and anecdotal information. Bonsai, as a subset of gardening, is no exception. But plants are still plants, regardless if they reside in the forest or in a pot. Their requirements don’t change because we stick them in a pot, prune them radically and bind them in wire. The basic laws of plant biology abide.

Now I’m not going to go into great detail on all the current and abundant soil recipes – they list in the hundreds – because, frankly, in my opinion, it’s not important. For too many years, there have been debates on what “Bonsai soil” should be composed of. I believe that we are asking the wrong question. We need to step back and ask “what is our goal” when we select a substrate for our leafy and bound wards. To begin with, we are confining our trees to very small pots, relative to their physiological needs, in just a few quarts of soil, for the most part.

Not a lot of medium to draw upon – and not a lot of room for error. Planted in a field or in the forest, or even our back yards, a tree’s roots can grow for many feet, searching for pockets of water and nutrients. Not so in their Bonsai pot. We have to bring the water and nutrients to them. We need a CONSISTANT and SIMPLE delivery system when we choose a substrate. If water drains too slowly, problems can arise by overwatering or overfertilizing – this is often the case of soils that are too rich in organic material. If water drains too fast, without any chance for the roots to absorb the water or nutrients, the plant will also be in trouble – this is the case of too much grit, sand or rock in our soil.

So, this points to an obvious solution – we need very good drainage AND moisture retention, with capacity for oxygenation for good measure, to also aid the growth of roots. We also need a good cation exchange coefficient (CEC), which affects its capacity to hold and release micronutrients. Rock is no good – it can’t hold anything. Dirt/compost is no good, it holds too much water, and breaks down too quickly for our normal repotting schedules. Therefore, we need a porous, stable material, of relatively uniform size, with a variation in the degree of porosity to allow good drainage and good moisture retention. Fortunately, there are materials that readily fit this description – pumice, lava, zeolite, haydite, etc.



We also need some organic material, not so much to “feed” the plants, but to act as a catalyst for holding and hosting micronutrients as a component of our fertilizing regimen. Regular soil/compost breaks down too fast, and can act as a sponge to let water and salts from fertilizer to build up. However, fir bark fills the bill. It lasts for 3-5 years and is a great host for Mycorrhiza and other beneficial microbes that develop in healthy, well-draining and aerated soils. Fine “orchid bark” is usually made from fir bark, and with a little more crushing/chopping, can be of the perfect size for our substrate.

It really can be this simple. Now I know people add more to their “soil” recipes – out of tradition, misinformation, conformity, what have you. But if you honestly look at the goals of what we are trying to achieve, there really is no scientific or practical need to add anything else. While other ingredients may not hurt, they also don’t necessarily help. If we can make our substrate a consistent, efficient no-brainer, then we have more time to focus on other important aspects of Bonsai.

Now, here’s the kicker – using porous, inorganic materials as a substrate for growing plants is nothing new. In fact, it is the result of years (actually centuries) of horticultural research and the basis of modern, efficient agricultural practices in this country and many others. It is called soilless culture, and is used for many large scale nurseries and in hydroponics. It is seen as a viable and superior answer for those areas facing loss of soil or poor quality soils. In fact, by using porous substrate, frequent watering and regular fertilization, we are actually practicing a form of open system hydroponics. What is most important about the use of soilless culture is that it easily meets our true goals, which are…

* Consistent control of our Bonsai’s substrate
* Less risk of over or under watering
* Non – collapsing particles
* Excellent CEC and fertilizing consistency
* Allows us the ability to maximize growth

Pumice is great, because it does not float or wash away, it’s not too light or heavy, it has good retention and release of water and a good CEC, and it has a good appearance. Pumice is often the material of choice used for large-scale soilless agriculture facilities.

Interestingly enough, none of the literature regarding modern agricultural practices that I have seen discusses the use of clay materials, which leads to my next blaspheming volley regarding soil. Why do we need Akadama? I hear the ruffling of feathers already. I understand that there are many books, Bonsai enthusiasts and senseis that sing the praises of Akadama. But, remember our goals, and remember that it’s not good to just do something because others do it, but because there is proof behind it. I respectfully submit that, other than anecdotes and tradition, there is no SCIENTIFIC proof that Akadama is a superior or the best material to be used for Bonsai. Actually, quite the contrary.



If we understand the goals and theory of soilless culture, Akadama is not an appropriate substrate. In the numerous horticultural research papers regarding soilless agriculture I have reviewed, not one mentions the use of clay materials as a suitable substrate. As mentioned above, soilless agriculture needs non-collapsing materials. The quality of Akadama can be inconsistent, with some beginning to break down in 6 months and most within two years, accelerating the need to repot sooner than we should for most bonsai material. Akadama also has a poor CEC (pumice and lava both have a better CEC). I have also heard claims that roots “grow through” Akadama, improving root growth. However, the roots are not necessarily growing through the Akadama, but rather I believe that the Akadama is collapsing around the root, just like when you pull a plant out of hard, adobe clay soils and there are mud balls clinging to the roots. I have seen this when transplanting trees out of Akadama.

Whether the root is growing through the Akadama or not, I don’t see root “dingleberries” as a good thing, especially when you have to transplant more often and the clay balls make it harder to comb out roots without injuring them. Having a consistent particle size of irregularly shaped material actually achieves the same goal of making feeder roots work harder and grow better.

Some even say that Akadama provides certain nutrients as it breaks down (I guess as an excuse for Akadama breaking down too soon). Again, there is no verifiable evidence of this. Furthermore, the goal of our substrate is to be a delivery system for the fertilizer and nutrients we add and control, not to be the nutrient itself. There is just not enough substrate in the average bonsai pot to perform that task, nor do we want it too. On top of all that, it is an expensive material.

I believe that, if someone insists on a clay particle in their soil, they would be better off using Turface MVP. Turface MVP is used for sports, such as baseball, where it is used in the infield. Turface is a clay material fired at very high temperatures, with very high absorption and retention rates. It has a consistent particle size and is angular in shape. Turface has a better CEC than Akadama, and can last for 10-15 years, which prevents soil breakdown.

So, over the last few years of research, my simple “soil recipe” is as follows:
* A mix of lava/pumice for my inorganic component
* Fir bark for my organic component
* For conifers/junipers, I mix 3 parts inorganic to 1 part organic (25% organic).
* For deciduous/non-conifers, I mix 3 parts inorganic to 2 parts organic (40% organic)

That’s it. Consistent. Simple. And very happy plants. Again, we really can make life simple when it comes to substrate, if we just focus on the goals we are trying to achieve, and use accepted horticultural practices proven by scientific method. I can worry about other stuff now. As a caveat, keep in mind that, with substrates, you need to be either in or out of one school or another. With soilless culture, you HAVE to water frequently and you HAVE to feed regularly and consistently. If you have more organics, grit, clay, etc. in your soil mix, you will have to devise your own watering and fertilizing schedule, because you have more inconsistent variables to deal with. By the way, most people do not feed their Bonsai anywhere near enough! But that’s another story ….

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  1. Jean-Christophe NEF says:

    There seems to be a good idea to add some part of charcoal intoo the mix, the charcoal have good texture for retening bacterias of the soil and keep water too.

    • Ricardo Almaida says:

      bonjour Jean
      you say charcoal? Under conditions of chunks or crushed into powder? I am very keen to try it.

    • Vern Smith says:

      I have seen claims made by companies that sell charcoal as a soil amendment, and then repeated in gardening blogs with anecdotal embellishments, but I have yet to see any significant, unbiased information on the benefits of charcoal as a soil amendment. It does not remove toxins from inorganic fertilizers, and is not a very good source of potash unless it is burned. It may add water and oxygen retention, but this can be accomplished with lava/pumice more efficiently and cheaper.

  2. Than Nguk Thong says:

    thank a lot Vern
    so … whether all this can be applied also for us who live in a tropical country like Thailand?

    • Vern Smith says:

      I am in a temperate/Mediterranean climate zone, and the plants are not as “tender” as tropical species. While the goals of moisture retention and oxygenation generally remain the same for all plants, and many tropical plants grow very well in lava-based soils, I would think a higher percentage of a finer sized organic component, native to the tropics, would be appropriate. One such “soil recipe” that I found at a tropical botanical garden site can be found at , and I’m sure a more thorough internet search would yield many more. However, I still believe a simpler, minimum ingredient mix that focuses on soil as a substrate instead of a source of fertilizer and nutrients is best for potted plants, and I will share my personal thoughts on fertilizer in a future article.

  3. Palo Alto Bonsai says:

    It can be practiced for all tree species? Or just for Juniper?
    Thanks you Oyakata Vern Smith, great article and very useful.

  4. Vern Smith says:

    I have had good luck with this substrate mix for all plants that I use here in California, adjusting the ratios of inorganic and organic components as discussed in the article.

  5. Luigi Art Bonsai says:

    You amazingly come with perfect writings. Regards for revealing your articles.

    • Vern Smith says:

      Thank you for your kind comments…the more I learn, the more I want to learn! I consider myself a student of Bonsai, and, if I’m smart enough, I always will!

  6. Shawn Silbaugh says:

    I would not use any organic component to be totally honest, yes for charcoal though as some tree’s need it & it retains water very well. The best tree’s with the most health i’ve seen have all been in soil with no organic. Now that said, if you lived in a VERY hot area then yes it might be ok but if you look at all the top tree’s in shows you’ll find very very few with organics.

  7. Daugh Russen says:

    I am extremely appreciated this articles. Its an informative subject. It assist me quite a lot to resolve some problems. Thanks you very much

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