In nature soil is a living and non-living union of organic matter, sand, silt, and clay. When we grow bonsai, we make our own “soil” which is a misnomer, a more appropriate name would be a growing “medium” rather than soil. Traditionally potted plants that you find at your local garden center are growing in a combination of ground up bark and components like perlite. This type of medium does well for containers but is not considered ideal for bonsai cultivation. When practicing bonsai, we are trying to grow trees in small containers for long periods of time which demands a very specific growing medium that promotes fast drainage and a fine root structure.  Garden center mediums generally develop course root structures, become compact over time, and often do not have adequate drainage. Over the decades bonsai mediums have become quite refined using mainly three components: akadama, scoria and pumice.

Akadama is a clay derivative from Japan that is responsible for retaining water and nutrients. There are five different clay types, but for the sake of simplicity I averaged their cation exchange capacity (CEC) which came out to 40-85 meq/100g (Havlin et al., 2013). CEC stands for the ability of a substance to hold onto cations. The higher the CEC the more nutrients a substance can retain. In the case of akadama it has above average CEC. Below is an image of akadama.

Scoria generally has a low CEC due to its inert nature. Black scoria has a CEC of 10.7, red scoria has a CEC of 28.5 and yellow scoria has a CEC of 60.1 (Bar-tal et al., 2019). Scoria often has a large particle size which provides drainage and aeration to the growing medium. Aeration provides oxygen to the tree’s roots which is necessary to facilitate proper growth. Aeration in the soil also prevents roots from rotting and combats pathogens like phytophthora. Additional to these benefits scoria often promotes a fine root system which is ideal for bonsai. Below you can see a photo of scoria.

Next, we have pumice which is a highly porous and airy form of lava rock. It has some water retention, provides great drainage and has a above average CEC. One study analyzed a sample of pumice and found the CEC to be 73 meq/100g (Kantiranis et al., 2007). Keep in mind there may be some variability in the CEC values of pumice because of the multitude of variables when analyzing different sources. When utilizing pumice for bonsai it provides a great middle ground between the qualities of scoria and akadama. Below we have a photograph of pumice.

Creating your Perfect Medium

Now that we have analyzed our main three bonsai medium components, we are ready to start making our mixture. You can customize this mediums ratio based on several variables like weather conditions, tree type and your watering schedule. For example, Los Angeles is an extremely warm climate with moderate humidity so the medium created should have higher amount of water retention. We can increase water retention by adding more akadama to the ratio. Next you must consider the species type because some trees like water more or less than others. Lastly, you must consider your watering schedule. If you are only able to water once a day, then you will want to create a medium that will hold enough water until the end of a hot day. If this was all confusing do not worry, a good overall growing medium ratio is 1 part akadama, 1 part pumice and 1 part scoria. I recommend experimenting with your mediums ratio with some practice trees until you find the perfect balance you desire.

References

Bar-Tal, A., Saha, U. K., Raviv, M., & Tuller, M. (2019). Inorganic and Synthetic Organic Components of Soilless Culture and Potting Mixtures. Soilless Culture, 259–301. https://doi.org/10.1016/b978-0-444-63696-6.00007-4

Havlin, J., Tisdale, S., Nelson, W., & Beaton, J. (2013). Soil Fertility and Fertilizers (8th ed.). Pearson.

Kantiranis, N., Filippidis, A., Vouta, S., Drakoulis, A., Koutles, T., & Tzamos, E. (2007). THE CATION EXCHANGE CAPACITY OF INDUSTRIAL MINERALS AND ROCKS OF MILOS ISLAND. Bulletin of the Geological Society of Greece, 40(2), 775. https://doi.org/10.12681/bgsg.16718