#6 Akadama as a Bonsai Soil Component

Akadama is a very common soil component for Bonsai mixes, but what exactly is it and how unique is it? 

First, it’s important to understand that Akadama is more of a product name like “Kleenex” than a scientific/geological name. In Japanese it means “red ball earth”, probably in reference to its reddish color. Geologically, it comes from a soil called Kanto Loam which is an Andisol (black soil). Andisols are a volcanic soil formed when ash and other material form during a volcanic eruption. Andisols are actually the rarest order of soil with only 1% of ice-free land being occupied by Andisols. In the US, we have some Andisol deposits in the northwest near our famous volcanoes like Mount Saint Helens. [1]

What is the Chemical Composition of Akadama?

Akadama is mostly made up of Silicon dioxide, Aluminum oxide and Iron oxides. The percentages in the table below only add up to ~58%, so I’m not sure what the other components are. It could be organic compounds that burn up in the composition analysis or just other materials not reported. The percentages below can vary based on the location from which it is mined. [2]

Figure 1: Akadama composition analysis

Aluminum, silicon and iron are the three most common elements in the earth’s crust, so this is not super unique. Most of these components are metal oxides (binary molecules with oxygen as one component) and the metals are not accessible to plants in their bound form. So even though there is a high percentage of aluminum in the soil (which is toxic to plants), it is not available to the plant to absorb. [3]

How is Akadama made?

Akadama is mined from the northern part of the Tochigi prefecture in Japan. It is about an hour from Tokyo. This article from a Japanese manufacturer of Akadama is excellent for seeing the different layers of the andisol that they use for different products like garden soil, akadama, and kanuma/pumice. [2]

After it is mined, it is dried in the sun under plastic. Once dry, non soil components are removed and it is sifted into different sizes. Once sifted, it is actually baked with an oven-like process to sterilize and harden the akadama for hard akadama variants. [4] Finally it is packaged and made available for sale! I highly recommend checking out the video linked in citation 4 as it is directly from an Akadama manufacturer and shows the end to end process.

Experiments

I did a few experiments that I’ll summarize below to gather some additional data on Akadama. I may discuss them in more detail in future articles so others can reproduce and share their results. I wanted to measure various properties of Akadama and compare it with other common soil components like Pumice and Lava Rock to understand Akadama relative to other soils we are familiar with.

Experiment 1 - pH

To measure pH of a soil component, people usually make a slurry of the soil + water and then stick in a pH probe. For my experiment, I mixed equal weights of Akadama + rodi water for 15 minutes and then took a reading. I got a reading of 6.5 for Akadama. This matches fairly well with the literature I’ve read showing Akadama is fairly neutral to slightly acidic.

Experiment 2 - Water Retention

This experiment involved weighing dry, packed Akadama, and then comparing it to fully saturated, packed Akadama to see how much water it holds per unit volume. I tested this at two different particle sizes for the soil. It is important to note that I was measuring the properties of the packed aggregates (particles as close together as possible for maximum packing density, similar to what you’d see in a bonsai pot). I poked drainage holes in my containers to allow water to drain from the packed mixture.

Figure 2: Amount of water held in equal volumes of soil across different soil types.

I was surprised that the aggregate Pumice held the least water, but was not surprised to see that Akadama held the most and that smaller particle sizes held more water. This is probably due to small pore spaces between particles allowing for more water to be held.

Figure 3: Results of reweighing the soil after 60 minutes.

Pumice lost almost 25% of water within 1 hour while small Akadama barely lost any moisture. This testing definitely aligns with the traditional belief that Akadama absorbs and holds a lot of water compared to the other bonsai substrates.

Experiment 3 - Crushability

I don’t have any fancy equipment for this, I just wanted to see which particles I could crush between my fingers. The Akadama crushed easily into a powder, Pumice would sometimes break, but would break into discrete chunks. Lava did not break before my measly might.

Properties of Akadama

Cation Exchange Capacity - 22-46 cmol kg-1 (From literature) [5]

pH - 6.5 - 6.9 (From testing and literature) 

Water Retention - High (relative to Pumice and Lava) (From testing)

Crushability - Easy to crush/breakdown (From testing)

Thoughts for Bonsai

It was interesting to learn more about Akadama, but ultimately there aren’t a lot of scientific studies on how it affects plant growth specifically since it’s such a niche product. I did want to compare it to other soil components so we can see what part it plays in our bonsai soil mixes. Akadama holds more water for longer than Pumice and Lava, so it contributes to a wetter environment for the tree in the pot. It also breaks down more easily than the other components. One feature of it breaking down is that it increases water retention over time as we can see by the medium particles holding less water than the small particles.

I have heard anecdotally that people believe this leads to creation of finer roots, but I haven’t found any studies yet to back that up or disprove it. I would love to do an experiment in the future where I measure density of rooting in different soil mixes to see if we can find evidence of this.

One thing to keep in mind is that while Akadama is higher in water retentivity and CEC than Pumice and Lava, it actually is much lower in both metrics than a traditional potting mix. The CEC of most of the main components of potting mix like humus, vermiculite or peat moss is closer to 200 (4-10x more than Akadama). [7] Pumice, Lava, and Akadama hover in the 10-46 range for CEC. In my water retention experiment, it was tough to measure the water retentivity of regular soil because it didn’t drain well in the containers I used, but it held at least 50% more water than Akadama in the same volume of soil. 

Akadama seems to maintain a good balance of retaining moisture while still allowing for aeration and easy drainage. The research in this article affirms that there are unique characteristics of the planting environment/soil for bonsai plants compared to regular gardening or agriculture that deserve more research. Hopefully understanding Akadama better can help us dial in our usage of it within the bonsai pot, increasing or decreasing the amount based on how wet the plant likes to be and how much storage of nutrients we want to enable within the soil.

Citations

[1] The Twelve Soil Orders. Soil Orders | Soil & Water Systems | University of Idaho. (n.d.-a). https://www.uidaho.edu/cals/soil-orders 

[2] Bonsai ～Akadama-Tuchi, kanuma-pumice～. Introduction of AKADAMA and KANUMA-TUCHI. (n.d.). http://www.tachikawa-heiwa.com/Introduction.html 

[3] Ofoe, R., Thomas, R. H., Asiedu, S. K., Wang-Pruski, G., Fofana, B., & Abbey, Lord. (2023, January 13). Aluminum in plant: Benefits, toxicity and tolerance mechanisms. Frontiers in plant science. https://pmc.ncbi.nlm.nih.gov/articles/PMC9880555/ 

[4] YouTube. (n.d.). 「產地直擊」. YouTube. https://youtu.be/UA8GF5MADAU 

[5] Andisols from four different regions of Iceland - Arnalds - 1995 - Soil Science Society of America Journal - Wiley Online Library. (n.d.). https://acsess.onlinelibrary.wiley.com/doi/abs/10.2136/sssaj1995.03615995005900010025x 

[6] Management, N. (n.d.). Nutrient management program lab. Overview - Nutrient Management Program - University of Florida, Institute of Food and Agricultural Sciences - UF/IFAS. https://soils.ifas.ufl.edu/nutrients/overview/ 

[7] Uidaho. (n.d.-b). https://www.uidaho.edu/-/media/uidaho-responsive/files/extension/topic/nursery/technical/cec-and-cn-ratio.pdf?la=en&rev=c2bb3db21bb0499f9792fe310c497a23