Published research on akadama clays

I’ve become a bit obsessed with Akadama lately and would like to share some potential insights into why it is such an important component of bonsai soils. Some of this may be new info while some of it has already been discussed in the excellent resource at the link below. I am very much an amateur when it comes to bonsai, so please let me know when I am off base. Sorry for the long post. If there is interest, I could write something a little more formal describing what I’ve found in the literature.

Akadama resource from DonPettite: https://www.arcgis.com/apps/Cascade/index.html?appid=6f0b256d0201451483f224d313109010

There has been much discussion in this forum lately on the microstructure of the akadama and how it may influence the branching of fine roots. I don’t have much to add to this discussion. However, my research took a turn when I started looking for information on the primary clays of imogolite and allophane which make up the akadama. Studies on akadama in English are quite rare, but imogolite is a bit of a hot topic right now in materials research and allophanic soils have become well researched over the last 20 years. These unusual clay minerals form during the weathering of the volcanic deposits creating unique soils properties. Imogolite comes in the form of long fibrous crystals (seen in scanning electron microscope images growing like worms directly from grains of volcanic glass, see picture below from Eswaran 1972), while allophane comes in the form of hollow spherules.

More advanced weathering of the volcanic deposits results in a variety of imogolite morphologies, as illustrated in the below figure (also from Eswaran 1972), which shows dense globular clusters (a,c,d) and less tightly packed fibers (b). Eswaran (1972) identified the imogolite fibers as being on the order of .2 to .8 microns in diameter and stated that the reported nanometer-scale diameter of the fibers are possibly the result of intense sample preparation techniques utilized in TEM research that may physically alter the amalgamation properties of the imogolite. The Eswaran study utilizes basic scanning electron microscopy which typically requires little to no sample preparation beyond coating the sample with a thin conductive surface. This suggests that the real-world form of imogolite (such as in our bonsai pots) is not the nanofibers that we have previously seen but amalgamations of fibers closer to 1-micron diameter and even more likely to occur as dense clusters of fibers tens of microns in diameter.

A potentially more significant aspect of the imogolite clays is their interaction with biological materials. A recent boom in research has explored the interaction of imogolite with biological molecules. For example, imogolite has been shown to bind with DNA (see Jiravanichanun et al. 2012 figure below), resulting from the affinity of the negatively charged PO4 of the DNA to the positively charged AlOH groups on the exterior of the imogolite tubes. This interaction likely occurs with a wide variety of negatively charged functional groups on biological molecules within the bonsai pots.

Imogolite has also been shown to be an extremely efficient enzyme immobilizer, which is an inorganic substance that can bind to the enzyme and concentrate it. So, using akadama in our bonsai pots may be allowing for higher than normal amounts of biochemical activity due to the concentration of enzymes within the pot. This is in addition to the typically discussed cation exchange capacity (CEC), which details the ability of the grains to hold exchangable ions. Enzymes are proteins which allow for drastically increased biochemical activity rates (life is not possible without them). For example, the imogolite/enzyme hybrid system described by Wang On Ya et al. (2010) suggests the formation of complex gels of imogolite and an enzyme (in their case pepsin, a digestive enzyme that breaks down proteins) which may result in higher activity than the pepsin alone due to the higher concentations when bound to imogolite. The imogolite/enzyme system seems to be theoretical at this point in the literature but would likely aid in growing beneficial microbial life within the bonsai soils by capturing the important components of fertilizer and producing the conditions for high biological productivity. Essentially, utilizing akadama in a bonsai pot may be creating a sort of bioreactor with high microbial productivity.

Kazue Tazaki (very well known in geomicrobiology research) described abundant and diverse microbial colonies inhabiting imogolite gels on weathering pumice in Japan (Morikawa and Tazaki 2003; Tazaki et al. 2006). The figures below are from Tazaki et al. 2006 and show microbial cells partially and completely encased in imogolite films or gels. These papers described microbial colonies living within imgololite gels within the pumice which were covered with poorley-ordered “bio-imogolite” particles precipitating on the negatively charged structural biopolymers of the microbial cell walls. This may be the original source of the imogolite within akadama soils horizons and this process likely continues during the use of akadama as bonsai soil. However, my bias as a geomicrobioloigst may be clouding my judgment on this point. To a hammer, everything is a nail. To me, all secondary minerals have some microbial influence.


Let me if the info of this post is particularly new or helpful to any of you. I’m sure more research can provide better information if there is a desire for it. I’ve only scratched the surface of the available literature on imogolite and didn’t even look into allophane.

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Wry interesting! Thanks

Thank you for detailing the hidden majestic microbial world found in our precious soils! Its amazing to see how much action is going on in one grain of “soil”.

On one hand this research makes me think we will be hard pressed to find an alternative to akadama; and on the other, perhaps the new akadama will be an artificial nanotech we can print at home. As long as the imogolite doesn’t bind to our DNA and turn us into clay zombies

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Very good summary that adds to our understanding of Akadama. I particularly liked the comments about the tube sizes as it relates to the sample preparation method and the “hammer and nail” analogy regarding how we view information. If the tubes in Akadama are in the submicrometer range rather than the single digit nanometer range then I can definitely envision root hairs and tips penetrating the interior to break them down as well as the interstices between the tubes.

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Thanks, Marty. The first two figures show standard SEM images of freshly broken pumice grains. So that is most likely what the form the imogolite takes in the bonsai pot and the roots can penetrate into the clusters and break them into individual fibers. TEM imaging preparation is very intensive to separate to create a single layer so the electrons can pass through them. Seems reasonable that this is altering or separating them in a way that is not how they would likely occur in the pot or in the natural soil.

Nobody wants clay zombies!

I have done both SEM and TEM on ceramics so I understand the sample prep. Luckily far more SEM.

Awesome! SEMs are the best.

It took 30,000-60,000 years to produce Akadama from Kanuma Pumice. I don’t think that will be replicated in a laboratory, and like artificial gemstones, the cost would likely be higher than the natural examples.

Thanks for posting a link to my Story Map above. I do believe that analogous materials may be present on the wet sides of the Cascade Mountains. The key is understanding how they formed now that we know what they are in hand specimen and microscope.

Unfortunately, most of the literature that could be scoured for clues is either behind science journal paywalls or has never been translated into English from the original Japanese studies.

What little I have gathered seems to point to airfall pumice deposits that weathered in lowland areas. I suspect it may be washed into old river valleys like the banks of the Toutle River in Washington. But there appears to be a groundwater leaching component to it.

I am always looking for new insights into the material or it’s source/setting…and so thanks for the deep dive. My summer will be spent scouting the Mount Hood drainages for old decrepit volcanic deposits.

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If you ever need a paper that is behind the paywall, you can either contact the corresponding author and they will send it to you or you can message me and I can get it. Its easy and free through university libraries.

Airfall pumice seems to be correct. I read several papers which mentioned remnant pumice grains found within the akadama grains. The first picture I posted with the imogolite growing from it’s surface is an example of this.

Sounds like a fun summer plan!

I was just watching the Repotting Vocabulary live stream. Ryan mentions that top dressing “protects the akadama from decomposition”. Does anybody have an idea why the top dressing would help with this? Does Ryan ever come onto the forum to answer questions?

He doesn’t really. But you could always ask during the live q&a.

I think top dressing helps because it helps keep the akadama hydrated at the same rate as the rest of the soil. The wet/dry/wet/dry cycle (as well as freeze/thaw) can lead to premature breakdown of akadama at the surface.

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That makes perfect sense. Thanks.

Also, perhaps just covering the akadama with something else prevents spraying water from shifting the surface akadama around. Less agitation, less friction, less wear, less decomposition (at least around the top surface of the soil mass that’s directly exposed to spray/agitation during watering).

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