Akadama Structure and Fine Roots

Ryan mentioned during a BSOP live stream that the tube-like microstructure of the akadama allows for small scale branching of the roots to occur. However, I study rocks and fossils with scanning electron microscopes and I am not aware of any study that shows this to be the case. Can anybody enlighten me as to where this idea comes from?

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I think he said in some podcasts or something that he sent some akadama to be studied by a university extension under a microscope that made that determination. As such, there might not be a published paper or study for that yet.

But, as mentioned in one of the recent podcasts (the one about worm castings) there was a webpage that was brought up that discusses akadama more. I looked at the webpage and it definitely has some interesting information for it. Now, I’m not a geologist by any means, so I can’t vouch for it 100% but sounds very well researched to me.

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

It does have photos of akadama tubes under a microscope and a good explanation of how it is created.

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That was an excellent read, thanks @nmhansen! I found some of the photos online:

Caption: “Micrographs of (A) allophane and (B) imogolite (external diameter of nanotubes is ∼2 nm)”

Caption: “Diagram of imogolite nanotubes and Al-rich allophane nanospheres, which have similar structures at the atomic scale”

I’m not sure if Akadama is “allophane” or “imogolite” or both (I’m new to these terms). But the Wikipedia page for imogolite says

"Imogolite consists of a network of nanotubes with an outer diameter of ca. 2 nm and an inner diameter of ca. 1 nm."

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“Akadama is dominated by allophane-imogolite clay minerals with subsidiary Al-humus complexes whereas kanuma is dominated by Al-humus complexes with subsidiary clay minerals.”

So, yes, Akadama has imogolite and allophane content, which is composed of nanotube structures.

houstonbonsaisociety.com/wp-content/uploads/2014/11/Inorganic-Components-Reference-Sheet.pdf

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Thank you for the excellent resource nmhansen! It was certainly a good read. From what I was able to gleam from the info, the tubules form from the weathering of the pumic (volcanic glass). This is standard weathering of silicates. However, the SEM images of the allophanes and imogolite show the scale to be ~2nm in diameter! That is WAY too fine scale to be allowing roots to grow through.

It is more likely that the alteration of the pumice grains is creating holes which may become interconnected enough to allow for the enhanced root branching effect that Ryan speaks of. Just a thought. It certainly needs more research!

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I think the roots grow between the tubules and it is the tubular nature of of the akadama that prevents it from turning to mush. The tubules act like the threads in a cloth to hold the structure together. Yes, the roots can penetrate the weak spots, but the remaining tubules will help keep the akadama in a particulate form (albeit smaller particles). For reference the minerals that make up the clay from which we make our pots are typically flat plates that are 0.7 - 2 nm thiick.

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I am glad the website (StoryMap) is getting some use…it has languished for 2 years since I built it. The impetus was largely to find suitable alternatives and an excuse to go see some ancient volcanoes.

The goals of the site were to document what I was finding on the origins and genesis of Akadama and Kanuma soils in an attempt to more easily find analogues here in Oregon…and to get feedback on either the actual mechanisms of its formation or possible locations to study here.

I didn’t make that easy by leaving out a means of easy contact…but now I would like to gain any insights/feedback via this site. So if you have any knowledge about the Japanese quarries or similar soils here in the state…let me know.

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This study of European beeches https://doi.org/10.3389/fpls.2017.01194 reported a minimum root diameter of 1 mm or a million nm.

Neat paper. I looked at several of the graphs and noted that the youngest roots that they reported upon are 2 years old. I am thinking that younger roots (and root hairs) are most likely smaller.

Lots of information on root hairs in Arabidopsis, a small weed which is the fruit fly for plant scientists, here https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3243358/#!po=52.3256

Arabidopsis root hairs reported to be 10 um or 10,000 nm. Tiny nm pores are not in play for plant roots or root hairs.

I think what @MartyWeiser was saying is that the tubular nature of the weathered akadama allows for the structure of the soil to stay intact and the fine roots can grow and branch their way through the fine mesh of tubules. Not that the roots are growing through individual tubules. I think this is the most likely scenario in which the roots interact with the akadama in a positive way. However, I would like to see some scanning electron microscope images of a weathered akadama grain to determine what the surface of the grain looks like after significant weathering. That surface morphology may impact the roots in some way as well.

Can someone explain how silicates weather? The pumice I have seen seems very indestructible. From the info I have read about akadama, layers of volcanic stuff were laid down, covered over by soil and organic growth. After eons of rain the pumice was weathered and left layers of akadama and kanuma. I am not clear now which one is always on top.

Knowing that volcanos can blow up a bunch of different stuff, Maybe it is a particular type of pumice that can be leached or weathered. So mstrange or DonPettit what is being weathered? And what is meant by “weathering” in this case.

Also, if weathering (or constant washing) is important here, how wet are the lowland valleys in Japan compared with eastern Oregon?
Thanks

A lot of good questions…will try to knock off a few.

Silicates weather differently depending on where/how they were formed, their composition and what they are exposed to. Some silicates weather simply because they are brought to the earth’s surface…and no longer stable at low pressure. Pumice is formed from extreme depressurization of a magma no longer “corked up” in the magma chamber. At their birth, they are essentially at normal surface atmosphere…so pressure is not the reason for their breakdown.

Pumice is a collection of volcanic glass shards, so they have tremendous internal surface area and low density. Compositionally, they are similar to the granite or andesite that most of the volcano is made of. The Newberry Crater and Mount Mazama (Crater Lake) pumice flows are relatively young. If what I learned about the Kanuma Pumice is correct, it is on the order of 60,000 years old, and in a very humid/temperate environment.

All pumice will weather in time, especially if exposed at the surface and exposed to the elements. The eastern Oregon pumice flows are not inundated with rainfall as the western slopes of the Cascades. The Oregon Cascades have many older volcanic deposits, but most are covered by younger deposits.

In the case of Akadama/Kanuma soils, they have been heavily leached of everything but silica and aluminum by rainfall or groundwater. Exactly how is what I am still trying to determine. The black organic soils likely contribute humic acids to help with that leaching. I had first thought that they might simply be airfall pumice deposited where they now are found, and were leached of their mineral content. I am thinking now that they may actually be more similar to a volcanic debris flow (primarily of pumice washed from the countryside) that settled into a valley setting and formed shallow terraces just above the water table.

Tying this down probably seems academic, but I think it will give us clues where to look. Are we more likely to find similar soils on the flanks of Mt. Hood (like Parkdale) or the lowlands of the Sandy River Valley…

If there’s akadama, it’s on top of kanuma as it is from a more recent pyroclastic event. Normally there’s kurotsuchi/kuroboku on top of it all. Kuroboku is sold in Japan, but I’ve never seen it exported.

The book “Clay and clay minerals of Japan” has a few paragraphs on Kanuma pumice and Imaichi pumice (aka Akadama).