"What's so special about akadama, anyway?"

     If you're like me, you've wondered that more than once. Articles in bonsai magazines, professionals giving talks, bonsai blogs and websites, all urge us to use akadama in our potting mixes. And testimonials back up the claim that our trees’ root systems will thank us if we do. But akadama is expensive (partly because outside of Japan it must be imported.) Is it worth the cost?

"Double Red Line," one brand of akadama.
There are others available as well.

I had the chance to find out just what it is that makes akadama different, and so good for plant roots, in a recent live-stream broadcast from Bonsai Mirai.  In a word, it’s the structure.

Akadamatsuchi, in Japanese, means “red ball earth.” Akadama, the soil-mix component, is a clay product, dried but not fired. (Despite what a label sometimes says.) Like all unfired clays, it has a good cation exchange capacity, or CEC. For any unfamiliar with the term, the essential thing to remember about CEC is that the higher a substance’s CEC, the more nutrients, in usable form, it can hold on its surface where plant roots can get at them.

But there’s more to akadama than that. Almost all clays, the world over, are composed of microscopic plates. The structure is not very flexible, and when particles swell and shrink due to wetting and drying, they tend to fracture along the plate boundaries. This results in a greater and greater number of smaller and smaller particles in a given volume, and that means an exponential increase in total surface area within that volume of clay. More surface area means more water is held. But because of the plate structure of most clays, the total air space within that volume does not increase along with the water retention. The result: soggy soil.

Mining akadama in Japan.

Akadama is mined in one small area on Honshu, the Japanese main island. There, a particular combination of soil minerals and heat from subsurface volcanic activity come together to produce a clay with a structure based on tiny tubes rather than tiny plates.

To borrow Ryan Neil’s imagery: imagine a game of pick-up-sticks, where every stick is a narrow tube. Toss the tubes and they come down in a disorganized pile, with individual tubes running every which way. Now pick up your pile of randomly-oriented tubes and squeeze out the spaces between the outer walls of the tubes. What you end up with looks like an (oversized) akadama particle.

The tiny tubes – tubules – are still large enough for the smallest root hairs to grow into their water-vapor-rich and nutrient-rich interiors. Once a root hair does that, it keeps growing until it fractures the tube from the inside, and fractures the particle. The two smaller particles created by the fracture each have their own tubule systems, and new root hairs emerging from the sides of the original root hair (now becoming a rootlet) grow into them. They grow thru them, grow large enough to fracture them, and their side-emerging root hairs start to colonize the new, smaller particles that result – and the process goes on and on. Air spaces are getting smaller at the same time, but the root hairs being produced in growing abundance are still small enough to make use of them.

The result of all this is a compact mass of densely-branched, thriving roots – just what is needed for healthy, vigorous bonsai!

Not all the akadama on the market is of the same quality. Given what akadama costs in many places, it’s a good idea to check the quality of a batch before you buy. Ryan Neil’s suggestion: look at how much dust is in the bottom of the bag. If there’s very little dust, it’s good akadama. If there’s a lot of dust – move on.

And not every plant species thrives in akadama. Common juniper, Juniperus communis, grows thruout the northern hemisphere’s temperate and cold zones, and can make a very good bonsai (especially if obtained as yamadori.) But it should never be potted in akadama: for reasons no one yet knows, akadama is the kiss of death to common juniper.

Sooner or later, the supply of akadama is likely to be exhausted, and it costs more than many hobbyists care to pay anyway. What can be used as a substitute? Diatomaceous earth (horticultural grade) has a CEC almost as high as that of akadama, and a somewhat similar tube-based structure. Ryan Neil is experimenting with it in his potting mixes, and others are as well. I’m looking for sources willing to sell it in small quantities. (No success so far.)

Is Japanese akadama unique on the planet? Not quite, it turns out. Ryan Neil told a little story: He sent a bag of akadama to a soil scientist at a California university, asking for a full analysis. After running all his tests, the scientist told Ryan that there is one other place on earth (just one) where the mix of minerals and the level of volcanic heat are such as to produce an identical clay: Mt. Hood, Oregon. A local bonsai practitioner then went up to Mt. Hood, dug out some local clay, and sent it to the same scientist for analysis. The result: there was no difference between that clay and Japanese akadama, none.

Hoodah thunkitt? 

(You can watch the Bonsai Mirai stream “Soils” for yourself, free, at this link. Ryan covers more than just akadama.)

:-)  :-)  :-)