two points ----- Is Akadama volcanic soil ?
secondly - I have noted that if you use a glazed, but porous clay pot, you can use a richer in organic mix as the pot wicks away extra moisture at the bottom [ the Chinese sell pots that are glazed on the outside, but not underneath.]
Additionally, if I use the above philosophy, the pot becomes an expendable item, throwing away every some many years, but easily replaced.
It is also possible to glaze with a glaze designed to look like stone, see Mayco, Duncan and anyone else.
Or you might be able to apply a coat of paraffin wax to the outside of the pot, which would penetrate the exterior to say 3 mm and not grow strange stuff.
The older books kept suggesting porous pot as the growing pot for bonsai. Stoneware, I wonder when that became the normal pottery body to use?
I will ask at the cement factory, what the material they use as an ingredient is called. It is absorbent, fast draining, plants do well in it and it is resistant to decomposition by roots. Additionally this is the material the world's finest quality cocoa grows in, there is one other place in the East Indies, on the opposite side of the world.
I suspect the US or simply North America has similar deposits.