Akadama

Akadama is a granular clay dug from volcanic soil in Japan, then dried or fired and screened into different particle sizes for use in bonsai soil. It is valued because individual granules hold water and nutrients inside their structure while still leaving air gaps between particles, so a mix built around it can stay moist without becoming waterlogged.

Growers usually blend akadama with pumice and lava rock rather than using it alone, adjusting the ratio for species and climate. Deciduous trees and moisture-loving species often get a higher akadama percentage, while conifers that prefer to dry out between waterings get more pumice or grit. Particle size matters too, with larger grades common in bigger training pots and finer grades reserved for shallow display pots.

One downside is that akadama granules break down over time, especially the softer, unfired grades, and as they break down they can compact and reduce drainage. This is one of the main reasons bonsai are repotted every one to a few years depending on species and pot size, rather than left indefinitely in the same soil.