Thanks for the answers so far! I thought somebody would have experience with this material as it is frequently used in bonsai growing, but maybe I can show it is. You can see what it is here http://bonsainostrum.com/akadama/… It is natural "loam" from Japan, "mined" from the soil where Cryptomeria grows. It is made of irregular grains of a very hard type of clay. pH is about neutral. The grains are very porous and absorb a lot of water and nutrients. They don't break down (they are also used for aquaria sand) and drainage is very fast. The advantage for me is that it is readily available, even if very expensive. The one I got is in granules from 2 to 7 mm in diameter. My problem with perlite is the "lite" part... it just floats away when I water it, and I am afraid that germinating seeds would be constantly dislocated. Fernando