@Joe Dupre' ,
I used to suggest the indestructible silica based gravel [ used down here for sand blasting and
mixing with cement to make concrete ].
Then I found out it might not be available all over the US.
I would use composted pine bark, because it is able to fulfill the biological section for
roots and microbes.
BUT I live in the Tropics and that might mess you guys up.
I would not use a clay based compound like Akadama, simply because, clay
down here encourages fat non feeder roots.
Good Day
Anthony
* Plus if you read the old books out of Japan, Akadama was a Tokyo zone
practice, other parts of Japan used volcanic based sand.