They are made of SiO2, which I would guess, if it could
dissolve in water it would be acid. Since in normal conditions
they're insoluble you could think of them as really tiny
perlite. When I mixed the diatoms half and half with peat moss
I got a mix that was similar in texture to concrete but much much
lighter. Since it didn't seem like it would drain very well, I
added alot of perlite. Filled a large pot up to about 4 inches
from the rim and planted the Cephalotus. This mixture, at first
seemed very wrong, looked great the next day. It seems that yes
the diatoms slowed the drainage of the water but after it finished
draining it hits the perfect wetness. The soil is very fluffy
and so I would guess has alot of air in it.
Dave Evans