Re: clay, laterite and vermiculite

To continue...

I use neither laterite nor any Dupla product. It seems reasonable to
assume that Dupla wouldn't have you adding drops to the tank water
if the iron was available from the laterite. Perhaps laterite is the
best product that they could come up with that held phosphates and
other useful anions under "wet" conditions until they are needed by
the roots. One can envision, say phosphate, being brought down into
the gravel via convection and being trapped there.

Another quote....
"Laterite soil is a soil of permanently moist tropical forests and
has a layer of laterite of some depth in the profile."

Note that laterite doesn't seem to contain much manganese.

Western Australia is home to lateritic sands and sandstones. Isn't
this the place that Horst Kipper founded Dupla and where he lives?
Someone should write him and ask him for a chunk of sandstone. By
the way, is laterite a reddish brown?