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mixed substrates and clay vs laterite

Kind Correspondents,

The question of just what is wrong with mixing different substrate
materials is a good one.  I can think of a few reasons for discouraging
mixed substrates:

	1)  All of the basic substrate recipes that we usually talk about
on this list have a proponent who is successful using the substrate and
well known for their advise; those proponents aren't as willing to provide
advice when problems come up with combinations of two or more of the basic
recipes.  When you mix things up yourself you can expect to wing it when
problems arise.

	2)  A few combinations of materials might be incompatible; peat
and laterite for instance aren't found intimately mixed in nature and if
they are mixed in an aquarium then acidic conditions set up by the peat
might attack the hydroxides in the laterite leading to problems like
(e.g.) aluminum toxicity.  In the specific case of aluminum toxicity,
acidic mixtures of peat with some clays also might cause problems.

	3)  Some mixtures are simply pointless because one or more of the
materials you might want to mix are actually mixtures themselves.  Clays +
laterite mixes may be an example of this because many clays already
contain some of the amorphous or finely crystalline iron and aluminum
oxyhydroxides which are the main part of laterite.  A better example might
be a mixture of soil with laterite or clay; you start with dirt and when
you mix in laterite or clay the result is [drum roll please] ...dirt.

As to the differences between clay and laterite:

	Your experience with clays dispersing in the water and laterites
not dispersing in the water will vary.  Some clay aggregates are cohesive
and won't come apart under water; in your substrate they will stay intact
and may flatten out like George's Duplarit K balls.  Some are not.  On the
other hand, I had kitty litter disperse in one of my tanks and cloud the
water.  I understand that Dupla laterite is coarsely granular and mostly
stays that way, but other people have used fine, powdery laterite products
and reported terrible problems getting it out of the water and keeping it

	Many clays and clay soils also contain some iron oxyhydoxides and
sometimes aluminum oxyhydroxides as well.  As a result, some clays will
share some of the chemical behavior of laterite.  There isn't a clear
dividing line between them.  However, I expect that grey, white or tan
kitty litter contains very few of the laterite components.  Red art clay
probably contains some.  Red clay soils probably show a gradation from
largely clay to mostly laterite components.

Roger Miller