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Re: Laterite

On Thu, 25 Feb 1999, Steve Pushak wrote:

> I've been slowly catching up on the old APD issues I haven't had a
> chance to read since my move and couldn't resist tweaking James on this
> post:

Welcome back, Steve.  Hopefully, since you're using the list for the
tweaking forum it's OK if someone else jumps in :).

James Purchase wrote:

> > Laterite is:
> >  1.)a type of clay
> >  2.)high iron content
> >  3.)has suffered heavy leaching
> >  5.)under tropical conditions
> >  6.)over geological time.
> >
> > There are many types of clay - the term clay only really refers to the
> > particle size of the material. A number of clays are available which have
> > high iron content - the red colour in some "pottery clay" is generally due
> > to the presence of iron oxides. The unique properties of laterite which made
> > Dupla (and several other companies) sit up and take notice are due to the
> > last three points - highly leached, formed under tropical conditions over
> > geological time.

Steve Pushak responded:

> Here I have to ask my question. Why is being highly leached important?

It makes the material stable and nonreactive.

> Why is being highly leached under tropical conditions

Leached with warm water hence very stable and nonreactive.

> over geological time

OK.  Very, very stable and nonreactive.

> important?

So that it remains stable and doesn't react?  I haven't read The Optimum
Aquarium, I thought it was justified there.  I don't see much inherent
value in laterite, so I can't justify it's importance.

> Surely this will cause the LOSS of almost all the
> nutrient value of the soil!

Yep.  Laterites are infertile subsoils.  Strip off the top soil, and not
much grows on the laterite underneath.  In fact, it's often so rock-like
that it can't be cultivated.

> Perhaps the only important thing about the
> laterite leaching is thus, the LOW concentration of phosphates and
> nitrates???

And most everything else but iron, aluminum, oxygen and hydrogen.  Even
silicon (a major part of most rocks and soils) is largely removed.  The
remainder is (nearly) inert.

> Why even pottery clay and kitty litter have this quality.

Pottery clay and kitty litter (and most other clay products) also contain
significant quantities of sodium, potassium, calcium, magnesium and
silicon and proportionately lower quantities of iron and aluminum; normal
clays shrink and swell depending on water content and composition; their
particles are charged and they may disperse in water or remain strongly
cohesive; they adsorb organic molecules and exchange a variety of ions.
Normal clay minerals can also be quite unstable when they're removed from
the environment they formed in.  Some of the additional elements present
in clays are present in exchangable sites and are available to plants;
others (notable magnesium and some of the iron) may be part of the
structure of the clay minerals and won't be available to plants unless the
clay breaks down.

> Surely the leaching for thousands of years is not critical to this
> factor!

A long period of leaching isn't critical to the absence of phosphates
and nitrates.  It is critical to the other properties of the materials.

You can use the word "clay" to refer to most any very fine grained
material and you'd be correct by one definition of the term, but I think
that usage is deceptive.  Laterite is often very hard, massive and
rock-like and is only fine-grained if it's ground finely.  Clay soils are
almost always very fine grained, plastic when wet and easily disaggregated
under water.  Laterite is made up of iron and aluminum hydroxides while
clay soils are mostly made up of aluminosilicate minerals, which are
completely different beasts.

I prefer *not* to describe laterite as "clay".

As to their fertility -- either laterite or clay taken alone makes a very
poor soil.  If you have clay soil for your garden you probably want to
ship in some top soil to improve things.  If you're a farmer and your
fields are eroded down to the laterite subsoil then you want to move
somewhere else.

Neither laterites or clays by themselves are going to give you a beats-all
substrate.  But either laterite or clay can be used to help when they are
used as part of a good substrate mixture/fertilization/maintenance

In what I've read here, what I know about the chemical differences, and
what I've seen in my own tanks where I have used clays (no laterite yet)
I'll speculate that if you use laterite you may need to maintain higher
nutrient levels in the water than if you use clays, but if you use clays
you may have worse problems with cloudiness than if you use laterite.

I think there are ample differences in these products, and instead of
reading more arguments about their similarities or differences, I'd much
rather read people constructively comparing successful methods for using
them in their aquaria.

Roger Miller