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Re: Clay (was Laterite)

On Fri, 26 Feb 1999, Steve Pushak wrote:

> Several of us here in Vancouver have tried a particular type of pottery
> clay called Terrastone which has the following Chemical Analysis:
>  BaO     0.4%
>  CaO     0.5
>  K2O     3.2
>  MgO     2.5
>  Na2O    0.7
>  TiO2    0.7
>  Al2O3  16.1
>  P2O5    0.3
>  SiO2   71.5
>  Fe2O3   4.0
>  LOI     6.5
> There is enough potassium and magnesium that we might be concerned if it
> were in a soluble form. I don't know if this analysis indicates that all
> of the minerals exist only as oxides. I notice that the total is more
> than 100% so perhaps a geologist or hydrologist or potter can give us a
> little more insight into what the analysis means. I don't believe it
> tells us weather these elements are in reactive or non reactive forms.

The analysis sites constituents as oxides because that is a standard
method.  It doesn't imply that the elements are there as the oxides.

The constituents other than LOI (loss on ignition) sum to 99.9%.  I find
that a little odd, because clay minerals contain several percent water
which doesn't appear in this analysis.  LOI is the change in sample weight
when the sample is heated to a high temperature.  Most of the loss is
water, some could be organic material.

Neglecting that problem, the analysis appears to be a mixture of illite
and quartz.  I think the quartz would be included as flux.  It isn't a

Illite is a very common clay mineral with a moderate to low CEC.  Illite
is stable in sea water.  Hence it is a major constituent of vast marine
shale beds and an important component of many soils that are partly
derived from those shale beds.  It can slowly degrade in soils or other
relatively acidic, fresh-water environments.  In this analysis, most or
all of the potassium would be unavailable to plants, as would some or all
of the magnesium.  The iron may or may not be available.  Some potassium
will be released as the clay degrades.  Commonly this degradation would
occur so slowly that it isn't noticable in aquaria. There probably are
exceptions to that, though.

> I have observed that this stuff, when I used it in relatively large
> quantities together with earthworm castings and gravel, did not
> initially seem favourable to certain kinds of fast growing plants such
> as Hygrophila polysperma. Other plants, with thick roots and slower
> growing, thrived in this stuff right from the start. After about a
> month, the Hygrophila species started to grow well. It could very well
> be that there was enough K or Mg in the interstitial substrate water to
> create some type of problem. I think this is a very important
> observation about pottery clay and we should make a distinction about
> the clay which is a component of surface soils.

Clays will always contain some cations in exchange sites: Ca++, Mg++, Na+,
K+, NH4+ (all considered "bases" is a soil vernacular), H+, and Al+++
(both considered "acids" in the same vernacular).  When you use clay to
build a substrate the exchanged ions will create a pH in the substrate
that may or may not be great for growing plants; it might be too acidic
(pH << 7), or might be too basic (pH > 7).  Over a period of probably
weeks to months the pH will adjust to conditions in your tank - and in
particular to the chemical conditions created near the roots of the plants
(which might change from plant to plant).  Perhaps in your case the clay
adjusted from a poor-pH condition to a condition that was better for
growing Hygrophila.

Probably anytime you use a clay or soil you should first check its
"reaction pH".  That is the pH of distilled water in contact with the
soil.  The following is *not* a standard procedure, but you might try it
anyway.  Color indicators generally won't work with this test because the
soils cloud and discolor the water.  Try using an inexpensive paper
indicator.  Moisten a small amount of the clay or soil with distilled
water.  After a few minutes, press the indicator paper to the sample
until it reacts, and then compare the paper to a color chart to determine
the pH.  Ideally it will be nuetral to slightly acidic (6-7).  If it's
outside that range you might consider using a different clay or soil.

Or laterite :).  After all, it's nonreactive

Roger Miller.