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Re: Soil Substrate

On 2-27-97 Rodney Dorville wrote:

>> From: Rodney Dorville/EC/SP_SF <RDORVILL at sp_ac.sg>
>> Date: 27 Feb 97 12:53:48 WAT
>> Subject: RE: Soil/substrate
>> I was talking to a couple of Civil engineers at a worksite near my home
>> is littered with red-coloured earth.
>> I enquired whether the soil was pure laterite - the answer was that they 
>> couldn't be sure until they did a full soil analysis.  I asked further
>> it was possible to determine whether a soil was laterite by the looks - they 
>> said "No".  Their reasoning is that there are a number of soils which have a 
>> red-coloured base.  The alternative method was to look at a geological
>> map which would show the composition of soils in particular areas.
>> Is this true or are they just spoofing me?

First, there are different types of laterites which are leached aluminum,
iron, and other mineral silicate oxide soils. Laterites are very uncommon in
North America. Because laterites are formed in tropical climates any
laterites in North America are exhumed ancient soils from a time when the
soils were in a tropical latitude. Aluminum laterites, such as bauxite,
found in Arkansas and some other southeastern states, and the Carribean are
the source of aluminum foil.  In California, some alumina laterites are
associted with very pure kaolinite and are mined for pottery.  These
aluminum laterites contain little iron.  Iron laterites are also uncommon
and do not occur in pure forms in the US to my knowledge.  Laterites from
the tropics would be a better source to ensure you are getting laterite.

Second, all the sources of these laterites are soils which are a mixture of
minerals, salts, and organic compounds including various amounts of
nitrates, phosphates, and a host of other macro- and micro- nutrients.

Using soil surveys, such as the USDA Soil Surveys prepared by the Soil
Conservation Service (now called the National Resource Conservation Service)
for many counties and parishes in the US, one might find areas of oxidized
soils, but the mapping covers broad areas and the site you pick will
certainly not be "pure" laterite.  The way soils are described and mapped
may not even indicate whether or not it is a laterite.  A complete chemical
soil analysis to determine quantities of all the eliments that need testing
will probably cost in excess of $100 per sample!

I suggest that before polluting ones aquarium with soil of unknown
composition you think twice.  If you believe laterite is what you want then
commercially available sources in the aquarium industry may be the source
since they are probably from a tropical source.  Since you probably want
iron laterite and not aluminum laterite it is another reason to question the
source. Just because the soil is red does not mean that it is pure iron
laterite or even laterite.  There are many red oxidized soils containing
iron that are not laterites.  Also, if you insist on using laterite or
something from the ground, wash it continuously with distilled or deionized
water for a long time to get rid of some of the available nitrates and
phosphates. The iron mineral will sink in the water since it through an
equilibrioum reaction with iron mineral and the aquarium water that the iron
ions will become available to the plants.

Good luck,

Niall McCarten, Ph.D.
Plant Ecologist
UC Herbarium
Valley Life Sciences Building
University of California
Berkeley, CA