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laterite, aluminum, silicates and plants
Thanks to Pat Jellison, Roger Miller and others who have helped
clarify my understanding of what laterite is. Let me cover a few
points as I understand them and correct me where I go wrong.
The term laterite is not precise. There is not a consistent scientific
definition nor should we expect commercial aquarium additives to
necessarily be labelled or identified precisely. A loose definition of
laterite would be soil primarily composed of iron oxide and hydroxides
which have little or no calcium, magnesium or carbonate compounds and
which are low in aluminum and silicates.
For the purposes for use in planted aquariums, it _may_ not be that
important that we use a laterite which has had every last bit of
aluminum or silicate leached out. The laterites commonly sold probably
contain some amounts of kaolinite (a low solubility aluminum silicate:
Al2Si2O5(OH)4). My suspicion is that one of the reasons for using
laterite is that it won't greatly affect the water hardness and pH.
1. Will the presence of silicates make any difference to aquatic
2. Will aluminum silicates be sufficiently soluble to pose aluminum
toxicity problems for plants if the substrate pH goes below 6?
I could only see a substrate with low pH if there were significant
amounts of organic material such as peat or humus. My questions on
aluminum toxicity are more to do with why laterite might be preferred
to various clays or soils and how we might evaluate the potential for
problems with cheaper commercial clays such as pottery clays (see my
next posting on Terrastone clay analysis)
3. Would these organic materials tend to form aluminum complexes,
mitigating any problems with aluminum plant toxicity?
4. Suppose there were aluminum ions in the substrate which diffused
into the water which had a pH of 6.5 or higher; would it be
precipitated or oxidized? Would it pose a problem for fish or
5. What is sesquioxidic Fe or Al? Fe2O3? Al2O3?
6. What are halloysite, Oxisols, Ultisols and Spodosols?
BTW, I did check my reference on silicon dioxide/aluminum silicate =
glass and my excuse is that it was late at night. The chemistry book
did mention some special types of glass with trace aluminum or other