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organic substrate layers and decomposition by-products
I've mentioned layering of substrate materials previously but didn't
explain why this is so important. The reason is this:
Almost no oxygen diffuses into the substrate much below an inch with a
fine gravel/sand substrate. The roots of aquatic plants mainly grow
and spread about an inch or two into the substrate (although many go
deeper in quest of mineral nutrients). In nature, virtually all macro
nutrients from organic materials are found in the top inch or so of
mud. You should maintain all organic materials (IF you use them)
within a layer no more than an inch thick and covered by no more than
an inch of 2-3 mm gravel or coarse washed sand. The organic material
should always be mixed with sand or soil. Ordinary garden soil is a
better source of organic material because it is well decomposed and
well leached of nutrients. Earthworm castings for example, seem to be
very fertile and should be used sparingly IF used. While advertised to
be a good source of humus, they are also a good (bad?) source of
nutrients and labile organic material.
When organic materials decay in the substrate below 2 inches or so,
the reduction-oxidation (redox) potential becomes very low since
oxygen, nitrate and other oxidizing chemicals are used up by the
bacteria. At this point, anaerobic bacteria take over using processes
such as fermentation which results in small (or large) amounts of
somewhat toxic by-products such as alcohols, ketones, esters and
aldehydes. Often there will also be various mercator (HS-) compounds
which have the distinctive sulfer smell of natural gas and which are
easy to identify. The gas bubbles which you get from organic
substrates are primarily methane but could also contain ethane as well
as nitrogen. The presence of iron in the substrate (iron containing
soils, iron oxide, laterite, red clay) helps to bind sulfide radicals
and thus reduce their mobility. Certain types of fish are much more
likely to be affected by small amounts of these compounds than are
plants. Regular water changes and carbon filtration are good
precautions. Activated carbon (if changed weekly) will remove most
dissolved organic compounds (DOC) such as the ones I mentioned above
so I would advise this if you are keeping delicate fish.
When you have a deeper substrate (3" or more) what you're attempting
to do with the lower layers is provide iron and manganese trace
elements at the low redox potential there. A shallow substrate can get
filled with plant roots to the point where the redox potential is too
high to provide enough iron for the plants. I suspect that it also
takes many plants like Hygrophila, a long time to develop large enough
root systems to get enough iron to satisfy their rapid growth. I've
always had to supplement chelated Fe to avoid chlorosis symptoms with
this plant in good nutrient/high light conditions. The only organic
material you might want at all below 2 inches is possibly a small
amount of humus which you can get from outdoor soil (not potting soil
or the like). Peat plates may be an exception but these are far less
subject to decomposition. Again peat plates should probably be no more
than 2-3" deep in the substrate.
Bottom layer: clay, sand, iron compounds, laterite, micronized iron,
iron oxides etc. trace humus.
Middle layer: (1") sand, silt, soil, peat(10%) or aged compost(<5%)...
Top layer: sand or fine gravel to keep things from getting stirred up.
You'll probably only use a single organic material such as peat or if
you use a combination, you'll reduce the overall amounts to limit the
total organic content. Aged compost or earthworm castings might be
better if limited to 2-3% by weight or better yet, leached for a month
or two in a bucket of water. The organic components of a substrate can
be left out and you can also get good results using a laterite or
similar additive in the bottom 1/3 of your substrate for iron.
Steve P in Vancouver BC where it's summer again...