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PMDD & Nitrate concentrations
I have a week old _experimental_ soil substrate which I have recently
set up. I have used PMDD at a reduced dosage (but not reduced
enough) for the last week. Recently I've been measuring various
parameters and find that I have approximately 100ppm nitrate in
this tank. Suffice to say that I won't be adding any more PMDD
to this tank until it becomes obvious that nitrates are not
accumulating. Some very important experience is to be gained
here:
o monitoring NO3 regularly is critical for proper use of PMDD!!
o a fertile organic soil substrate will contain considerable
nutrients which are going to make nitrogen available to the plants
esp in the substrate itself. That substrate nitrogen will primarily
be in the form of ammonia which means that the plants are not
going to use _any_ nitrate from the water at all. Suspend the
addition of nitrate and probably all nutrients and micronutrients
in a soil based substrate until it becomes clear that the
plants are becoming nitrogen limited.
o in closed systems where water changes are minimal, we need to
be cautious about nutrient additions. These can probably be
accomplished (with possible exception of Ca) by regular feedings
of a balanced fish food diet (or other organic materials in
fishless tanks)
o in systems where we're pumping in lots of nutrients such
as with PMDD, my feeling is that it's critical that a regular
schedule of water changes is followed to prevent build-up
of some nutrients.
o with a fertile organic soil substrate I think it would be a
good policy to allow the plants to grow for a week without
any fish and measure NO3 at that time. If an excess is found,
you can easily do large water changes without fear of the
colder temperature water adversely affecting the fishes.
The lone occupant of the tank, a female platy, goes back with
her friends tonight and I'll change a fair amount of the water.
The plants by the way, appear to be quite happy and growing
rapidly although 100ppm NO3 is a lot. No algae evident following
the complete bleaching method described by Krombholz. So far
air borne algaes have not colonized the tank.
> From: Jonathan_Kirschner at Energetics_com (Jonathan Kirschner)
>
> Based upon the collective wisdom I have gleaned from the List over
> the past several months I have decided to start with a layer of peat
> plates, then a fine gravel/laterite/Dupla root tablet layer, then an
> additional layer of fine gravel (about double the thickness of the
> gravel/laterite mix). Finally, I plan to add a fairly thin layer of
> coarser gravel which can be vacuumed during water changes. Any
> comments about using the peat plates under the gravel? My
> motivation for using them is that I have read that crypts, and
> perhaps other plants, will do better with some organic matter in the
> substrate.
Several points:
o I don't believe peat should be in a bottom layer when using
an iron rich substrate. For the iron to become soluble it must
be in a reducing environment which means in the lowest layer
and in an environment with little or no sulfer or sulphate
compounds. I think those using peat should stick with a regime
which has been shown productive such as what Neil Frank uses.
o Peat is the only organic substance aside from well composted soil
which I suspect will not develop redox approaching 0 mV which
may cause problems with H2S in a deep substrate (i.e. > 2 inches)
The acidic nature of peat also helps inhibit bacteria.
o The processes of decomposition of organic material to liberate
nutrients and the solubalization of Fe occur best at different
redox potentials. I think there should be two layers, the
lower one should have Fe and a small amount of organic matter
probably humus at 2%. The organic layer should be near the
surface and possibly no more than 1 to 1.5 inches thick. I'm
concerned that de-nitrification could occur here but don't have
an answer about the redox potential this will occur at.
o I think the purpose of gravel in a top layer is to prevent
fine materials like soil or clay from being stirred up during
planting not to be used for gravel vacuuming. My feeling is that
for the plants benefit, this layer should be as thin as possible.
Probably fine sand is superior to gravel. A secondary use for a
gravel/sand top layer is to inhibit nutrient diffusion out of
the substrate particularly upon initial submergence of a soil
substrate. Since you're not using fertile soil, this won't be
an issue.
In a Dupla system I believe the substrate is intentionally gravel
and thicker to provide the reducing environment near the bottom
where the laterite is concentrated.
Heating coils help prevent very low redox potential by producing
low circulation and this allows detritus and excreta to work into
the substrate and be used by the plants as a nutrient source.
Once this material is below the surface of the substrate,
the lower redox potential greatly improves the availability
of the nutrients.
Soil and peat substrates operate somewhat differently and while I'd
like to go on at length, time doesn't permit and there are some
issues which aren't yet clear to me. I hope we can give a lot
more guidance on improved substrates but it's going to take some
time and probably some controlled experimentation.
> Concerning the substrate... I have Tetra Hilena Crypto tablets.
> Should these be added to the gravel/laterite/Dupla tablet mix, or is
> this overkill? Should they be used at all?
I don't think we have established clearly how much artificial
fertilization is appropriate in a substrate. Literature from Diana
Walstad's research indicates that organic fertilization is far
superior to artificial. Excessive fertilization can certainly
create problems particularly when using sulphate compounds in
an environment where the redox potential may be low (ie. in
an organic, fertile substrate that is inches deep).
Certainly, I believe it is highly advantageous to provide
nutrients in the substrate, particularly organic nutrients.
We can also use certain kinds of fertilizers with caution
in infertile, gravel-only substrates with dramatic results.
My advise is stick to a single method unless you know what
you're doing or like experimenting. If you use small containers
for your Crypts or other plants, you can experiment with
much more freedom and I would encourage people to use this
approach and share their results (good and bad).
Steve