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[APD] role of CEC
The total of the cation exchange capacity provides a buffering effect
upon the concentration of all cations in solution. I don't think its a
very large capacity so if you had to rely upon it as the sole source of
a particular nutrient, it wouldn't last very long. The buffering
capacity is moderately small.
An interesting trick is to boil some peat in a strong solution of a
particular nutrient to charge most of the CEC sites with that nutrient.
Once in the tank, these ions would be released and typically replaced
with calcium and magnesium cations. The effect would be quite short
lived so the trick isn't very useful. If you increase the concentration
of a particular ion, then the proportion of CEC sites holding that ion
will increase proportionally. Other ions get released as a consequence.
Ammonium, NH4+ is a cation which can be charged into peat or other
media. I've also heard of charging nitrate into some other medium but I
can't locate the name of it presently. I've mentioned it here in the
past. Its something I learned about chatting with one of the local
hydroponics shop's chemist guru. Believe me, these BC guys really know
how to grow plants indoors! ;-)
I think you can really charge nutrients into a medium, especially when
you dry it out because the drying effect chemically bonds ions from
solution into the medium. Once formed, those bonds tend to remain. I'm
surprised that no one has thought of trying it out yet...
Plants take advantage of CEC sites because they can release hydrogen
ions to displace nutrients from adjacent sites. The CEC sites increase
the local density of nutrients.
If all of the nutrient input is coming into the tank hydroponically, and
there is little or no circulation through the substrate, its not going
to concentrate nutrients. Aquatic plants are able to draw water through
their systems via a transpiration stream but I don't think its enough to
effectively "sponge" up nutrients from the water into the substrate.
Terrestrial plants have the advantage of spilling water into the
atmosphere by evaporation and rely upon a much stronger transpiration
stream to induct virtually all nutrients through the roots and stems.
I think it gives aquatic plants a bit of an edge to have a high CEC
substrate but its not enough of an edge to make a big difference in
competition with algae for instance.
I'll digress a little on other substrate factors:
In a substrate with enough labile organic matter, you'll get a low
enough redox potential to liberate iron (& manganese) into solution. A
mixture of an iron source and earthworm castings for instance will
provide a good source of iron for a hungry plant such as a sword plant.
When the plant has all the other nutrients that it wants, such as with
nitrate hovering around 5 ppm and CO2 up around 20 ppm and calcium,
magnesium, potassium up there, then the iron supply can become the
governing growth factor. Potted with a source of iron and you can get
some pretty super charged growth.
Fertile organic matter, can also contain phosphates (as well as
nitrogen) which can help fuel plant growth for a good while. Phosphate
is also more available under reducing conditions. Probably 90% of that
supply is going to be used up within 6 months or so. If you have a lot
of OM, then it can really be more phosphate than your system can
Peat is not going to contain any N or P unless you pre-charge it.
Earth worm castings are quite rich and would supply more than enough for
anything I could imagine.
Compost will be even richer. Raw compost is things like partly rotted
vegetables or fruit. That's way to labile to use in an aquarium but you
can use a compost box to perform nutrient extractions. Those little
soil-tea or compost-tea solutions can be very potent. Its really best if
you can arrange to have algae-free, close to sterile conditions for the
plants if you're going to mess around with high nutrient concentrations.
There were a whole series of articles in the back issues of TAG, The
Aquatic Gardener publication of the AGA, about soil+compost nutrient
extraction by Paul Krombholz. He also mentioned getting calcium and
perhaps magnesium from egg shells. I found it a bit messy and dry
calcium carbonate is available in large bags and virtually mess free
(unless you spill it).
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