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Re: Nutrients in Substrate or Water?
This is in part from my aquarium experience, and in part from reading in
chemistry and limnology.
In most aquaria it is inevitable that at least some of the plant's
nutrient requirements will be in the water: carbon from CO2 or HCO3-,
nitrogen from NO3- or NH4+ (because the animals eliminate much of their
metabolic wastes in solution); sulfate (for sulfer) and Mg++, Ca++ and K+
are almost inevitably present and in most instance (excepting unamended
distilled or RO water) present in significant quantities.
Phosphorus and iron are the only major nutrients I can think of off hand
that are missing from that list. It is possible to have low phosphorus
concentrations in the water, but more than adequate supplies in the
substrate. Phosphorus tends to become attached to solid surfaces; it also
forms poorly soluble compounds. Iron also forms very insoluble solids
that too probably end up in the substrate.
Rooted plants have a great advantage over algae (most of which get their
nutrients directly from the water) when phosphorus and iron are present in
the substrate but the water holds them in only low concentrations. Their
advantage is even greater if the substrate can bind Ca++, Mg++ and K+ so
that the rooted plants can get them at higher concentrations than is
provided by the water. That capability is provided by clays (soils, kitty
litter, laterite), iron oxides (soils, laterite) and many complex organic
molecules (soils, peat, and probably even the "mulm" that accumulates in a
tank over time).
Nutrients in the water (within reason, anyway) shouldn't be a problem as
long as the dissolved concentrations of one or both of the critical
nutrients (phosphorus and iron) remains low enough to keep algae at a
competitive disadvantage. If those nutrients are low or absent in the
water, then you must insure that they are present in the substrate.