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Re: [APD] distasters in the hobby
>Does anyone know whether the macronutrients are consumed at
>roughly the same rate? In other words, if I start out with 5
>mg/L NO3, 10 mg/L K,
>and 0.1 mg/L PO4, and I wait until my NO3 drops to 1 mg/L; is
>accurate to assume that my K will be about 2 mg/L, and my P
>about 0.02 mg/L?
>Sorry I can't test this myself at the moment. Thought I would
>ask in case people already knew.
Depends on the species.
7:1:8 is an average I found review the mineral components of
roughly 45 aquatic plant species.
The problem is that depending on the concentration and the ion
in question is that they change as concentration changes.
Enzyme kinetics determines this.
As concentration increases the enzyme has more chances to
transport the ion into the cell as well as osmotic pressure and
If you want to know if the ratio of NPK is taken up at the same
rate, I'd say no. Plants will remove things like PO4 and NO3
before some like K+.
So there is no simple answer for your question, you can focus
the question down to a species. The other issue is that the
plant is often CO2 limited and species that use malate acids and
direct/indirect bicarb usage will beat out other plants and the
amount of each of the macro nutrients they remove varies. Light
will also influence this to large degree as will bacteria and
organic and inorganic complexes.
N and P are incorporated into plant tissues, K+ is not.
So there are a few things that will make this tough to say much
about in a tank.
The 7:1:8 ratio seems to be fairly robust but as the nutrients
get down to the levels in your example, the plants become very
limited, all submersed macrophytes are limited at 0.02ppm PO4
and few test kits(no hobbyists kits) can reproduce and detect
0.01ppm of PO4 without large error. Run a few low level PO4
test and you'll find this out. Big error bars.
I'd suggest maintaining the NPK are fairly stable ranges and
seeing how much is lost. You will still have the issues, but the
test kits will work better and the plants will not change as the
nutrients begin to drop and the plants starve/nutrient stress.
You can have quite different NPK ratios in a tank and have the
plant's internal level being 7:1:8 also, so nutrient status
prior also can play a role as well as fish waste in the form of
NH4, which is missed by a NO3 test kit if removed by bacteria
NO3=> N2 gas or by plant NH4 uptake.
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