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Re: Ratios

Tom Wood wrote:

> I've wondered if it might be useful to think in terms of ratios when dosing
> NPK. To that end, I asked one of the list botanists what ratio these
> elements were found in plant tissue. He answered:
> "An average N:P:K ratio, based on the composition of leaves, would be about
> 16:1:8."

Are these molecular ratios, or weight ratios?

Diana Walstad cites "critical concentration" values that give weight
ratios of 11:1:6.  She measured concentrations in mixed stems and leaves
from her tank that gave weight ratios of 7:1:8.

So I guess there's a lot of variation.

> Then I asked a list chemist to help me with the math to get to that ratio:
> N - KNO3 MW=101 g/mol N=14%
> P - NaH2PO4 (Discus Buffer) MW=120 g/mol P=26%
> K - K2SO4 MW=174 g/mol K=44%
> KNO3 =26/14x16=30, MSP=1, K2SO4=26/44x8=4.7, so you would add these
> in a 30:1:5 (roughly) ratio to get a 16:1:8 N:P:K ratio."
> Is it just me, or does that calculation not account for the K in the KNO3?

It doesn't appear to account for the K in KNO3, and I think it treats
the ratio as a weight ratio.

> So, if I mixed a powder of the compounds above, in the ratio shown above,
> and set the dosing level by measuring N (or NO3), then I should be able to
> provide these nutrients in the right ratio. I suspect that I can delete the
> P since I feed heavily.
> Any thoughts?

If you want to tailor a macronutrient dose to match your tank then you
need to account for the macronutrients that you are already adding with
the fish food and water changes.  If you feed heavily then you probably
don't need to add more than two of the macronutrients.  The ratios you
need to add them in may not have much to do with their ratios in the
plant.  Diana Walstad ran through the kind of "model aquarium"
calculations that you would need to do to make the estimate.

An alternative would be to dose your tank with excess K (as excess K
doesn't seem to hurt anything) and with whichever of N or P that
disappears first.  If nothing disappears then you don't need to dose
with either.  That implies considerable trust in your test kits.  If
everything disappears and the plants are still healthy then you still
don't need to dose with anything.

Roger Miller