Looking for dry ferts to dose nitrates/phosphates/potassium individually - The Planted Tank Forum
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post #1 of 6 (permalink) Old 07-30-2017, 09:56 PM Thread Starter
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Looking for dry ferts to dose nitrates/phosphates/potassium individually

I'd like to try dosing the 3 macro nutrients individually. I'm auto-dosing my tank and I'd like to really dial in the ferts. Right now I have a bunch of KH2PO4 and KNO3, so I could dose phosphates and nitrates individually, but the potassium would be dosed from both solutions. Are there other compounds I could use to achieve this?

Thanks!
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post #2 of 6 (permalink) Old 07-30-2017, 11:30 PM
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K2SO4 potassium sulfate.
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post #3 of 6 (permalink) Old 07-31-2017, 12:04 AM
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post #4 of 6 (permalink) Old 07-31-2017, 08:55 AM
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PO43- and NO3- are always bound to a cation, in this case potassium(K+). See Common Compounds of Nitrate NO3 -- EndMemo. This is a list of common salts of nitrate, as you can see many are toxic to fish in the levels required to produce a rise in NO3- in the water. The same goes for phosphate.

K+ compounds(e.g. KNO3, KH2PO4, K2SO4) are the most common simply because they introduce potassium(a relatively safe cation) into the water that plants rapidly uptake. This likely leaves an excess of potassium, which is relatively non-toxic and removed with 50% weekly water changes(per the EI method).

Examining KNO3 closer for example we can see that K+ and NO3- are added in roughly a 2:3 ratio, i.e. in a 100gm sample of KNO3 there will be roughly 60gm of NO3- and 40gm of K+. Repeated for KH2PO4, we see an approx 3:7 ratio. And finally SO4 is closer to 1:1. Applying this to EI(dosing only KNO3 and KH2PO4) over a 1 week period 29ppm for nitrate, 8.7ppm for phosphate is accumulated. The K+ accumulated is 36ppm. This is assuming no plant uptake.


This level of potassium accumulation is somewhat negligible when we're targeting NO3 and PO4 levels which are much more sensitive and contribute to algae growth in the wrong levels. Its just not economical to dose them individually(as K, N, or P), considering potassium exists as a highly reactive metal, nitrogen as a gas and phosphorus which is never found as a free element, buying them in elemental forms would be a waste of money.

Hence why the popularity of KNO3, KH2PO4 and K2SO4, which are relatively cheap, easy to obtain and non-toxic compounds at the recommended dosage levels.
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post #5 of 6 (permalink) Old 08-06-2017, 05:43 PM Thread Starter
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Lots of good info, thanks. I wish I went further in chemistry in college, maybe I'd remember some stuff.

I'll try just dosing the KNO3 and the KH2PO4 separately. I was hoping that if I were to dial it in specifically for the tank, then maybe the weekly 50% water change wouldn't be necessary.
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post #6 of 6 (permalink) Old 08-07-2017, 08:40 AM
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Quote:
Originally Posted by Roarke View Post
Lots of good info, thanks. I wish I went further in chemistry in college, maybe I'd remember some stuff.

I'll try just dosing the KNO3 and the KH2PO4 separately. I was hoping that if I were to dial it in specifically for the tank, then maybe the weekly 50% water change wouldn't be necessary.
Yeah people generally dose potassium sulfate if their nitrates and phosphates are already where they need to be. So you dose potassium sulfate to get whatever extra potassium you need and sulfate is relatively safe to decent levels. Plus plants can also utilize some of the sulfate.

Many people only use KNO3 and the KH2PO4, but say you were in a situation where you had a really heavy bioload. Your nitrates might already be at sufficient levels without dosing KNO3. This might be a case where dosing potassium sulfate would be handy.
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