Question on Dosing Potassium - The Planted Tank Forum
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post #1 of 7 (permalink) Old 12-03-2004, 02:53 PM Thread Starter
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Question on Dosing Potassium

Got my stuff from Greg Watson and read Rex Grigg's web page. My question is, Chuck Gadd's calculator gives me the K from KNO3 and the K from K2SO4, so I can figure out how much K2SO4 to dose. He omits the K in KH2PO4, but I'm assuming he knows what he's doing, but I'm curious as to why we can ignore the K in KH2PO4? Is it because that K is not usable?
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post #2 of 7 (permalink) Old 12-03-2004, 03:00 PM
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I"m no chemistry whiz, but I think the KH2 is bound and not usable. That and when you consider how little you dose, even if it was available, you'd be dosing .3ppm of it's a negligible amount, especially since K over doses are uncommon and don't have dire consequences.
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post #3 of 7 (permalink) Old 12-03-2004, 05:11 PM Thread Starter
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OK, thanks. That makes sense.
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post #4 of 7 (permalink) Old 12-03-2004, 05:22 PM
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Malkore is correct about the part of very little K being dosed from KH2PO4. Unavaliablity though is not the issue. KH2PO4 dissociates into K+ and H2PO4- (no KH2). The K+ is useable. Since phosphates are a buffer, depending on pH the other ions present will be H2PO4-, HPO4--, or PO4--- in varying concentrations. I have no reason to believe H makes nutrients unusable.
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post #5 of 7 (permalink) Old 12-03-2004, 05:29 PM Thread Starter
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Thank you, Rolo!
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post #6 of 7 (permalink) Old 12-03-2004, 06:25 PM
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The K+ is available and usable. However, if you dose KH2PO4 to give you the desired level of K+, your phosphate will be way too high - and an algae bloom will be the result. As Malkore said, so little KH2PO4 is needed to get 1ppm PO4 in the water, the amount of added K+ is too small to matter.

Example (using a theoretical 1-L tank):

If I add:
1mg KH2PO4 in 1Liter = 0.7ppm PO4
1mg KH2PO4 in 1Liter = 0.3ppm K

Since the target level of potassium is 20ppm, the 0.3ppm is insignificant (1.5% of the needed amount).



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post #7 of 7 (permalink) Old 12-03-2004, 07:05 PM Thread Starter
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Ah so, I just noticed the "Percentage of compound" reference on Chuck Gadd's web-based calculator, so I just plugged in 1 gram and 1000 ml and got the 0.7ppm for PO4, so the K must be 0.3ppm. Thank you!
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