Using KNO3 as the sole source of K+(no K2SO4/KCl needed

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plantbrain

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This is a repost of mine:

If you dose a fair amount of NO3 via KNO3, keep nice tank parameters, don't have much fish load, I'd suspect you don't need to add K separately.
Steve and I did this for awhile, there were times when folks could not get any K2SO4 around the area, so we did without.

We never really found much difference.Steve suggested we did not need any extra K as it was being supplied via KNO3.

If you have 6.4 ppm of so of NO3, then you add 4 ppm of K using KNO3. Some products have K+, like TMG.
I find it unlikely you really need MORE K+ to grow plants well and ************it's one less thing that folks need to dose which is my goal here****.
Looking at a mass balance of N/K+ ratios, __most/many__ plants have about 1.5 :1 ratios of N:K(Epstien 1972).

Now take into account the O3 part of NO3 and to get elemental N and you suddenly find you have plenty of K+ for plant growth.
So 1.5/4.4= .34 N's for every K+ you add.

So you should theoretically have 3x as much K as you need relative to N x the 1.5/1 ratio = 4.5x as much K+ relative to N per dose.
.
There are other sources of N, plant decomposition, fish waste but some K+ comes from these pools also.
I really do doubt that adding K2SO4 to such a tank will make any significant difference.

Unlike NO3 and denitrifying bacteria NO3=>N2 gas, there is not much to the K+ cycle except for plant uptake/leaching.

So it certainly is something folks can skip if they add KNO3 as their main source of N.

Now if you have a good fish load, over feed Discus, slow growth/non CO2 etc, adding K+ from KCl/K2SO4 is advisable since you already will have plenty of NO3.

But for many, KNO3 as the source of both N and K should do the trick.

I'd say you simply don't need but 3 things, KH2PO4, KNO3 and traces.
And then the other two parts: Light and CO2.

I'm glad Ghori brought it up as I've not thought about it since I guess 1997 or so. But going through it, it should not be a problem even with all the different plant species needs and possible extra K+ needs a plant might have.

I think it's be EVEN easier for the new folks and us too, to delete one more nutrient from the mix.

That is a GOOD GOAL.

The goal is to reduce down with simpler designs/methods/dosing routines for folks that still work very well.

Importantly:
Just remember if you have NO3 issues from the tap/fish load, you will still find a use for K+ dosing from KCl or K2SO4.

Regards,
Tom Barr
 

Slappy*McFish

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Thanks for the heads-up on K dosing.

High-light/fast growth/low fish-load = Dose KNO3 only(for nitrate)

Low-light/slow growth/heavy fish-load/NO3 in tap = Dose KCL/K2SO4
 

OnyxFishies

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Just a question. I'm hardly an aquatic plant expert, nor am I a chemist. But, I've noticed something that (to me) doesn't seem to match up. As far as I can tell, the general consensus on the ammount of K to have in your tank is around 20 ppm. Also, I find that the recommended ammount of NO3 to have/dose in your tank (either dose it or from fish load) is around 5 ppm. I understand that for NO3, this is a minimum, more is not terribly bad (within reason). I also have seen many people say that an overdose of K is not a terrible thing, within reason. (As opposed to an overdose of phosphate!)

According to my calculations, and from Chuck Gadds aquatic plant website, and a few other places I've seen, the ratio of NO3 to K in KNO3 is around 5 parts K to 8 parts NO3. See below for my calculations, if your interested. My question is this: If you use KNO3 as your sole source of K, how do you get even close to the recommened amount of K in your tank, if you measure the NO3 to around 5 ppm? 5 ppm of NO3, *all* from KNO3, should yeild around 3.1 ppm K. (0.625 * 5 ppm) This is further compounded in that most of us get at least some of the ammount of NO3 from the end result of having fish in the aquarium. Are there other sources of K that I'm missing? I don't mind dosing to 10, or even 20 ppm of NO3, but even 20 ppm of NO3 from KNO3 should yeild only 12.5 ppm of K. (according to my thinking!)

Calculations:
molar mass of NO3 = 62 grams/mole (N is 14g/m, O is 16g/m)
molar mass of K= 39 grams/mole
(Both from my periodic table. I rounded off to whole numbers. For example, on the table that I have the molecular mass of K is 39.0983amu)

So, the ratio of K to NO3 in KNO3 is 39/62, or approx 0.625 to 1.

I'm not trying to say that Tom is wrong, just trying to say I don't understand, and showing what I am thinking so I can be corrected. :eek: One thing I suspect is the recommended amount of K.. if 20 ppm is recommended purely because it is WAY more than the plants will need, (and that much of an overdose won't harm anything) and the actual ammount the plants need is say 3 or 4 ppm, that would explain it all.
 

happychem

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You're on the right track. But remember that it's not NO3 that the plants want, it's N. Before they use it they strip the 3 oxygens.

So while your 10g of KNO3 adds about 3.9g of K and 6.13g of NO3, you're really only adding about 1.4g of N.

Based on the N/K ratio of 1.5/1, if you were adding this 1.4g N on purpose, the required amount of K would be about 0.9g, so by having about 3.9g, you've got about 4.4 times as much K as you need.

This is why ppm sucks and the world of chemical concentrations should be done in moles and moles/L!

...or at least equivalents
 
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plantbrain

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I typically will round down to add a buffer of error and call it 4:1, N:K based of Epstein's 1972 anaylsis.

Not all plants will have the same N:K make up, but with a 4:1 ratio buffer, adding KNO3 should easily handle any differences between plant species and aquatic species differences(generally these have lower N/K ratios).

So the practical effect is you don't need to add more K+ in almost every case unless you have very high bioloading, too high IMO.
50% of the N from a CO2 enriched tank is pushing it.
Also tap water can have a fair amount of NO3 so that can influence whether you need K2SO4.

Regards,
Tom Barr
 

ChicoRaton

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This is why ppm sucks and the world of chemical concentrations should be done in moles and moles/L!
I must agree wholeheartedly... but so many people get scared away by the word "nitrate" That if we start saying "blah blah mol/L blah blah mole 6.022x10^23" I don't think any more people would join the hobby :p
 

happychem

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Yeah, but you don't need to think about the number of molecules in a mole, just that a mole is a) indicative of the number of molecules as opposed to weight and b) you can follow moles through a reaction, you can't follow grams or ppm
 

plantbrain

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I like moles, they are nice and furry. Oh darn, now I'm being a biologist, not a chemist:)

Yes, moles are far better units as are micro moles of light but few people use these units in this hobby.

ppm is not too bad for most.

Regards,
Tom Barr
 
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