I'll repost an old post based on ratios for K+ and N(dating back to 1998-1999 or so):

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, KH2PO4, and GH booster.

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). Aquatics are pretty close to this also.

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. you'd have to have over 75% of the N coming from others sources(yes, it's possible, but not that likely)

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.

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.

So while I did not address the math or the calculators(I do not use them), using ratios and indirect relationships with KNO3........I know my K+ will be at least well above a non limiting level. I also can say the upper bound is safe also, since Erik and myself went looking for upper K+ issues and low Ca++, and found none at either 60ppm(me) and 100+ppm for Erik for several months on specific pants that where suspects for sensitivity to K+/Ca issues(we both had large healthy groups of A gracilus).

Regards,

Tom Barr