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