I asked a similar question to this about 6 months ago. I enquired if anyone knew at what level potassium started interfering with calcium uptake.
Tom Barr gave me this reply:-
"Not that Ca++ issue again, Erik and myself both tested that notion to 75ppm for myself and Erik had like over 100ppm K+ and we both used the test plant that is suppose to be sensitive, Ammannia gracilius.
Now my plant grew great, his central theme plant won the AGA contest for that year..............
If it was K+, then why did we not have the same problem?
We both had lower Ca++ levels as well so that would error toward the notion that high K+, caused Ca++ blocking.
Ca++ blocking simply does not exist in this context, something I know and can easily justify with repeatable results as well as many folks maintaining very high K+ levels for over a decade.
I'm not saying what those folks claimning it were having, could be poor CO2, NO3 etc.
But I do know it was not high K+ blocking Ca++.
K+ over 100ppm might be bad, but no one has shown it causes issues in planted tanks directly.
K+ can cause salt stress in plant at a certain point, but that is rather high levels, beyond 100ppm.
Folks that use K2SO4, KNO3, and Seachem EQ heavily may have quite high K+."
Hope this helps answer at least part of your question.