From what ive seen the only good source for K is potash and using that directly seems risky. Natural PH of around 12+!
Im no expert but it does seem that K is only used in small amounts throughout the lifecycle at least until near the end of flower.
it seems most other admendments have SOME K, just very small amounts. Hopefully enough to keep a plant at least vegging nicely.
Even with chem nutes you'll see really low K numbers until you hit the final phase nutes where it seems like its used for bud hardening and trichrome production.
So with all that (observations not facts) it seems that a decent soil mix has enough K to get through most the plants life with an expectation of getting some late in life K from other sources.
It does seem most non macro foods also contain K so it seems a good K source is not going to be from macro feeds but actually from micros and carb additions.
let me restate this is FROM MY OBSERVATIONS and none of this should be taken as fact, I found my head in the same position as yours but since I have not noticed any defs after a few months of using my own organic mixs it isnt a priority for me to figure out right now.
I also have been using Snow Storm in flower for some time which is specifically a K suppliment so that could be why I have not issues because of it. I have three mothers that are about three months old each using my soil mix and other then a monthly top dress of bone and guano I have not noticed any K related issues, the mothers get almost no chemicals at all other then the occasional humbolt Honey or liquid Karma jolt.