Thanks for the explanation.

Frankly, I wouldn't mess with muriatic acid or muriate of potash. Just
buy standard garden fertilizers which have the N-P-K numbers that suit
your needs.

The problem is that my soil is already loaded with excessive amount of
phosphate, and most commonly available fertilizers always have some
amount of phosphate. I am afraid that if I use regular fertilizer to
increase the level of potash (like 10-10-10 or 21-5-11), I will end up
getting too much phosphate. This is the reason why I want to find a
way to add potash to the soil without also adding phosphate.

Jay Chan