As far as I know most people are not monitoring potassium. I see more people testing for phosphate than potassium As to iron I like the hanna instruments HI746 checker. Yes it cost more than most test kit at $50 but it is very accurate. But you do need distilled water use it. I have RO / DI system at home that reliably produces water with a TDS of 1. They also have very good phosphate, ammonia, and alkalinity checkers. Sadly they don't make a nitrate checker.