Some chelating agents may remove potassium as well as other cations, so that's the "K" But the phosphorous is not as easy. And nitrogen isn't even only one chemical. The N-P-K jargon of plant care has sent you to a false conclusion, fir the most part, there isn't a chelator for these nutrients,in part because of their nature, and how they're delivered as well as their much greater concentration. There's no "carbon chelator" or "oxygen chelator" that can be applied to a living thing and selectively rinse those away, either. Simply because they're not delivered in the same way, to give a silly example. In fact, that silly example gives you the best way to remove the N-P_K from media or root surfaces, just rinse it away.