plants do not absorb nutrients, they absorb cations of those nutrients, positively charged particles...separate atoms of nutrients, you CANNOT flush those cations, and testing the sap in a plant is nothing like a valid experiment...that sap didn't ever contain raw nutrients...it contained cations of those nutrients. also, the plant doesn't excrete excess cations from its roots, they get used up by the plant and turned into vegetable matter of some kind, cellulose, or sugar, or chlorophyll, or t.h.c. ..then the water minus the used cations is sweated out by the stomata on the leaves during transpiration.
you can force too many cations into a plant, and they burn, in different places, depending upon which type of cations they are...but you can't wash them out, the whole purpose behind flushing is to remove an excess of nutrient cations from your medium, so they can't burn your plants tissues...not to remove those excess cations from your plants...because you can't