The formation constant for FeEDTA is 25.1 (log Kmy). The formation constant for Mg is 8.7, for Ca 10.7, for Cu 18.8, and for Zn 16.5 (under standard conditions). As these are log values it is clear that EDTA binds iron MUCH more tightly than any other cation ... I dont think you have to worry about Fe being displaced. Info from Skoog and West, Fundamentals of Analytical Chemistry, 2nd Ed, 1969, p. 345. Dr. dave.