I can't help with the plant part, but I can with the chem part.
Chelated iron isn't going to precipitate out easily. It's locked up inside another molecule. To get any to precipitate out, you would also to add a ton of extra iron, have incredibly basic water (we're talking pH >12, maybe even higher), and/or mix it with something that would precipitate it out (i.e. concentrated hydrochloric acid, NaOH, etc).