Let's say you had a cast-iron pan and an apple tree that you were going to plant. Let's say you dug a large hole, threw the pan in and planted the tree near the pan.
One might imagine that the tree would use iron from the pan when it needed iron, and ignore the pan when it didn't. However, what is the truth of this matter? How would the tree treat the pan? Would it entirely ignore it, get too much iron from it, get the proper balance or what?
Let's say the pan and the tree were there for 20+ years and the soil was just slightly iron-deficient to begin with.
This is a hypothetical question. I don't have a tree I'm planning to plant any time soon. You can substitute apple with pear, peach or whatever. You can substitute the cast-iron pan with an unpolished brass (copper/zinc) turtle or some other mineral chunk. If the answer would differ based on the kind of tree and the mineral, go with an apple tree and iron.