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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.

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Unfortunately, you cannot substitute the iron for just any mineral. Iron is a big part of soil, see my answer here:

Rust is iron oxide, which does not harm plants in moderate amounts, because it is not water soluble unless the soil ph is very low. In fact, oxidized iron is what gives most red subsoils their color.

Unless the pH is far too low the plant will be unable to uptake the iron oxide, and will not be useful for correcting deficiencies. It will not harm the tree if it is really only iron, not coated with anything. Now, even after it rusts, it will not necessarily be absorbable by plants. See here:

Although Fe is one of the most abundant metals in the earth's crust, its availability to plant roots is very low. Fe availability is dictated by the soil redox potential and pH. In soils that are aerobic or of higher pH, Fe is readily oxidized, and is predominately in the form of insoluble ferric oxides. At lower pH, the ferric Fe is freed from the oxide, and becomes more available for uptake by roots.

So iron objects won't really affect the plant, other than obstructing roots/messing with drainage. This is not true of just any mineral/metal chunk.

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  • I focused on iron for this answer, and suggest asking a separate question for each mineral in question, as different minerals have different properties, and you will more likely get a better answer.
    – J. Musser
    Nov 22, 2014 at 1:22
  • Would you suggest doing brass separate from copper and zinc (or would one answer all three)?
    – Brōtsyorfuzthrāx
    Nov 22, 2014 at 10:41

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