Comment: Here's more information.

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In reply to comment: or rusty nails? (see in situ)

Here's more information.

http://5e.plantphys.net/article.php?ch=t&id=289

Photos of plants with Mg, Mn, Mo, N, P, S, n, B, Ca, Cl, Cu, Fe and K deficiencies.

Use these maps to find the wood ashes you need.

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