Answer: First, lets dispell the myth - bark mulch won't deplete the iron in plants. I wonder if the rumor you heard has been misinterpreted? It is possible for wood chips and sawdust to use nitrogen during decomposition, which would mean that plants might need an additional application of nitrogen.
I would use peatmoss as a mulch around acid-loving plants. As the peat decomposes it will leach acidified nutrients into the soil.
Best wishes with your garden!
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