I replaced a fiberglass urn where annuals thrived with a beautiful cast iron urn. The same annuals that thrived in the fiberglass urn were much less happy in the cast iron urn. I was careful about watering, and am wondering if there could be a problem with the iron leaching from the urn itself. I have gone from having stunning flowers and a so-so urn, to having a fabulous urn and so-so flowers.

Any thoughts?