Are you [url=https://findarticles.com/p/articles/mi_qa3931/is_200205/ai_n9060354/]SURE[/url] you want to add iron?
Perhaps your soil has enough already, and you are not aware. If so you can set up a toxic situation for plants that can actually stunt growth, usually heralded by bronze speckles on the leaves...
This is a good example of where we can try to "outsmart" Nature and just end up messing things up. If you do a test and find you are low on iron (I've RARELY seen it), then there are products to do it safely and without additives like chrome (quick build up and extreme phytotoxicity) that without assays you cannot rule out in a bag of shavings or can of screws.
While I have bored most people here with my steady refrain of "Compost, compost, compost.", it still remains the best way to mitigate any soil deficiency you can name, even water.