I had an idea the other day and I shared it with my regular forum, but I
guess nobody really has any insight into this, so I will try here.

My
idea came when I was stripping rusted nails from old lumber I was
cutting for biochar. I was about to toss the nails into the garbage
when I thought that perhaps adding them to my rainbarrel would help to
add iron to the water. This thought came to me because I remembered
watching a show a couple years ago about adding old, used-up steel wool
scrubs to a jar of water and letting them rust down and then using the
resulting mixture to deal with chlorosis in houseplants. None of my
yard plants show any signs of chlorosis and of my potted plants, only my
Monstera is JUST starting to show signs. Visiting my mother's house
yesterday, I noted that her or own Monstera (which I had gotten my pups
from) was much further along in regards to possible chlorosis (yellowing
leaves with pronounced green veins). So if adding rusted nails to my
rainbarrel (and hers) wouldn't have any negative effects, we could
slowly and gradually keep our potted plants happy, iron-wise. Granted, I
believe it would be wise not to throw copper nails/staples into the
rainbarrel, but I don't think I have any of those around.