Rust is iron oxide. Probably not in the right form for plants to use.
You'll see a lot of mention about putting iron nails/screws/bits into the bottom of gravel/hydroton beds in aquaponic forums b/c the anaerobic conditions/bacteria create chelated iron that plan roots can use.I'll have to go through my notes again and find these articles. I didn't see mention of the washers, but definately saw them mention putting nails under one of the layers that was used.
Not to be a jerk, but this pH wouldn't kill anything on its own. Many discus and apisto keepers keep there pH near 4.... just one more example of the insignificance of pH reading. Stability and consistency are about all that matters....plants probably cant use the rust b/c its not soluble unless your pH is well below 5 or [email protected] point most of your plants/fish will probably be dead unless you have [aquatic] blueberries :hihi:
H+ (a cation) is from the loss of an electron giving the Hydrogen atom a net positive charge. Sorry to be nitpicky. If it's any consolation I lol'd at how "secretion of protons" sounds so dirty. :biggrin:This secretion of protons (H+)