I wonder if you could apply ferrous sulfate to decomposing citrus or carambola (citric and oxalic acid are natural chelating agents) and use it to treat iron deficient plants?
It would work and chelate somme iron .
But once the chelated iron gets into the soil and if the soil its not acid,then it will imediatly be lost( iron turns to rust).
For maximum 6-7 ph should work.
There is a chelate that its stable up to 10 ph and thats the best but also the most expensive.
Cheaper chelates are only good in low ph where you wouldnt normally need to add iron.
The cheap chelates do work well as foliar in neutral and alkaline soil grown plants ,but so does iron sulfate wich you can use directly as foliar spray instead of chelates.
FeEDDHA its the best chelate that can resist alkaline soils above 7 ( up to 10) without becoming rust.It is the most expensive form of chelated iron thogh.