Yeah have had Epsom salts suggested, along with iron chelates and even nails in the trunk but from what I have read the best long term solution is to apply agricultural sulphur that will also lower the PH of the soil.
The article in the link below is the one that made me realise what the problem was and the solution
It's been a big leaning curve for me, I started out with gardens that had mostly native trees and shrubs and nice patch of lawn out the back. When the put deep sewage in they used my back yard to drill in 3 direction and in the process destroyed my lawn. Unfortunately and unknown to me at the time they used Kikuyu seed to restore the lawn, that took over became difficult to maintain and with my lower back pain I gave up on the garden.
Some years latter my back came good and took to clearing the overgrown mess of native trees and shrubs along with the Kikuyu grass which was a challenge in it's self, with the idea of turning the gardens to fruit and vege.
LOL initial I was looking small scale and looked at putting in rain water tanks but when I saw the cost of rain water tanks I thought why not look at getting a bore put in. Well the cost of the bore $3k, was not that much more than rain water tanks and I would have as much water as I wanted regardless of if it rained or not.
So with a bore and having as much water as I wanted, I went over the top with fruit trees/shrubs, I have about 2 dozen of them now. But I didn't realise how bad the soil is and while I manage to get most at least some what established, things like blue berries that like acidic soil have struggled to survive.
I'm getting there and just have to accept that some things are just not going to grow here no matter what
