I do remember seeing a trial on organic acid chelation agents, I think the used oxalic, citric and acetic acid and they found that citric acid was the best, I have also read that the organic acids are better then the synthetic ones when it comes to plant performance.

I know that John kempf is a big fan of citric acid as almost all the aea products use it. I have used it for a few chelates and it is quite cost effective.

Just my two cents and I am far from an expert. Info on this topic is hard to find. I would love to have someone with a strong chemistry background explain all this.