I have read that iron sulphate (regular iron) isn't very stable at high pH levels.

A Chelate is simply an organic molecule that wraps itself around a metal ion and prevents it's precipitation, increasing it's solubility.
This can be an advantage in hydroponics, but probably not necessary in soil.
Daniel has much more on this at: scienceinhydroponics.com,
But his site has become tricky to navigate.

Try this;
http://scienceinhydroponics.com/2010/05/preparing-your-own-chelates-improving-your-hydroponic-nutrients.html
Good Luck
~Ken~

Thanks Ken I'll check it out.