I was under the impression that, as I have 100% flourite as the substrate,
then this already has all the iron and trace elements that I may need

Flourite only provides iron. Which is what laterite provides. The other
traces must be added, whether you use Flourite or laterite. Plus, some plants
don't have much in the way of roots, and draw nutrients through their leaves.
Flourite only feeds the roots.


Leigh

http://www.fortunecity.com/lavender/halloween/881/