Paul,
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 - it
is fractured clay. I believe the chemistry is OK for trace elements.

Martin

PS. I am actually getting laboratory grade KNO3 and K2SO4 now as I don't
(100%) trust the gardening stuff not to have some nasty trace elements in it
(like pesticides) (it won't go to waste though).