Ok, first of all the Iris have most likly used up the soil, this is why big time growers bring in big loads of new soil and make new fields with it about every 5 years. They should also be thined out so that there is new room for the toes to grow out again. You can use just about any good fert. to till into and around the iris. You should remove the dead fans, but leave the rest as even during winter they are making food for the next springs blooms and new offshoots too.
-- "In this universe the night was falling,the shadows were lengthening towards an east that would not know another dawn. But elsewhere the stars were still young and the light of morning lingered: and along the path he once had followed, man would one day go again."
Arthur C. Clarke, The City & The Stars