I use the fertilizer that Cooley's Iris Garden sells

For bearded iris I use granular low nitrogen, ie. bulb formula.

At planting time I always dig in some superphosphate and/or bonemeal.

horse manure and lime.

Alfalfa pellets, lime, and bulb food (or superphosphate).
Laetitia

I buy a 40 lbs bag of 6-10-10 granular fertilizer from Home Depot(about $6). I mix about 1/4 cup underneath the iris, mix it in, put about an inch or two layer of soil on top and plant the rhizome on top of that. If it's a new bed, I topdress with a little Miracle Gro Weed Preventer and Plant Food. Then I don't add anything else for a few years, when I'll add some bonemeal after bloom.

Neil