Japanese irises are heavy feeders. I give mine a feeding of 10-10-10 or equivalent, 12-12-12, etc. in the spring, and water them every two weeks alternating Miracle Gro, and Miracid. I also give them composted manure once a year, anytime.

I don't think it's necessary to give them a specialized fertilizer. Most important is to give them all the water you can.