All sources I have found recommend that the nitrogen (first number) be small.  A 6-10-10 or bone meal might be better.  Here is a copied paragraph from a commercial iris farm: FERTILIZATION - Modern iris varieties may need more fertilizer than older varieties due to the increased number and size of blossoms. We fertilize in mid to late April with bone meal, superphosphate, or a fertilizer low in nitrogen such as 6-10-10. Fertilizers high in nitrogen tend to cause bacterial rot and lush, but weak, foliage growth. When selecting fertilizers for irises, be sure that the 2nd and 3rd numbers are bigger than the 1st. These numbers stand for the amounts of phosphorus and potassium in the mixture. Phosphorus and potassium are the key nutrients in root and bloom production. Another dose of fertilizer at half strength after fall dividing and planting will slow release throughout the winter and early spring. Though fertilizing can only help, it is certainly not necessary for growing a garden of beautiful irises.