I have used bone meal. It was recommended by one of our nurseries. They suggested that I put bone meal into the newly dug hole before planting each rhizome. I did that, and have to say that it made a big difference in the health of the rhizomes, and the bloom the following spring. The same nursery told me that Irises really don't need anything more. I don't know if that's a general rule, or if that's just true for the soil in our area.
