6-9-7 is not a problem. I would use it without concern except I would avoid direct contact with the rhizome. Avoid things like 21-0-0. Organic matter will improve your soil but I would dare bet it is better than your soil tester indicated. I'll bet that there are county agents in Idaho who will help you get a soil test at one of the universities which would give you more accurate results and let you know if you need to add something. Addition of fertilizer in the early spring is more effective than in the fall. I know there are a number of iris growers in Idaho who would be happy to give you advice for your area. Where in Idaho are you and I might be able to suggest someone.
