Kent has pretty well given you the answers I would have, but I would add that , for your iris, like your other plants, ...if they are growing well, and bloom nicely ....they are probably pretty happy with what you have been doing, as to fertilization, and culture. Before you add any kind of amendments ........you should have a REASON to do it ! ... poor color, low bloom count, or obvious signs of stress. If you don't already have one, a soil test kit is pretty cheap, and will show what,....if anything....your soil might be lacking.
