I "generally" use whatever fertilizer "mix" that my soil tests say I need. I also use a "higher than normal" amount of nitrogen, than is usually "suggested" for iris. Over the years I've found that what "works well for me", might be the" WORST thing YOU could do. Climates, soil types, etc. are so variable across the country , that what works for one, may not work for all. The old adage is that the first "#" in the ratio...nitrogen...is what "grows the leaves", the second "#"...phosphate...is what "makes the flower", and the third "#"....potash..is what "makes the root". That is basically true, and many of the "bloom buster" type fertilizers have a "high" second number, to promote more flowers....but, phosphate use by the plant is tied directly to nitrogen use, by the plant i.e. , without the proper amount of nitrogen, the plant can't utilize the phosphates ( in our area, at least, the high use of phosphates are the big driver of a lot of the "water pollution"....the plants can't use it all, and it leaches into the water shed).
If you do have a "commercial" soil test...or ask for some insight from your local Extension agent, they will point out several things about "fertilizer ratios", as well as how soil ph (acidity / alkalinity) is tied to the plants use of nutrients, that will explain it much better than I can ! In my years of gardening.. most have been a "learning experience"!!! Sometimes, you "do", and regret it !
. One final bit of advice.....anything that's IN the soil..or that you PUT in the soil, is REALLY hard to get OUT of the soil ! So, ONLY add what a soil test tells you you need !
