The "ideal" soil mix depends on who you talk to, and what kind of soil you have, in the first place, IMHO. Too "organic' holds a little too much water, and too "mineral" generally 'sets up", almost like concrete. A "mix' of the two types usually seems about the best, and, if drainage is a problem, raised beds.....or even planting on a small "mound" of soil will usually be all that's needed. Bear5ed iris seem to be pretty darn hardy, and adaptable , so ,as yiu grow them for a while, and can actually "see' which plants, in which areas of your garden ,do the best.....you can amend the conditions in other areas of your yard . One thing I would recommend that everyone do, is test their soil.....both for nutrients, and for ph. Test kits are fairly inexpensive, and ,IMHO, you can't add what "you need", if you don't KNOW what you need !