I would , if I were you, get some 'garden lime"....the finely ground stuff....looks like flour....and "dust" every rhizome, before planting....and a 'small hand full" scattered over, and around, any of the newbies that are already planted. The bacteria that cause 'soft rot' don't like "extremes" of ph....too acid, or too alkaline will stop them ...dead. It's pretty easy to get 'too acid" for iris (bearded).....that leaves them open to fungal infections , and ties up needed nutrients. So "too alkaline" is the better choice (if you don't already know your soils ph). IMHO, it's better, by far, to PREVENT the problem, rather than to "treat" it.
