Last year all of my bearded iris rhizomes developed a rot, which I understand to be caused be a bacterial infection when I investigated their demise last summer. Now I am wanting to plant in that area again and I remember reading that the bacteria can still exist in the soil. I have tuberose that will be perfect in that warm, protected area. How much soil is necessary to remove and replace in order to do new planting in the bed? Thank you!

Sheila