When I've had overwintering pots lose volume in potting media in winter I have replaced the soil with fine mulch. I kept the rhizome exposed but covered any roots that were showing. My thought was using something airy and light would be safer. I would also have considered using sand if I'd had it on hand. Typically that was replacing soil that washed out from the top during heavy rain, so I could just add mulch to the top. In some cases the media just decomposed and the iris was sitting lower in the pot. Then I removed the root ball and added soil to the bottom of the pot and brought the rhizome back up to the surface of the pot.
