As well-drained as you can get it, they will grow in almost pure gravel if they have to. They will rot in a moist moisture-retentive soil, so if you're on clay, dig in grit or gravel. They don't need added feeding.
Bear in mind that they make annual roots from flowering until July, and at the same time the previous years roots wither away, so you need to do the irises sooner rather than later, and be very careful not to damage the fleshy roots coming from the back of the rhizome.

Thanks TD. They didn't flower this year as the bed was overrun with plants, hence the clear out.