The ideal soil for Iris is a 7 ph: High potassium, high phosphorous. They need at least 1/2 sunshine.
Clay Soil: Clay soil is harder for water to percolate through the soil; your best solution is to add organic material like compost or peat moss. This breaks up the soil, making oxygen retention possible, and aerates the soil.
Sandy Soil: Water percolates rapidly through sandy soil and the odds are there isn't much organic material in it. To retain moisture you again need add peat moss or compost--organic material.
A note on using compost; make sure it has broken down well and reached a hot enough temperature to kill organisms. As a general rule, remove iris leaves from the garden and do not use them as compost.
In the Southwest, it is always a good practice to dig a hole for the iris, fill the hole with water and let it soak in, and then plant the iris, watering it again after it is planted. The first week after planting your iris should be watered at least three times. Try to allow time for the iris to dry out between waterings (to avoid rot) but water often. Iris are suitable for planting in the hot southwest, but they need watering. They can be planted in hot weather as long as they are watered.