Your hens will love to dust bath! They dig a shallow hole, loosen up all the dirt with their feet and proceed roll around in the dirt and get themselves absolutely as dirty as they possibly can (don’t worry they shake off the dirt later). Dust baths are necessary to keep you girls healthy by cleaning feathers, removing excess oil and suffocating parasites such as mites or lice that might be present in your bird's feathers and legs.
If your run doesn't have a dry patch, you can provide an artificial one by adding a box or tire that is at least 12” deep, 15” wide and 24” long. Your dust bath should contain equal amounts of sand, wood ash, peat moss, and/or soil. You can also add 1-2 cups of food grade diatomaceous earth if external parasites (such as mites or lice) are a problem. Be sure, however, to mix it in well as it can be harmful if inhaled in large amounts. By mixing it in, it has less probability to become airborne, while still preventing external parasites.