There are many materials that can be used on the ground in a chicken run but, after 30+ years of keeping chickens, my favorites are dirt, sand, and peat moss. Each has its own advantages and disadvantages, so I use them in different spots of my very large run.
If you’re like me and you live in the middle of nowhere, on land that has never been lived on, then you can just use the dirt that is naturally there. If the land is grassy, the chickens will enjoy scratching through the grass. If you have areas of weeds, they will scratch in those as well.
However, if you moved to a house where you don’t know if the previous owners sprayed or treated their yard with chemicals, like pesticides, you want to lay down a layer of topsoil.
I use the same kind of sand that I use inside the coop (see litter).
Sand drains well and is easy to work with. However, it can get very hot in the summer sun. So, I only put sand in some spots of the run.
I also have sandboxes full of sand underneath my chicken coops where my chickens love to have their dust baths.
Peat moss is absorbent, so it helps to keep the run dry. So, if you find that the run has some spots where it stays muddy after it rains, spread a thin layer of peat moss on top of the soggy ground and it will help keep the run dry.
Once you find the area is starting to get muddy again, just layer some more peat moss. Luckily I only have to do this twice a year.
Peat moss is also relatively cheap and easy to find (hardware stores, gardening centers, etc), making it a good option for many chicken owners.
Tip: Build your chicken run or fenced-in chicken yard on top of a hill or slope, where puddles won’t build up when it rains.