What is the particular reason for wanting to prevent them "totally denuding and poisoning the soil"?
When we started thinking of getting a few chickens for fresh eggs, we knew we didn't want to muck out a coop & yard daily or weekly. We didn't want a pronounced coop smell, and we didn't want the birds so crazy bored in a bare dirt pen that they'd start pecking themselves during the winter when we won't be home before dark to let them run around for a bit.
Originally, we planned on having 3 chickens in a tractor and moving them over just a little bit every day, so that eventually, they would have moved from one side of our 50' x 150' fenced-in yard to other. If we didn't have problems with predators, we thought we might move them outside of the fence to the meadow area next to the trees at the back of our property before starting all over again.
The soil here is clay and slightly uneven. We've been wanting to amend it and even it out slightly, so the idea we had was to use the chickens to till & fertilize it for a day, then move them. We figured that we'd put a little pile of DE/peat moss/sand/topsoil in the tractor run for them to dust bathe in, and at the end of each week, we'd lightly rake over the top of the areas they had worked to smooth it out.
This way, we'd have about a dozen fresh eggs per week from chickens with access to fresh greens (we are lucky to have lots of chickweed, dandelions, clover, & burdock in our "lawn") that are supposed to make for extra delicious & nutritious eggs, and they'd have also done the work of tilling in some amendments for us.
Then we found y'all and caught chicken fever.
So now we're trying to figure out how many chickens (up to 12) we can keep without the chickens stripping their regular areas bare. A bare run means that we would have to go forage for them... which is a lot less fun for all of us.
We've thought about just keeping a few flocks in different tractors, but it seemed like keeping all of them together would mean more socializing & regular running around space for them and less work for us. So we're considering more chickens with a permanent coop & run IF we can figure out a way to keep the smell down and the run healthy enough to support a lot of green.