If you
fence off a small section at a time and sprinkle pellets, wheat or other grains on the ground, it will give them a good reason to scratch around.
You might also wish to dig a few holes to help them get started scratching around.
A couple of buckets of arborist's mulch will attract insects, worms and fungi that the chickens will also enjoy scratching through and improve your soil biology.
We regularly rake out the soil from our chicken run and top up with mulch, the girls go crazy scratching through the piles and spread it through their area.