How do you know their crops are "full" of dirt? If your run is bare soil, it's time to put in a deep litter. IMO, bare soil is unhealthy soil, especially if it has a flock of chickens on it. When you clean the coop, toss the litter into the run. Add leaves, wood chips, grass, garden debris, basically anything that is compostable with the goal to build up a 6" layer of goodies for them to scratch through. You can increase their thrills by tossing some scratch into the mix now and then. The DL will attract lots of organisms, including microscopic beneficial bacteria and fungi, as well as beneficial insects. All of these critters will keep the pathogens in check, including helping to eliminate internal and external parasites, and will give your birds a healthy gut flora, in effect giving them a stronger immune system and decreasing your feed bill.