Yep, I think the key is to get something under the roost to catch their poo. They poo an amazing amount during the night--I think much more than during the day. But the poo shouldn't smell that much anyway with a few chickens--what brand of food are you using? I wouldn't use lime or DE on the grass--it changes the pH and might make it too alkaline, especially over time. It really should not be necessary to use anything on the grass, I would start with their diet, and cleaning the overnight poo.