gross rear feathers

these are pictures of our two hens. right now, they are fed grain, dried worms, a small amount of a corn & seeds mixture, along with a variety of fresh produce every day. we have been supplementing with peck purity and flock armour. we also let them out to bathe in the "spa" every day, though they only choose to do it about twice a week. lately we have added diatomaceous earth to their spa dirt, hoping it might help. without fail, they end up with poop dried to their butt feathers like this. we are currently mitigating the issue by bathing them in warm soapy water on warm, sunny days but we're wondering if there is another way to handle this? and what causes this? what should we be doing instead? so far, they are totally healthy but we're obv afraid of mites, etc. any help is appreciated.