My buddy does that with deer heads. He buries them in the ground and puts a wash tub over them, with a couple of ducrete blocks to weigh it down. ( He has had stray dogs did them up and haul them away, so he takes this measure to keep from losing them.) In the acidic soil of western PA, by the time spring rolls around and the snow melts off, the skulls are bare bones, but discolored. Hair bleach is the cure for that. His total cost is whatever the beauty supply store is asking for the bleach. One small box does a couple of heads.