Keeping the slave dirty and muddy helped to protect it from the strong sun while working in the field. Treating it like crap, overthrows his self-esteem, his willpower, rebellion and resistance. He becomes more resigned, humiliated and obedient.
His wounds and bruises were covered with shit and mud without problem. After the beatings and whipping, the boys used to piss on the wounds to disinfect them.
He was only washed when the workers wanted to fuck him. His mouth and ass were washed before they were penetrated by the farm men.