dirty, firm, fat
髒 centers on physical uncleanness or impurity, extending to something that is dirty or soiled. The senses of firm and fat are less common and may be separate applications, but they can be seen as describing a solid, thick, or unrefined state.
髒 combines 骨 (bone) with 葬 (bury), suggesting something buried with bones or associated with decay and dirtiness. The exact historical development is uncertain, but the component combination points to impurity or uncleanness.
Bones 骨 being buried 葬 in the ground get dirty and covered in soil. Picture a skeleton being interred, its bones becoming filthy and grimy.
For ソウ, imagine the dirty bones being buried in the soul (ソウ) of the earth: soul -> ソウ, and the burial makes everything unclean.