"The Song of the South" derives from Joel Chandler Harris' "Uncle Remus: His Songs and His Sayings" It is a compilation of stories derived from the oral folklore tradition of former slaves as collected by white journalist. Harris is often credited with retaining the dialect in the stories, which was done in an effort to preserve authenticity. Mark Twain, who used the same device in "Huckleberry Finn" notably credited it for this.
There are questions about exactly how accurate his retellings were and many scholars have criticisms of his role as an authority. More pointedly he has long been criticized for creating patronizing and damaging stereotypes that romanticize the antebellum era -- which can be seen in the Disney film.
Alice Walker's 1985 essay "Uncle Remus: No Friend of Mine" is one of the most pointed criticisms of Harris's works. I cannot find an online version of that but the link below is from a talk she gave in 1981 to the Atlanta Historical Society on the same topic. She includes her feelings regarding the Disney movie in this essay.
https://books.google.com/books?id=m...=snippet&q=atlanta historical society&f=false