Symbolism, Imagery, Allegory
Like the porch columns, the bell in the backyard of Skully's Landing is a reminder of the plantation's past, but with a slight difference. It has the explicit memory of slavery built into it, because it was used to call the slaves in from the fields (from back when there were slaves working in the fields).
While that past, with its violent legacy, is still very much alive in the memories of Landing residents, the bell shows how old-fashioned and backwards the slavery model is: "the metal had turned a mildewed green, and the platform on which it rested was rotten" (1.2.127). The bell is disintegrating, and its foundations are rotting away. Maybe Skully's Landing is disintegrating because its foundation—slavery—is rotten.
That doesn't mean that people are ready to let the past go, however. Check out this scene from the end of the novel, when Joel returns from the abandoned hotel to find
Zoo, crouched near the broken columns, […] tugging at the slave-bell, trying, it seemed, to uproot it, and Amy, her hair disarranged and dirt streaking her face like war paint, paced back and forth, directing Zoo's efforts. (3.12.50)
Zoo tries to escape the deep roots of slavery while Amy tries to repeats it. Gross.