Lime and arsenic are some of the story's creepiest symbols. Lime is a white powder that's good at covering the smell of decomposing bodies. Ironically, it seems that the lime was sprinkled in vain. The smell of the rotting corpse of Homer Barron stopped wafting into the neighborhood of its own accord. Or maybe the town just got used to the smell. The lime is a symbol of a fruitless attempt to hide something embarrassing, and creepy. It's also a symbol of the way the town, in that generation, did things.
We lump it together with arsenic because they are both symbols of getting rid of something that smells, and in the case of "A Rose for Emily," it happens to be the very same thing. Remember what the druggist writes on Emily's packet of arsenic, under the poison sign? "For rats." Faulkner himself claims that Homer was probably not a nice guy. If Homer is planning to break a promise to marry Emily, she, in the southern tradition, would most probably have considered him a rat.
The arsenic used to kill a stinky rat creates a foul stench, which the townspeople want to get rid of with lime. (If you want to read more about arsenic, click here). We should also note that arsenic is a favorite fictional murder weapon, due to its reputation for being odorless, colorless, and virtually undetectable by the victim. Director Franz Capra's 1944 film Arsenic and Old Lace is good example of this.