Adam Clarke on Amos 2:1

For three transgressions of Moab and for four - See an explanation of this form Amo 1:2. The land of the Moabites lay to the east of the Dead Sea. For the origin of this people, see Gen 19:37.

He burned the bones on the king of Edom into lime - Possibly referring to some brutality; such as opening the grave of one of the Idumean kings, and calcining his bones. It is supposed by some to refer to the fact mentioned 2Kgs 3:26, when the kings of Judah, Israel, and Idumea, joined together to destroy Moab. The king of it, despairing to save his city, took seven hundred men, and made a desperate sortie on the quarter where the king of Edom was; and, though not successful, took prisoner the son of the king of Edom; and, on their return into the city, offered him as a burnt-offering upon the wall, so as to terrify the besieging armies, and cause them to raise the siege. Others understand the son that was sacrificed to be the king of Moab's own son.