When Gold & Gems adorn the Plow To peaceful Arts shall Envy bow (101-102)
This one is pretty simple. A plow is the opposite of weaponry—it's a peaceful tool, used for agriculture. So, Blake's saying that when people finally get their act together, they're going to spend lots of cash and gold and gems on peaceful arts—science, poetry, agriculture, and stuff like that—and less on war. Envy is a source of conflict, but it will eventually humble itself, and let the peaceful things that need to happen happen. It's a PBS-style plea for the arts.