For example, sargassum seaweed, which floats freely in the ocean, is rich in both phosphorus and ammonium. There are already efforts in the Caribbean to turn s

For example, sargassum seaweed, which floats freely in the ocean, is rich in both phosphorus and ammonium. There are already efforts in the Caribbean to turn sargassum into compost, because there's so much of it washing ashore that it's become a nuisance. Medieval Irish farmers relied on seaweed fertilizer to turn much of the island into rich farmland.