What keeps the Amazon's thin soil from starving?

Answer: Dust blown ~5,000 km from the Sahara

Dust blown ~5,000 km from the Sahara ✓ — Correct! Trade winds lift 182 million tonnes of Saharan dust every year, and about 22,000 tonnes of phosphorus settles on the Amazon — roughly matching what the forest loses to runoff. Without a river of African dust crossing the Atlantic, the rainforest's soil would steadily deplete.

Fungi recycle nutrients faster than rain washes them away — Not quite. Amazon fungi really are remarkable — they break fallen leaves back into the system within weeks, and mycorrhizal networks keep trees fed. But heavy tropical rain leaches phosphorus and iron out of the soil faster than internal recycling alone can replace them.

Volcanic ash drifting east from the Andes — Not quite. The Andes do produce volcanic ash, but South America's prevailing winds move westward, carrying Andean dust toward the Pacific — not eastward over the Amazon basin. The crucial input comes from Africa, not from the mountains next door.