What helps replace the phosphorus that heavy rain washes out of the Amazon's thin soil each year?

Answer: Dust blown across the Atlantic from the Sahara

Dust blown across the Atlantic from the Sahara ✓ — Winds lift phosphorus-rich dust from the Sahara, much of it from a dried-up lake bed in Chad, and carry it across the Atlantic to settle on the forest. A NASA satellite study estimated that about 22,000 tonnes of phosphorus arrive this way each year, roughly what the basin loses to rain and flooding. Later measurements suggest the dust share may be smaller, with smoke from African fires adding phosphorus too.

Fungi making new phosphorus as they rot fallen leaves — Fungi cannot make phosphorus; they only free what fallen leaves already contain so that roots can take it up again. That recycling feeds the forest from day to day and moves far more phosphorus than the dust brings. But whatever the rain carries off to the rivers is gone, and only a supply from outside can replace it.

Volcanic ash drifting east on the wind from the Andes — The winds over the Amazon blow the other way, from the Atlantic westward toward the Andes, so ash from the mountains is not carried out across the basin. Andean minerals do reach the lowlands, but as river mud that fertilises the floodplains, not the vast forest between the rivers.