The Amazon is fertilised by dust from the Sahara

Satellite measurements trace a plume of African dust across the Atlantic and onto the Amazon, carrying on the order of twenty thousand tonnes of phosphorus a year. The forest's soil is poor. The desert makes up some of the difference.

The Amazon basin recycles a great deal of its nutrients, but phosphorus is lost to the rivers and the harvest of growth, and the soils are old. The Bodélé depression in the Sahara, a dry lake bed, is one of the dustiest places on Earth. In winter the wind lifts it. Lidar on the CALIPSO satellite, read by Hongbin Yu and colleagues, followed the plume for years: tens of millions of tonnes of dust a year deposited on the basin, and within it a phosphorus flux of roughly twenty-two thousand tonnes a year, comparable to what the rivers carry out. The number moves with the wind and the year. The direction of the trade does not.

The forest is not a plantation fed by Africa, and the dust is not its only source. Fires, weathering, and the recycling of leaves do most of the daily work. What the measurement adds is a dependency nobody standing in either place would guess. A lake that dried in Chad helps set the fertility of a forest in Brazil. The two landscapes are one system, joined by a wind you cannot see from the ground at either end.