Why is the soil under the world's richest forest so poor?

The soil under a rainforest is poor because the same rain that feeds the forest also washes nutrients out of the ground. The abundant rainfall leaves leached, nutrient-poor soils, and the old soils of the eastern and central Amazon are mineral poor. So why is the forest on top so rich? Because most of its nutrients are not in the soil at all. They are locked inside living trees and pass through a very fast loop.

Here is the loop. When leaves and dead animals fall, the heat and moisture speed up decay, and bacteria, fungi and termites break them down. Plant litter rots rapidly and releases nutrients that plants take up immediately. Rapid bacterial decay even prevents humus from building up. Roots know the trick: most nutrients sit near the surface, so many big trees spread broad buttress roots across the top of the soil instead of reaching deep.

The loop still leaks, because heavy rain and floods wash some nutrients away, and phosphorus is scarce. One surprising top-up comes from far away. Wind carries dust from the Bodélé depression in northern Chad, in the Sahara, and more than 56 percent of the dust fertilizing the Amazon comes from there. NASA's CALIPSO satellite measured an average of 182 million tons of dust leaving the Sahara each year, 15 percent of it falling over the Amazon basin. That dust contains phosphorus, and each year it replaces the equivalent of the phosphorus washed away.

Not all rainforest soil is poor: the young soils of the western Amazon are mineral rich. For a long time, poor soil led people to think that the Amazon could never have supported many people, though recent findings suggest it was once densely populated. Farmers who clear the forest learn the other side of the story: their fields are productive for a short period before the nutrients are consumed or leached away.