Just 20 years ago, the soils of the Amazon basin were thought unsuitable for large-scale agriculture, but then industrial agriculture -- and the ability to fertilize on a massive scale -- came to the Amazon. What were once the poorest soils in the world now produce crops at a rate that rivals that of global breadbaskets. Soils no longer seem to be the driver -- or the limiter -- of agricultural productivity. But a new Brown University-led study of three soybean growing regions, including Brazil, finds that soils have taken on a new role: mediating the environmental consequences of modern farming. The study focuses on the relationship between soils and phosphorus（磷）, a key agricultural nutrient. Typically in short supply, particularly in tropical soils, phosphorus is unique among fertilizer requirements. It is finite, irreplaceable and mined in just a few places around the world.

"If that suggests scarcity, which is a concern, the overuse of phosphorus can also pose another problem, causing harmful algal（海藻） blooms in waterways," said Stephen Porder, assistant professor of biology in the Department of Ecology and Evolutionary Biology and co-author of the study in the January 2013 edition of BioScience, posted early online. "It's a bit of a Goldilocks problem -- too much and our waterways are choked with algae, too little and we cannot produce enough food."

The new study compares the production of a single crop, soybeans, in the three places they are grown most -- Iowa in the United States, Mato Grosso in Brazil, and Buenos Aires in Argentina. What the authors found was an example that illustrates how the combination of management and soil type frames the phosphorus-related concerns associated with these massive agricultural enterprises.

"Here are three regions where the crop that comes off the farm field is the same, but the fertilizer that goes in and the effects of this fertilizer on the environment are very different," said lead author Shelby Riskin of Brown University and the Marine Biological Laboratory.

In Brazil, farmers must use a tremendous amount of the fertilizer, researchers found. In Iowa, historical overuse of phosphorus continues to harm waterways.

"Having a one-size-fits-all approach to our understanding of interaction between people and their environment via agriculture is going to lead us to some erroneous concerns and conclusions if we don't take the regional biophysical setting into account," Porder said. "If you are concerned about the global phosphorus supply, Brazil is your problem -- they are using a ton of it. If you are concerned about lakes and rivers being filled with algae, then Iowa is your problem, and learning how to mitigate even very small amounts of loss after decades of overfertilization is a real challenge."