"If you have too much phosphorous, you get eutrophication," explains Carpenter of the cycle of excessive plant and algae growth that significantly degrades bodies of fresh water. "Phosphorous stimulates the growth of algae and weeds near shore and some of the algae can contain cyanobacteria, which are toxic. You lose fish. You lose water quality for drinking."

The fertilizer-fueled algae blooms themselves amplify the problem as the algae die and release accumulated phosphorous back into the water.

Carpenter and Bennett write in their Environmental Research Letters report that the "planetary boundary for freshwater eutrophication has been crossed while potential boundaries for ocean anoxic events and depletion of phosphate rock reserves loom in the future."

Complicating the problem, says Carpenter, is the fact that excess phosphorous in the environment is a problem primarily in the industrialized world, mainly Europe, North America and parts of Asia. In other parts of the world, notably Africa and Australia, soils are phosphorous poor, creating a stark imbalance. Ironically, soils in places like North America, where fertilizers with phosphorous are most commonly applied, are already loaded with the element.

"Some soils have plenty of phosphorous, and some soils do not and you need to add phosphorous to grow crops on them," Carpenter notes. "It's this patchiness that makes the problem tricky."

Bennett and Carpenter argue that agricultural practices to better conserve phosphate within agricultural ecosystems are necessary to avert the widespread pollution of surface waters. Phosphorous from parts of the world where the element is abundant, they say, can be moved to phosphorous deficient regions of the world by extracting phosphorous from manure, for example, using manure digesters.

Deposits of phosphate, the form of the element that is mined for agriculture and other purposes, take many millions of years to form. The nations with the largest reserves of the element are the United States, China and Morocco.

The new study was supported by grants from the U.S. National Science Foundation and the Natural Sciences and Engineering Research Council of Canada.