Eutrophication

Farmers around the world have come to depend on manufactured inorganic fertilizers containing key plant nutrients phosphorus, nitrogen and potassium to enhance soil fertility, especially in the otherwise poor soils of most tropical settings. But while all three are relatively abundant in nature, commercially viable sources of phosphorus to make these fertilizers could be exhausted just a few decades from now. That prospect, which remains a source of heated debate, has spurred a drive to recover the significant quantities of this element that disappear in the waste streams of cities and farms.