Phosphorus has a number of indispensable biochemical roles, but it does not have a rapid global cycle akin to the circulations of C or N. Natural mobilization of the element, a part of the grand geotectonic denudation-uplift cycle, is slow, and low solubility of phosphates and their rapid transformation to insoluble forms make the element commonly the growth-limiting nutrient, particularly in aquatic ecosystems. Human activities have intensified releases of P. By the year 2000 the global mobilization of the nutrient has roughly tripled compared to its natural flows: Increased soil erosion and runoff from fields, recycling of crop residues and manures, discharges of urban and industrial wastes, and above all, applications of inorganic fertilizers (15 million tonnes P/year) are the major causes of this increase. Global food production is now highly dependent on the continuing use of phosphates, which account for 50-60% of all P supply; although crops use the nutrient with relatively high efficiency, lost P that reaches water is commonly the main cause of eutrophication. This undesirable process affects fresh and ocean waters in many parts of the world. More efficient fertilization can lower nonpoint P losses. Although P in sewage can be effectively controlled, such measures are often not taken, and elevated P is common in treated wastewater whose N was lowered by denitrification. Long-term prospects of inorganic P supply and its environmental consequences remain a matter of concern.

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Nitrogen was the most commonly yield-limiting nutrient in all pre-industrial agricultures. Only the Haber-Bosch synthesis of ammonia broke this barrier. The rising dependence on nitrogenous fertilizers, which represents the largest human interference in the biospheric N cycle, has two different roles. In affluent nations it helps to produce excess of food in general, and of animal foods in particular, and it boosts agricultural exports. But for at least a third of humanity in the world's most populous countries the use of N fertilizers makes the difference between malnutrition and adequate diet. Our understanding of human N (protein) needs has undergone many revisions and although some uncertainties still remain it is clear that average protein intakes are excessive in rich countries and inadequate for hundreds of millions of people in Asia, Africa, and Latin America. More dietary protein will be needed to eliminate these disparities but the future global use of N fertilizers can be moderated not just by better agronomic practices but also by higher feeding efficiencies and by gradual changes of prevailing diets. As a result, it could be possible to supply adequate nutrition to the world's growing population without any massive increases of N inputs.