The other major nutrient added to fields as fertilizer is phosphate (PO₄³⁻), and here the problem is a different one. Like nitrate, much of the phosphate added to fields as fertilizer leaves as runoff, and phosphate runoff can be an important contributor to eutrophication. The Everglades of south Florida provide a good example. This ecosystem is built of plants that have adapted to low phosphate levels. However, phosphate levels in parts of the Everglades have increased up to a hundredfold. Chemical tracers show that these increased phosphate levels stem from phosphate fertilizer used on agricultural lands to the north. As a consequence, phosphate-loving cattails and other introduced plants are expanding at the expense of native plants, decreasing the diversity of this remarkable ecosystem.

There’s no question that overuse of phosphates has detrimental effects on certain natural ecosystems, and yet we need to be concerned that we will eventually run out of phosphates to use as fertilizers. Unlike for nitrate, there is no industrial process to produce phosphate. Thus, the only source of phosphate fertilizer is mining. Only a few places in the world have sedimentary rock with phosphates in high enough concentration to be worth mining, and these stores are being depleted.

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Globally, phosphate production peaked about 1990, raising a serious question of how the world’s need for phosphate fertilizer will be satisfied in the future. In many ways, the problem of phosphate availability parallels that of petroleum production. As abundant sources become depleted, we will have to mine poorer and poorer resources, raising production costs and ultimately calling into question our capacity to sustain agricultural production.

Human encroachment on the nitrogen cycle and long-term phosphate availability, then, converge on a single question: Can we manage future agricultural production in ways that feed the growing human population in a sustainable fashion?