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You’ve heard of peak oil-the idea that the globe’s easy-to-get-to petroleum reserves are largely cashed, and most of what’s left is the hard stuff, buried in deep-sea deposits or tar sands. But what about peak phosphorus and potassium? These elements form two-thirds of the holy agricultural triumvirate of nitrogen, phosphorus , and potassium (also known as NPK, from their respective markers in the Periodic Table of Elements). These nutrients, which are essential for plants to grow, are extracted from soil every time we harvest crops, and have to be replaced if farmland is to remain productive.

For most of agricultural history, successful farming has been about figuring out how to recycle these elements (although no one had identified them until the 19th century). That meant returning food waste, animal waste, and in some cases, human waste to the soil. Early in the 20th century, we learned to mass produce N, P, and K-giving rise to the modern concept of fertilizer, and what’s now known as industrial agriculture.

The N in NPK, nitrogen, can literally be synthesized from thin air, through a process developed in the early 20th century by the German chemist Fritz Haber. Our reliance on synthetic nitrogen fertilizer (as its known) carries its own vast array of problems-not least of which that making it requires an enormous amount of fossil energy. (I examined the dilemmas of synthetic N in a 2011 series on Grist.) But phosphorous and potassium cannot be synthesized-they’re found in significant amounts only in a few large deposits scattered across the planet, in the form, respectively, of phosphate rock and potash. After less than a century of industrial ag, we’re starting to burn through them. In a column  in the Nov. 14

Nature, the legendary investor Jeremy Grantham lays out why that’s a problem:

 These two elements cannot be made, cannot be substituted, are necessary to grow all life forms, and are mined and depleted. It’s a scary set of statements. Former Soviet states and Canada have more than 70% of the potash. Morocco has 85% of all high-grade phosphates. It is the most important quasi-monopoly in economic history.

 What happens when these fertilizers run out is a question I can’t get satisfactorily answered and, believe me, I have tried. There seems to be only one conclusion: their use must be drastically reduced in the next 20-40 years or we will begin to starve.