Jeremy Grantham sounds an unnecessary alarm about the “impending shortage” of phosphorus and potassium fertilizers (Nature 491, 303; 2012). With phosphorus constituting 0.1% of the lithosphere and potassium 2.5%, supplies are likely to outlast our species and possibly even the planet itself.

Grantham assumes that mineral-reserves numbers are indicators of resource availability. However, 'reserve' is an economic and legal concept that has nothing to do with the quantity of material available (see

A reserve represents the amount of an ore or element that has been drilled, tested, measured and defined, and which can be extracted using current techniques and profitably at current prices. It costs a great deal to confirm all those points for a particular mineral deposit, so it is done only for those likely to be used in the coming decades.

'Resource', by contrast, denotes the amount of the same ore or element that is out there, with prior knowledge of roughly where it is, how much there is and how it might be extracted. Resources are transformed into reserves by spending money — when that is strictly necessary. Every generation exhausts its reserves of almost all minerals, because the tendency is to convert only enough resources into reserves to last for a generation.

Resources of phosphate and potassium fertilizers are sufficient for thousands of years of current usage. On that timescale, total element availability is probably more important as a limit.

Jeremy Grantham replies: Phosphorus in Earth's crust is indeed relatively plentiful, but not in the context of feeding a future 9 billion people. Only about 0.5 parts per million of phosphorus occur in phosphate rock deposits that can be extracted economically (D. Cordell and S. White Sustainability 3, 2027–2049; 2011), and the richest deposits are rapidly being depleted.

The price of phosphate rock has risen 4.3-fold in 10 years. The 'big agriculture' style of US farming is demanding ever-increasing quantities of phosphates. This must change, or millions more people will be priced out of the fertilizer and grain markets.