NATURE GEOSCIENCE | VOL 6 | NOVEMBER 2013 | www.nature.com/naturegeoscience 897 FOCUS | COMMENTARY A key component of DNA, cell membranes and cellular energy transport, phosphorus is essential to every form of life. Yet most soils contain only low concentrations of this nutrient. Phosphorus inputs in the form of fertilizers are therefore essential for food production around the globe. e majority of phosphorus for agriculture is mined from finite, and therefore exhaustible 1,2 , sedimentary phosphorite deposits. Growing demands for food and biofuels mean that these reserves could become exhausted in the next 40 to 400 years 1,2 . As such, phosphorus is expected to become economically inaccessible to low- income and food-deficient countries. Discontent over soaring food prices has revitalized interest in food production, and raised awareness of the issues around dwindling phosphorus supplies 1,3 . According to an emerging consensus, scarcities in phosphorus supplies and temporary constraints in accessibility will lead to marked rises in the cost of phosphorus. For instance, when China introduced an export ban on minerals including phosphorus in 2008, the global price of phosphorus rose 50% faster than the price of any of the main staple crops. An elevated price prevails today. Here, we identify three broad groups of nations — rich phosphorus consumers, poor and food-insecure phosphorus consumers, and phosphorus producers — with competing interests in the longevity, equitability and cost of phosphorus supplies. We argue that continued structural inequalities in phosphorus use and supply could exacerbate the divergent agendas of these three groups of nations, and potentially render a sustainable management regime of finite phosphorus resources impossible. Structural inequalities High prices have not prevented farmers in rich countries from buying phosphorus- containing fertilizers to maximize crop yields. Indeed, North America, western Europe and eastern and southern Asia, including China and India, account for more than 80% of the global consumption of phosphorus fertilizers (Fig. 1), despite the fact that less than 55% of the world’s croplands are located in these regions 4 . In fact, phosphorus has oſten been used in excess in many of these regions, leading to phosphorus pollution of freshwater and marine ecosystems 4 . Up to 80% of the phosphorus supplied to crops is estimated to be lost before consumption, largely due to the erosion of agricultural soils 1 . In contrast, expense has hindered use of phosphorus fertilizers in low-income and food-deficient countries, where phosphorus deficits occur across some 30% of the cropland area 5,6 . e location of the world’s phosphorus reserves is equally uneven. Although precise data on the amount of mineable phosphorus are still lacking, the world’s phosphorus reserves are highly concentrated, with Morocco estimated to hold 85% of the global share, followed by China with 6% and the US with 3% 5 . National interests We contend that the interests of rich phosphorus consumers, poor and food- insecure phosphorus consumers, and phosphorus producers are, to a large degree, conflicting. If so, any possible solution to limited supplies would conflict with the interest of at least one group (Table 1). Wealthy phosphorus consumers are already using their political and economic weight to secure future supplies from phosphorus producers. We project that rich phosphorus importers will try to continue to source relatively cheap, long- term and exclusive supplies by setting up preferential bilateral trade arrangements with phosphorus producers. Low-income food-deficient countries, many of which are now endowed with phosphorus-deficient soils 6 , oſten as a result of unsustainable management practices, will want to see inequalities in access to phosphorus supplies addressed. e challenge for these countries will be to convince richer countries to mitigate the consequences of their phosphorus deficits for food production. Assisted measures of mitigation could range from food aid to technological transfers that improve agricultural production. Phosphorus-producing nations are likely to increasingly manage their reserves to maximize profit for both their domestic mining and farming industries. To raise prices, phosphorus-producing nations may try to establish multilateral negotiations with a cartel of producers, as the petroleum- exporting countries tried to do with oil. It could be argued that an increase in price would help to conserve this exhaustible natural resource in the long-term. However, increased costs would also limit phosphorus The phosphorus trilemma Michael Obersteiner, Josep Peñuelas, Philippe Ciais, Marijn van der Velde and Ivan A. Janssens Mineable phosphorus reserves are confined to a handful of countries. Reductions in wastage could free up this resource for low-income, food-deficient countries. P fertilizer input for wheat 7 0–5 5–10 10–15 15–20 20–30 >30 P fertilizer kg ha –1 Figure 1 | Global map of phosphorus-fertilizer input for wheat (kg P per hectare), estimated for around the year 2000. In general, the low-income and food-deficient countries in sub-Saharan Africa, central Asia and Latin America suffer from low P inputs (0–5 kg ha −1 ) to their agricultural production systems (this pattern is also true for other crops, alongside wheat). Most northern soils are saturated with P because of historically high applications. © 2013 Macmillan Publishers Limited. All rights reserved