The legacy of phosphorus: agriculture and future food security

Growing global demand for food leads to increased concern regarding phosphorus (P), a finite and dwindling resource. Debate focuses on current production and use of phosphate rock rather than on the amount of P required to feed the world in the future. While the time scale of P depletion is debatable, a critical question beyond the physical availability of P is whether P resource depletion can be managed by sustainable consumption of P. We quantified P demand for cropland as well as grassland in 2050 at global scale. Methods employed included an extensive analysis of historical, long-term P application and agricultural production data and a modeling approach that considers major P flows in a soil-crop system. We applied a simple two-pool soil P (DPPS) model to reproduce historical crop and grass P uptake as a function of P inputs from fertilizer and manure and to estimate P requirements for crop and grass production in 2050. Accounting for legacy P in estimation of the required fertilizer in cropland leads to a reduced fertilizer requirement compared with other studies that did not account for residual soil P. In Europe, Asia, Latin America and Oceania, crop production can benefit from the residual P. In contrast, in Africa more than a five-fold increase in P application is needed to achieve the target P uptake in 2050. I conclude that the future P requirements from fertilizer in cropland increase less than crop production increases, whereas in grassland the opposite is true. This is because much of the P in animal manure spread in cropland originates from grassland. The transfer of (manure) P from grassland to cropland is not compensated with the transfer of P in livestock feed from cropland to grassland – resulting in soil P depletion of grassland. To achieve the target crop and grass production in the next four decades a global cumulative P input from mineral fertilizer and manure of ca. 1200 and 1215 Tg is needed in cropland and grassland, respectively. The amount of mineral fertilizer P needed in cropland and grassland systems in total is estimated to be 1380 Tg until 2050, corresponding to 10700 Tg phosphate rock. This amount of phosphate rock is about 16% of the total phosphate rock reserves currently thought to be available on the planet. Thus, we will not immediately run out of P, but mineral fertilizer prices may increase, which may pose a serious challenge to regions with low soil P stocks. Finally, to provide an example of potential solutions to the global P scarcity, China as a key player in P consumption and production is studied and the feasibility of efficient use of P in China’s agriculture is discussed.