Large Scale Expansion of Organic Farming at Risk of Phosphorus Limitation in the Long Run.

Debates about the agronomic feasibility of feeding the world organically have often focused on potential nitrogen (N) limitations, overlooking the challenges raised by the availability of phosphorus (P) in soils. Here we address this missing piece by simulating, over a 100-year time span, a 100% organic world, in which mineral fertilizers are banned. We show that, in the short term, soil available P would barely limit organic crop production due to (i) existing soil P stocks, (ii) a replenishment of the available P pool by more stable pools, and (iii) a strong N limitation that reduces crop yields and consequently plant P demand. Conversely, in the long term (~100 years), the simulated 60% drop in cropland soil available P would result in a -12% reduction in cropland production globally, ranging from -4% in Eastern Europe to -29% in Latin America. This P limitation would be greater in places with high yields and a high proportion of legumes in crop rotations, highlighting the need to address N-P interactions simultaneously to design more sustainable food systems. The expansion of organic farming therefore runs the risk of slowly but surely depleting soil P unless measures are taken to recycle all P flows within the agri-food system.