Recovering phosphorus from soil: a root solution?

value of the current global soil P resource is appreciating the range of complexed P forms in soil and their specific issues for crop P accessibility. Soil organic P is especially poorly considered in terms of amounts and future utility. Learning to better utilize all of the different residual P forms in soil is essential to providing a sustainable cycle between agricultural inputs and plant uptake, akin to more natural systems. Residual soil P (that becoming stabilized within soils) includes a range of chemical forms. Most research related to improving phosphorus recovery has considered only the accumulated inorganic orthophosphate forms in the soil. Yet a considerable amount of phosphate becomes incorporated into organic compounds such as monoesters, diesters (e.g., DNA), and polyphosphates (e.g., ATP). Some orthophosphate monoesters are stabilized in soils due to a high charge density or formation of metal ion salts, such that large reserves have accumulated in many soils. A review of global arable and intensive grassland topsoil P data shows the accumulation of soil P in inorganic and organic forms (Figure l). Countries such as the United States, Canada, and Germany, where intensive agriculture relies on large fertilizer inputs, have soils that contain large proportions of inorganic orthophosphate, but worldwide, organic P forms make important contributions to topsoil stocks. UK data (Figure 1, dominated by pasture soils) show a large monoester contribution of 54% to the extractable P total. Global averages across arable and pasture soils ranged from 24% (Germany) to 59% (Ethiopia). Organic P therefore warrants attention as a quantitatively important soil P resource. Developing agricultural systems to use these soil P reserves will mark the start of a process of reducing the dependency on excessive applications of mined P fertilizers to sustain production. However, new P use strategies must be able to maintain food production to sustain the world's population, predicted to rise to 9 billion by 2050. Evidence is required to show that withholding fertilizer from agricultural soils will not suppress crop yields, because productivity can be fueled by utilization of residual inorganic and organic P forms. Both these P forms accumulate in soils but require different approaches to realizing their use. Many staple crops appear poor at accessing organic P, having been bred and selected for compatibility with Phosphorus (P) is crucial to modern agricultural production, but access to geographically concentrated global reserves of phosphate rock is politically sensitive and economically challenging. The use of mined rock phosphate as a principal P source for chemical fertilizers facilitated last century's agronomic revolution, but increased our dependence on this nonrenewable P source to unsustainable levels. The plant availability of phosphate fertilizer is reduced by sorption and organic complexation in the soil, so fertilizer applications greater than the amount required by the crop are used to counteract the strong binding of the phosphate to the soil matrix. This has greatly increased the P content of managed soils, many of which now contain sufficient P reserves to potentially buffer threats to food security for many decades. Overapplication of P fertilizer to soil is in itself wasteful, but the transport of P to watercourses by erosion is also causing widespread problems of eutrophication. We argue that research should be targeted urgently toward capitalizing on the accumulated soil P reserves by improving P bioavailability and utilization by crops, while simultaneously developing more efficient use strategies for rock phosphate, manures, composts, and wastes. A fundamental stage in realizing the magnitude and