Phosphorus is a scarce finite resource on earth and means of production other than mining are unavailable because of its non-gaseous environmental cycle. The predominant source of phosphorus comes in the form of phosphate rock and in the past guano. According to some researchers, Earth's phosphorus reserves are expected to be completely depleted in 50–100 years and peak phosphorus to be reached in approximately 2030.Others suggest that supplies will last for several hundreds of years. The question is not settled and researchers in different fields regularly publish different estimates of the rock phosphate reserves.
The peak phosphorus concept is in line with the concept of planetary boundaries. Phosphorus, as part of biogeochemical processes, belongs to one of the nine Earth system processes which are known to have boundaries that, to the extent that they are not crossed, mark the safe zone for the planet.
Estimates of world phosphate reserves
The accurate determination of peak phosphorus is dependent on knowing the total world's phosphate reserves and the future demand for rock phosphate. In 2012, the United States Geological Survey (USGS) estimated that phosphorus reserves worldwide are 71 billion tons, while world mining production in 2011 was 0.19 billion tons and this has been taken to mean that there were enough reserves to last for at least 370 years and possibly a lot longer. These reserve figures are widely used, but others suggest that there has been little external verification of the estimate.
There are many different views as to the extent of world phosphate resources. The International Fertilizer Development Center (IFDC) in a 2010 report estimated that global phosphate rock resources would last for several hundred years.This is disputed by a recent review which concludes that the IFDC report "presents an inflated picture of global reserves, in particular those of Morocco, where largely hypothetical and inferred resources have simply been converted to “reserves". Another review suggests that it is "not very likely" that there would be significant depletion of extractable rock phosphate by 2100.
"Reserves" refer to the amount assumed recoverable at current market prices and "resources" mean total estimated amounts in the Earth's crust. Phosphorus comprises 0.1% by mass of the average rock (while, for perspective, its typical concentration in vegetation is 0.03% to 0.2%), and consequently there are quadrillions of tons of phosphorus in Earth's 3 * 1019 ton crust, albeit at predominantly lower concentration than the deposits counted as reserves from being inventoried and cheaper to extract.
Economists have pointed out that there do not need to be shortages of rock phosphate to cause price fluctuations, as these have already occurred due to various demand and supply side factors.
Rock phosphate shortages (or just significant price increases) would have a big impact on the world's food security. Many agricultural systems depend on supplies of inorganic fertiliser, which use rock phosphate. Unless systems change, shortages of rock phosphate could lead to shortages of inorganic fertiliser, which could in term affect crop growth and cause starvation.
Exhaustion of guano reserves
In 1609 Garcilaso de la Vega wrote the book Comentarios Reales in which he described many of the agricultural practices of the Incas prior to the arrival of the Spaniards and introduced the use of guano as a fertilizer. As Garcilaso described, the Incas near the coast harvested guano. In the early 1800s Alexander von Humboldt introduced guano as a source of agricultural fertilizer to Europe after having discovered it on islands off the coast of South America. It has been reported that, at the time of its discovery, the guano on some islands was over 30 meters deep. The guano had previously been used by the Moche people as a source of fertilizer by mining it and transporting it back to Peru by boat. International commerce in guano didn't start until after 1840. By the start of the 20th century guano had been nearly completely depleted and was eventually overtaken with the discovery of superphosphate.
Phosphorus conservation and recycling
A huge amount of phosphorus is transferred from the soil in one location to another as food is transported across the world, taking the phosphorus it contains with it. Once consumed by humans, it can end up in the local environment (in the case of open defecation which is still wide-spread on a global scale) or in rivers or the ocean via sewage systems and sewage treatment plants in the case of cities connected to sewer systems. An example of one such crop in South America that takes up large amounts of phosphorus is soy. At the end of its journey, the phosphorus often ends up in rivers in Europe and the USA.
In an effort to postpone the onset of peak phosphorus several methods of reducing and reusing phosphorus are in practice, such as in agriculture and in sanitation systems. The Soil Association, the UK organic agriculture certification and pressure group, issued a report in 2010 "A Rock and a Hard Place" encouraging more recycling of phosphorus. One potential solution to the shortage of phosphorus is greater recycling of human and animal wastes back into the environment.
Reducing agricultural runoff and soil erosion can slow the frequency with which farmers have to reapply phosphorus to their fields. Agricultural methods such as no-till farming, terracing, contour tilling, and the use of windbreaks have been shown to reduce the rate of phosphorus depletion from farmland. These methods are still dependent on a periodic application of phosphate rock to the soil and as such methods to recycle the lost phosphorus have also been proposed. Perennial vegetation, such as grassland or forest is much more efficient in its use of phosphate than arable land. Strips of grassland and or forest between arable land and rivers can greatly reduce losses of phosphate and other nutrients.
Integrated farming systems which use animal sources to supply phosphorus for crops do exist at smaller scales, and application of the system to a larger scale is a potential alternative for supplying the nutrient, although it would require significant changes to the widely adopted modern crop fertilizing methods.
The oldest method of recycling phosphorus is through the reuse of animal manure and human excreta in agriculture. Via this method, phosphorus in the foods consumed are excreted, and the animal or human excreta are subsequently collected and re-applied to the fields. Although this method has maintained civilizations for centuries the current system of manure management is not logistically geared towards application to crop fields on a large scale. At present, manure application could not meet the phosphorus needs of large scale agriculture. Despite that, it is still an efficient method of recycling used phosphorus and returning it to the soil.
Sewage treatment plants that have an enhanced biological phosphorus removal step produce a sewage sludge that is rich in phosphorus. Various processes have been developed to extract phosphorus from sewage sludge directly, from the ash after incineration of the sewage sludge or from other products of sewage sludge treatment. This includes the extraction of phosphorus rich materials such as struvite from waste processing plants. The struvite can be made by adding magnesium to the waste. Some companies such as NuReSys are already using this technique to recover phosphate.
Research on phosphorus recovery methods from sewage sludge has been carried out in Sweden and Germany since around 2003, but the technologies currently under development are not yet cost effective, given the current price of phosphorus on the world market.
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