Altogether, phosphorus flows now add up to an estimated 37 million metric tons per year. Of that, about 22 million metric tons come from phosphate mining. The earth holds plenty ofphosphorus-rich minerals—those consideredeconomically recoverable—but most are not readily available. The International Geological Correlation Program (IGCP) reckoned in 1987 that there might be some 163,000 million metric tons of phosphate rock worldwide, corresponding to more than 13,000 million metric tons of phosphorus, seemingly enough to last nearly a millennium. These estimates, however,include types of rocks, such as high-carbonate minerals, that are impractical as sources because no economical technology exists to extract the phosphorus from them. The tallies also include deposits that are inaccessible because of their depth or location offshore; moreover, they may exist in underdeveloped or environmentally sensitive land or in the presence of high levels of toxic or radioactive contaminants such as cadmium, chromium, arsenic, lead and uranium.
Estimates of deposits that are economically recoverable with current technology—known as reserves—are at 15,000 million metric tons. That is still enough to last about 90 years at current use rates. Consumption, however, is likely to grow as the population increases and as people in developing countries demand a higher standard of living. Increased meat consumption, in particular, is likely to put more pressure on the land, because animals eat more food than the food they become.
Phosphorus reserves are also concentrated geographically. Just four countries—the U.S.,
China, South Africa and Morocco, together with its Western Sahara Territory—hold 83 percent of the world’s reserves and account for two thirds of annual production. Most U.S. phosphate comes from mines in Florida’s Bone Valley, a fossil deposit that formed in the Atlantic Ocean 12 million years ago. According to the U.S. Geological Survey, the nation’s reserves amount to 1,200 million metric tons. The U.S. produces about 30 million metric tons of phosphate rock a year, which should last 40 years, assuming today’s rate of production. Already U.S. mines no longer supply enough phosphorus to satisfy the country’s production of fertilizer, much of which is exported. As a result, the U.S. now imports phosphate rock. China has high-quality reserves, but it does not export; most U.S. imports come from Morocco.
Even more than with oil, the U.S. and much of the globe may come to depend on a single country for a critical resource. Some geologists are skeptical about the existence of a phosphorus crisis and reckon that estimates of resources and their duration are moving targets. The very definition of reserves is dynamic because, when prices increase, deposits that were previously considered too expensive to access reclassify as reserves. Shortages or price swings can stimulate conservation efforts or the development of extraction technologies. And mining companies have the incentive to do exploration only once a resource’s lifetime falls below a certain number of decades. But the completion of old mines spurs more exploration, which expands the known resources. For instance, 20 years ago geologist R. P. Sheldon pointed out that the rate of new resource discovery had been consistent over the 20th century. Sheldon also suggested that tropical regions with deep soils had been inadequately explored: these regions occupy 22 percent of the earth’s land surface but contain only 2 percent of the known phosphorus reserves.
Yet most of the phosphorus discovery has occurred in just two places: Morocco/Western Sahara and North Carolina. And much of North Carolina’s resources are restricted because they underlie environmentally sensitive areas. Thus, the findings to date are not enough to allay concerns about future supply. Society should therefore face the reality of an impending phosphorus crisis and begin to make a serious effort at conservation.
The standard approaches to conservation apply to phosphorus as well: reduce, recycle and reuse. We can reduce fertilizer usage through more efficient agricultural practices such as terracing and no-till farming to diminish erosion [see
“No-Till: The Quiet Revolution,” by David R. Huggins and John P. Reganold; Scientific American, July 2008]. The inedible biomass harvested with crops, such as stalks and stems, should be returned to the soil with its phosphorus, as should animal waste (including bones) from meat and dairy production, less than half of which is now used as fertilizer. We will also have to treat our wastewater to recover phosphorus from solid waste. This task is difficult because residual biosolids are contaminated with many pollutants, especially heavy metals such as lead and cadmium, which leach from old pipes. Making agriculture sustainable over the long term begins with renewing our efforts to phase out toxic metals from our plumbing.
Half the phosphorus we excrete is in our urine, from which it would be relatively easy to recover. And separating solid and liquid human waste—which can be done in treatment plants or at the source, using specialized toilets— would have an added advantage. Urine is also rich in nitrogen, so recycling it could offset some of the nitrogen that is currently extracted from the atmosphere, at great cost in energy. Meanwhile new discoveries are likely just to forestall the depletion of reserves, not to prevent it. For truly sustainable agriculture, the delay would have to be indefinite. Such an achievement would be possible only with a world population small enough to be fed using natural and mostly untreated minerals that are low-grade sources of phosphorus. As with other resources, the ultimate question is how many humans the earth can really sustain. We are running out of phosphorus deposits that are relatively easily and cheaply exploitable. It is possible that the optimists are correct about the relative ease of obtaining new sources and that shortages can be averted. But given the stakes, we should not leave our future to chance.
Source of Information : Scientific American(2009-06)