Just 8% of the mineral phosphorus input into US domestic agriculture in 2007 ended up being eaten by Americans that year. The rest was retained in soil, lost post-harvest, used in biofuels, or exported in the form of crops or meat. That is according to scientists from Canada and the US.
"Increasingly, phosphorus is being thought of as a vital non-renewable resource because there are finite reserves of high-quality phosphate rock," Graham MacDonald, formerly of McGill University, Canada, and now at the University of Minnesota, US, said. "Although the lifespan of these reserves is the subject of debate and considerable uncertainty, the largest reserves are concentrated in just a handful of countries."
Unlike nitrogen and carbon, said MacDonald, phosphorus is unique in that it does not typically get transported in large quantities through the atmosphere. "So the effect of taking phosphorus fertilizers from where phosphate rock was mined and redistributing it elsewhere, as is the case with fertilizer traded among countries, could really alter the distribution of phosphorus across the landscape."
Together with colleagues from McGill, University of Wisconsin-Madison, US, and University of Minnesota, US, MacDonald analysed the use of phosphate fertilizer in the US to produce food crops, livestock and biofuels. The team also examined the amount of phosphorus in food exported from or imported into the nation.
"Given these dimensions of phosphorus – as a key fertilizer, a potential pollutant and a non-renewable resource that is being redistributed by humans – we felt that it was important to consider how phosphorus moves through the food system," said MacDonald. Phosphorus in agricultural run-off has been implicated in causing hypoxia in regions such as the Gulf of Mexico.
The US is the leading exporter of phosphate fertilizer worldwide but some forecasts indicate that depletion of the country's phosphate rock reserves could occur within decades. One-quarter of the phosphorous fertilizer used in the US goes towards goods that are later exported, primarily crops such as corn, wheat and soybeans. This takes about 338 Gg of phosphorus out of the country each year. But other imports, such as red meat, rely on phosphate fertilizer use abroad.
"Food systems in some countries, particularly highly developed nations like the US, appear to be rather 'leaky' in terms of phosphorus – that is, we tend to see considerable potential for losses of phosphorus between the field and the dinner plate – whether it be nutrient excesses in livestock waste that get concentrated on some farms, phosphorus losses during processing and transportation, or food waste at the consumer level," said MacDonald.
The team suggested several solutions to reducing mineral phosphorus use: achieving more effective recycling of livestock waste, lessening phosphorus use linked to red-meat production, curbing some consumer waste and ensuring that fertilizers are targeted precisely to the crops that need them most. Farmers could also produce a more diverse set of agricultural commodities and grow biofuel crops that are more phosphorus-efficient.
The researchers estimate that changes in domestic farm management and consumer waste could together halve the amount of phosphate fertilizer required for US food consumption. They say this is on a par with the phosphate fertilizer reduction attainable by cutting domestic meat consumption.
"US export-oriented agriculture, domestic post-harvest phosphorus losses and global demand for meat may ultimately have an important influence on the lifespan of US phosphate rock reserves," wrote the scientists in ERL.