One way to feed the globe's growing population is to ramp up intensive farming in tropical regions, but doing so will require a lot of fertilizer - particularly phosphorus. This is not only because it is often present at very low levels in tropical soils, but also because many of these soils bind added phosphorus fertilizer, making it less available to crops.

A new study in Nature Plants estimates that intensifying farming on the world's phosphorus (P) binding soils could annually sequester in soil 1 to 4 million metric tons of P fertilizer. For comparison, approximately 2 million metric tons of fertilizer phosphorus are used in North America each year.

Furthermore, the authors found that even after fertilizing for decades, farmers on P-binding soils will still be forced to pay this "P-tax" every year, tying their success or failure to the production, distribution and cost of a finite resource found largely in just a handful of locations around the world. By 2050, the "P-tax" could double if the expansion of the world's tropical cropland area continues.

Eric Roy, now an assistant professor at the University of Vermont, led the study with his former advisor at Brown University, Associate Professor Stephen Porder. They and a team of colleagues at Brown and in Brazil interviewed farmers in Mato Grosso, Brazil, a state with very poor soils that has become a global agricultural powerhouse by using large quantities of phosphorus fertilizer.

Farmer surveys, and Brazilian government statistics, suggest that even after decades of fertilizer inputs in excess of what is taken up by crops, these P-binding soils still sequester about 50 percent of the phosphorus fertilizer added.

"Tropical soils are really different and so the costs, consequences and considerations of trying to do this (intensive agriculture) in the tropics are different," Porder said. "Our study is the first to ask the question about any of these costs: What is it really going to take to do this at a large scale?"

Phosphorus problems
Roy said the phosphorus tax needs to figure into projections about whether tropical agricultural intensification will be economical and sustainable over the long term. Like oil, phosphate rock is in finite supply and the world's reserves are concentrated in a small number of places (about 72 percent of current reserves are in Morocco and Western Sahara). Unlike oil, phosphorus cannot be replaced by something else, Porder said.

"There are very real economic and political considerations to building a global food supply based around a finite resource that is mostly found in a few countries and which we will be dependent on for decades because these soils are not going to saturate anytime soon," Porder said.

Roy said he is not worried that the world is going to "run out" of phosphorus. Rather, food security could become more vulnerable to geopolitical dynamics and the volatility of phosphate rock prices.

Agricultural alternatives
Roy and Porder described several ways to mitigate the phosphorus tax.

One way would be to recycle more phosphorus-rich livestock manure to tropical croplands and reduce the need for synthetic fertilizer made from phosphate rock. However, in Brazil this has been limited because most of the soybeans farmers grow are exported to feed animals reared for meat in China and Europe.

Globally, however, this is an important solution, Roy said. Researchers around the world are developing ways to "close the loop" and safely recycle phosphorus from human waste back to croplands.

A second possibility would be to rethink high-meat diets, which require more land in agriculture - and more phosphorus - than a low-meat or meatless diet would, Porder said. A lot of cropland is currently devoted to growing feed, including corn and soybeans, for meat animals. By growing more crops for people, rather than livestock, Roy said, less land and other resources would be needed for agriculture overall.

Third, a lot of the world's food is wasted, Porder said. Reducing food waste would also slow the presumed need to intensify tropical agriculture.

A final option may be technology, if scientists can develop a way to recover phosphorus from the soils that currently bind it tightly.

These changes are important to start now, Roy and Porder urged.

"If we don't make these changes to diets and we don't make these changes to food waste, we will need more land in intensive production," Porder said. That would incur other resource costs beyond phosphorus, he said, like using more pesticides and fossil fuels as well.

In addition to Roy and Porder, the paper's other authors are Peter Richards, Luiz Martinelli, Luciana Della Coletta, Silvia Rafaela Machado Lins, Felipe Ferraz Vazquez, Edwin Willig, Stephanie Spera and Leah VanWey.