Phosphorus shortage: How to safeguard the agricultural system’s future.

The Dirty but Necessary Way to Feed 9 Billion People

The Dirty but Necessary Way to Feed 9 Billion People

The citizen’s guide to the future.
Dec. 25 2013 11:41 PM

The Dirty Way to Feed 9 Billion People

The solution to a major agricultural problem is a little gross, but it’s necessary.

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What stands between our current world of mined fertilizer and increasingly nasty water and the sometime-in-the-future alternative worlds of scarcity and mayhem versus abundance and sustainability?

The best opportunity for lowering our demand for mined P is to recover and reuse P from agricultural and human wastes. Animal manures, food-processing wastes, and human sewage constitute about half of the P on the conveyor belt to the environment. These waste streams offer the most immediate route to recovery and reuse because most of the P is in slurries of organic solids that also contain high amounts of energy. Anaerobic digestion, in which specialized microbes chew up organic matter in the absence of oxygen while producing methane gas, or microbial electrolysis cells, in which bacteria generate an electrical current that leads to hydrogen gas, are excellent means to convert the organic materials into highly valuable energy outputs. These microbial processes release the P as phosphate, which can be captured in clean, concentrated, and convenient forms for reuse in agriculture. Using microorganisms this way would give us three valuable things: renewable energy, concentrated P, and water with most of its pollution removed. All three contribute to economic, food, and environmental sustainability. These technologies aren’t yet reliable and cost-effective, but their eventual deployment could create whole new industrial and job sectors. Implementing these technologies allows fertilizer production to become regionally distributed and self-sustaining, and thus resilient against global geopolitical perturbations.

That still leaves the phosphorus that comes from erosion and drainage from agricultural fields. The best option would be to take all measures to reduce erosion and maintain healthy soil, as capturing and reusing this P once it has escaped the field is a much greater challenge. The P is in low concentration, attached to soil particles, and not accompanied by the energy-rich organics that enhance economic viability. Also, erosion and runoff often come in sudden, large flows associated with storms. Methods to remove and capture P from these flows are not as close at hand as technologies for the organic-rich slurries. Continued improvement of precision fertilizer application will also be needed, assuring that P finds its way to the crop, where it belongs.


Another strategy to help close the P cycle is to shift the human diet away from meat consumption. Only 10 percent of the phosphorus that animals eat ends up in our meat—much of the leftover goes onto the P conveyor belt to spoil our water. But even if we eventually recycle all manure, lowering meat consumption will still help reduce pollution by reducing farm runoff because we won’t need to grow so much feed for livestock.

Click on the animation above to see how different recovery and reuse strategies can cut the input of mined P from its current rate of 14 million metric tons per year to almost zero, while lowering P pollution to the environment by about 80 percent. Complete energy and P capture from the organic slurries can lower mined-P demand and P pollution by almost 50 percent, while providing renewable energy. Then, reducing P loss from erosion and drainage by 50 percent—realistic through a combination of more precise fertilizer use and capturing P from contaminated waters—achieves almost an 86 percent reduction in mined-P demand. By adding the third step—reducing meat consumption by 50 percent—we can get the mined P to a minimal level and cut P pollution by almost 80 percent. (Achieving all of these measures will be made easier if human population peaks at lower levels in coming decades, closer to 8 billion than the daunting 11 billion included in some projections.) As we take these steps, upward pressures on fertilizer prices will ease, enhancing fertilizer access for farmers in the developing world so they can raise their crop yields and achieve food security.

So, what world future will be realized? Will our descendants be buffeted by a global resource battle over a dwindling fossil P supply that also threatens their drinking water? Or, will they prosper in a food- and water-secure world, nourished by a distributed, resilient, and sustainable fertilizer supply? We have tools for food and water security. Will we use them?

This article is part of Future Tense, a collaboration among Arizona State University, the New America Foundation, and SlateFuture Tense explores the ways emerging technologies affect society, policy, and culture. To read more, visit the Future Tense blog and the Future Tense home page. You can also follow us on Twitter.

James J. Elser is Regents' Professor in the School of Life Sciences at Arizona State University.

Bruce E. Rittmann is Regents’ Professor of Environmental Engineering and director of the Swette Center for Environmental Biotechnology at Arizona State University.