Peak Phosphorus

Demand for this essential plant nutrient is rising, yet supplies are finite. Will we run out?

June 28, 2013

Phosphate mining, as seen in this 1961 photo, fuels modern industrial agriculture.In the early decades of the 20th century, the world's farmers launched a global experiment in industrial agriculture. Fueled by vast stores of chemical and mineral reserves, farmers began pushing the limits of production in their fields and orchards. Now, industrial agriculture may be losing one of its key ingredients: phosphorus.

Phosphorus, one of six elements that are essential to all forms of life, is a necessary component in DNA and cell membranes. Plants need phosphorus to develop healthy roots, to bloom, and to form fruits and seeds. While a phosphorus deficiency will not halt photosynthesis, it will hinder the plant's ability to use photosynthesis-generated sugars—essential building blocks in plant cells. This leads to a buildup of sugars in the leaves, which causes a darkening of the tissues in some plants. Phosphorus-starved tomatoes develop those characteristic purple leaves because they simply cannot move sugars from their leaves quickly enough.


Despite its importance, phosphorus is relatively scarce and is often a limiting factor for plant growth in gardens, farms, and natural ecosystems. Historically, farmers and gardeners replenished soil phosphorus levels by recycling crop debris and manure or by applying natural sources, such as bone meal.

The discovery of phosphate-rich rock changed all this. Modern industrial agriculture was made possible by the phosphate mining industry that was born in the late 1800s. Cheap and accessible phosphorus fertilizer broke the closed loop of farm fertility. It became easier to transport processed phosphorus from elsewhere than to reuse the phosphorus already on site.

"Today's high crop yields would not have been possible without the use of processed mineral fertilizers," warns Dana Cordell, Ph.D., principal researcher with the Global Phosphorus Research Initiative, and her colleagues in a 2009 research paper. "We are effectively addicted to phosphate rock." This dependence does not bode well for the future of industrial agriculture. Like petroleum, phosphorus is a limited resource that becomes increasingly more difficult and expensive to mine as easy-to-extract reserves are depleted. Ultimately, as with any nonrenewable resource, we will reach the moment of peak phosphorus, defined as the point when the world maximizes its harvest of the resource. Once past peak, supply will decline even if demand continues to rise. The result may be chaotic and rapidly rising prices for phosphorus fertilizers, as well as the agricultural products fueled by those fertilizers—vegetables, fruits, and meats.

Scientists disagree about the proximity of this peak. Many scientists, including Cordell, estimate peak phosphate will arrive in this century, possibly just 2 decades in the future, while some claim the moment is hundreds of years away.

Peak phosphorus brings ramifications for both organic and nonorganic growers, but the impact will be felt first and hardest by industrial agriculture. While pure rock phosphate—essentially crushed phosphate-rich rock—is allowed under USDA organic standards, it is absorbed very slowly by plants. Forms that are more readily available to plants, such as triple super phosphate, are treated with sulfuric acid, phosphoric acid, or nitric acid, which renders them unacceptable for organic growers under current USDA standards. Industrial nonorganic farmers who rely heavily on these processed fertilizers will likely feel the impact of peak phosphate first.

Unlike oil, phosphorus is reusable. The challenge lies in collecting once-used phosphorus and safely returning it to the soil. Because phosphorus is an element, it is not destroyed when used by animals or plants. It can be reclaimed through composting dead plant and animal materials, as well as the excreted phosphorus in manure and urine. Gardeners recycle small amounts of phosphorus every time they add spent plant materials or kitchen scraps to their compost heaps.

As the global reserves of accessible phosphate rock decline in the coming decades, the cost of phosphate may climb so high that nonorganic farmers will be compelled to alter their soil management practices—and ultimately their entire approach to agriculture. Because, without cheap processed phosphorus, today's ongoing experiment in chemical-fueled agriculture may not be viable.

Originally published in Organic Gardening Magazine August/September 2013.