Phosphorus Fields

Phosphorus and nitrogen fertilizers drive modern agriculture, but they are also poisoning the planet.

By Michael Tenneson|Monday, November 09, 2009
RELATED TAGS: POLLUTION

From the air, they stand out as enormous iridescent white rectangles filled with translucent blue-green ponds. Bizarre and unearthly, these deceptively beautiful structures, known a gypstacks, look like something that might show up on Mars or Europa. In reality they are found mostly in west central Florida, the unfortunate result of mining phosphorus to make fertilizer.

Phosphorus, one of the key ingredients behind the green revolution of the 20th century, has helped maximize farm yields worldwide. But obtaining raw material for fertilizer is hardly a green process. Miners pull phosphate ore out of the ground, crush it, and throw it into an acid lake, where it separates into white gypsum stacks and a liquid acid that is then processed further to feed crops. The stadium-size gypstacks are especially abundant on the Bone Valley formation in west central Florida, one of the world’s richest sources of phosphorus. The Peace and Alafia rivers flow through here, collecting toxins that spill into Charlotte Harbor and Tampa Bay.

Gypstacks represent just one form of fallout from the ever-escalating use of fertilizers to boost crop production. Another example shows up about 400 miles west at the mouth of the Mississippi River; here a colossal green mat of algae, nearly the size of New Jersey, blooms in response to a flood of nitrogen created in factories and delivered as runoff from midwestern farms. The nitrogen serves as a nutrient for the algae, which explode in number. As the algae die and decompose they become food for bacteria, which consume so much dissolved oxygen that resident fish and marine crustaceans cannot survive. A giant dead zone is the tragic result.

Demand for phosphorus and nitrogen fertilizer is rising steadily as the world grows more crowded. “Over the next 50 years, the human population is predicted to grow from nearly 7 billion to 9 billion,” says Abdullah Jaradat, research leader at the soil lab run by the U.S. Department of Agriculture in Morris, Minnesota. “To feed those people, we must double our agricultural output, and that means more nitrogen and phosphorus.”

The average crop requires about ten units of nitrogen to one unit of phosphorus for robust growth; living plants must maintain a delicate balance between the two nutrients to produce at their best. “If that balance is disrupted, we have a problem,” Jaradat says. Phosphorus propels cell mechanisms and is crucial to the structure of DNA and RNA. Nitrogen helps plants produce proteins, including the key molecules that control photosynthesis, the process by which the sun’s energy is captured and converted into the carbon compounds that are the raw material for growth.

The United States is one of the world’s leading producers of the rock that is the source of phosphate for fertilizer. Extensive mining and agriculture in west central Florida—America’s primary phosphate-producing region—have lowered the water table enough to turn springs into sinkholes along the Peace River, which each day loses about 11 million gallons, or 8 percent of its flow, to these underground caverns. The entire upper part of the Peace River has gone dry in seven of the last ten years. On June 27, 1994, a gaping sinkhole developed in a 400-acre, 220-foot-high gypsum stack containing a liquid impoundment of acid and ammonium. “About 4 million cubic feet of phosphogypsum and an undetermined amount of acidic water disappeared through that shaft into the ground,” says Ann Tihansky, a scientist with the U.S. Geological Survey. There are approximately 20 gypsum stacks located in the sinkhole-prone region of west central Florida today.

Disposing of gypsum waste is another challenge. Some scientists have proposed using the material as drywall filler or road base, but the EPA says it contains too much naturally occurring radioactivity and so forbids its use. The Southern Environmental Law Center is suing the state of North Carolina over its approval of an 11,500-acre expansion of phosphate ore mining by PCS Phosphate in wetlands along the Pamlico River. Says staff attorney Geoff Gisler, “Eventually these mines will close down, and we will be dealing with the impacts for years.”

At least the effect of the gypstacks is fairly local; the damage caused by farm runoff is far broader. Studies show that only about 30 percent of the nitrogen fertilizers used in agriculture are actually taken up by the crops they are intended to feed. The rest remains in the soil or is carried off in rainwater. Phosphorus runoff creates algal blooms in freshwater lakes and streams. Nitrogen runoff gives rise to grander algal blooms at the mouths of rivers, like the one at the mouth of the Mississippi. Some 200 dead zones have been charted worldwide.

To address the problem, Jaradat says, we need a second green revolution—one that does less damage to the planet while preserving our remarkable agricultural gains. Using carefully controlled amounts and proper proportions of fertilizer would largely achieve this, he says. Jaradat’s lab is working to determine the optimal amounts of fertilizer needed to boost production without spoiling the environment. It is also experimenting with no-till and minimum-till agriculture, techniques in which farmers inject seeds into the ground, avoiding the plow and thereby minimizing nitrogen and phosphorus runoff. Other solutions include planting winter cover crops that retain soils and cultivating beans that create their own nitrogen.

“More farmers need to look at how their actions affect the environment as a whole. And that’s not an impossible task,” says Dan Richter, professor of soils and ecology at Duke University. “No more excuses. We can do a better job.”

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