Synthetic nitrogen fertilizer has become one of the foundations of modern industrial agriculture — and it’s leading to alarming consequences.
Nitrogen is essential to life — it’s in protein, RNA and DNA. Photosynthesis depends on it. In the root nodules of legumes, such as clover and beans, nitrogen-fixing bacteria transform atmospheric nitrogen into forms that nourish plants and, in turn, animals. At least, that was how the nitrogen cycle worked until 100 years ago, when chemists discovered they could create synthetic nitrogen by using natural gas to transform atmospheric nitrogen into anhydrous ammonia. This substance was used in the munitions industry during World War II. Postwar, with production facilities still humming, farming became its next chief application.
As soil fertility declined because of repeated tillage and other industrial practices, farmers began using more synthetic nitrogen. About 25 billion pounds of it have been applied to crops each year in the United States for the past few decades. That’s almost five times the amount that was applied in 1960. Now, scientists say it’s leaching into and affecting groundwater. A 2015 study from Environmental Science and Technology Letters linked nitrate from fertilizers to the uranium contamination of drinking water from two major U.S. aquifers. Some water samples in the Midwest registered at 89 times the Environmental Protection Agency’s maximum level for safety, and water in California’s Central Valley was even worse, with some samples revealing uranium levels up to 180 times the uppermost safety limit.
The study explains that nitrates set off bacterial and chemical reactions that dissolve naturally occurring uranium from rock and sediment into groundwater. Nearly 2 million people live within 1 kilometer of this uranium-contaminated drinking water, and by consuming it they’re at risk for kidney damage, elevated blood pressure and cancer. Because other aquifers worldwide are also located in agricultural areas, this contamination likely isn’t isolated, says study co-author Karrie A. Weber at the University of Nebraska.
Aside from our drinking water, our food is affected, too. The Central Valley produces much of the nation’s fresh produce, and the Midwest samples were taken near the Corn Belt, where farmers heavily fertilize subsidized crops with synthetic nitrogen. The study found that food crops irrigated with contaminated water accumulate the dissolved uranium. Past research corroborates this. A 2002 study published in the Journal of Environmental Radioactivity tested various crops and found that uranium concentration in plants, particularly in lettuce and radishes, increased linearly as researchers increased the uranium concentration in irrigation water. A 2012 French study published in the Journal of Environmental Monitoring reported similar findings when researchers tested lettuce and wheat grown with uranium-contaminated irrigation water.
Another problem with a reliance on nitrogen applications is that it acidifies soils. In the Northwest, for example, wheat yields in the intensively farmed Palouse region are falling. Soils of this former prairie once had a neutral pH and thus rarely required agricultural lime, but continual nitrogen application has led to acidification. Over the years, we’ve depleted the organic matter that once provided buffering, says Paul G. Carter, a Washington State University Extension specialist. Carter also notes that, so far, the Farm Bill hasn’t offered much support for alternative methods of boosting soil fertility. But the alternatives do work. “We know we can get by with a lot less nitrogen fertilizer without sacrificing yields, as long as we integrate nitrogen-fixing cover crops into the system,” says G. Philip Robertson, Michigan State University distinguished professor of ecosystem science.
With mounting scientific evidence against the overuse of synthetic nitrogen fertilizer, consumers may want to press for policies that limit its use. They can also opt to purchase food from local, small-scale, diversified farmers who skip synthetics and use compost, manure and cover crops for fertility. Or, they can simply look for the Certified Organic label — current standards prohibit the use of synthetic nitrogen fertilizers.
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