Published online 11 February 2010 | Nature | doi:10.1038/news.2010.67

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Overuse of fertilizers is imperilling food supply.

Nitrogen fertilizers increase rice yields but can acidify the soil. elwynn1130/iStockphoto

Chinese farmers' rampant use of fertilizers could soon endanger the nation's ability to feed itself.

A survey of more than 8,000 samples from across the country has shown that overuse of nitrogen fertilizers has caused soil pH to plummet over the past 20 years. Left unchecked, the acidification could cripple Chinese agricultural production, says Fusuo Zhang, a soil and plant scientist at the China Agricultural University in Beijing and lead author of the survey, which is published online this week in Science1.

Since the 1980s agriculture yields have grown in China, as has the nation's use of chemical fertilizers. In 2007, China consumed 32.6 million tonnes of nitrogen fertilizer, a 191% increase over 1981.

Acid test

To understand the consequences of this increase, the researchers compiled 8,875 soil pH analyses from existing data to calculate the change in soil acidity between the 1980s and 2000s. The data came from various sources, including a national soil survey carried out between 1980 and 1984, 912 published research papers and long-term field monitoring studies.

The team's results show that extensive overuse has caused the pH of soil across China to drop by roughly 0.5, with some soils reaching a pH of 5.07 (nearly neutral soils of pH 6-7 are optimal for cereals, such as rice and grain, and other cash crops). By contrast, soil left to its own devices would take at least 100 years to acidify by this amount.

The acidification has already lessened crop production by 30-50% in some areas, Zhang says. If the trend continues, some regions could eventually see the soil pH drop to as low as 3. "No crop can grow at this level of acidification," he warns.

The speed at which the soils in China are acidifying is "astonishing", says Alan Townsend, a biogeochemist at the University of Colorado at Boulder. He agrees with Zhang's assessment that continued use at these rates could threaten China's food security.

Lean and green

The solution could be as simple as educating farmers. In Europe and the United States, fertilizer it is now used sparingly owing to an awareness of its environmental effects, including the pollution of rivers and lakes. But Chinese farmers are often unaware of the consequences of over-fertilization.

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"They see the green leaves but they don't see the impact on the soil. If they have a poor crop they think more fertilizer is needed, making matters worse," Zhang says. Farmers routinely apply double and sometimes triple the necessary amount, he says. Better education could provide a simple solution to the fertilization problem.

Upendra Singh, a soil scientist at the IFDC, a non-profit public organization based in Muscle Shoals, Alabama, that carries out agricultural research, adds that Chinese farmers could also leave dead crop biomass on the fields to help rebalance the soil. In the worst cases, lime could be spread across fields, he says.

In 2005, the Chinese government launched a scheme to educate farmers on issues including fertiliser use and techniques to rebalance the soil, but the programme has yet to reach many, says Zhang. He hopes the government will enhance its efforts to deliver knowledge and new technologies to the wider population of farmers.