China has long struggled to feed one-fifth of the world's population on 7% of the world's arable land. Adding to the challenge are the side effects of rapid economic development: air pollution, contaminated water, and encroaching urbanization, all of which threaten Chinese farmland. Now, a new study has tacked unnaturally acidic soil onto the list, caused by excessive fertilizer use over the past 30 years.
Soil pH is critical to plant growth. Most crops thrive in soils that are neutral, with a pH value of 7, or slightly acidic. When soil's pH value creeps downward, it becomes prone to diseases and pests that stunt plant growth. Heavily acidic conditions also prompt the leaching of toxic metals into nearby bodies of water. So when pH values plunge, as they have in China, scientists start to worry.
The trend has been charted by soil scientist Zhang Fusuo of China Agricultural University in Beijing and colleagues. They compared the results of a national soil survey taken in the 1980s with surveys conducted over the past decade. They also collected data spanning 25 years from the closely monitored agricultural zones the Chinese government maintains throughout the country. For nearly all soil types found in China, soil pH has dropped 0.13 to 0.80 units since the early 1980s, they report online today in Science. A drop on that scale "normally takes hundreds of thousands of years," Zhang says. Even soils considered resistant to acidification showed a decrease in pH.
Soil acidification can be the result of acid rain. But overuse of certain types of nitrogen fertilizer is another cause, and this is what the researchers identify as the culprit in China. Beginning in the 1970s, Chinese farmers applied ever-increasing amounts of fertilizer with the hope that it would lead to bigger harvests. Instead of high yield, however, they got water and air pollution. Today, agricultural experts estimate that in many parts of China fertilizer use can be slashed by up to 60%.
A drop in pH on the level identified in the study could have some very harmful effects. Acidic soil is paradise for nematodes, parasitic roundworms that destroy crops. And at least one group of Chinese soils is approaching the pH values at which aluminum and manganese start leaching into surface water, with potentially toxic results.
"It is very important work," says Ganlin Zhang, a soil scientist with the Chinese Academy of Sciences in Nanjing. He agrees that the acidifying effects of nitrogen fertilizer deserve more attention in China. But the study's data raise some questions, he says: "It's just a collection of samples without exact site information." Alfred Hartemink, a senior soil scientist at ISRIC-World Soil Information in the Netherlands, echoes that concern. "You don't know where the samples came from," he says, adding that even soil scientists with little knowledge of China could have predicted acidification simply by looking at the high amounts of fertilizer applied there.
Zhang Fusuo dismisses these concerns, saying his study uses "large-scale sites from each province and each soil system." And his critics agree with him on one matter: the paper points to the importance of reducing fertilizer use in China. Acidic soil can be neutralized by adding lime, but that's too expensive for poor Chinese farmers. A better solution, says Zhang: "Stop overuse."