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The extent of China's soil pollution, long guarded as a state secret, was laid out in an official report that confirmed deep-seated fears about contaminated farmland and the viability of the country's food supply.

Nearly one-fifth of the country's arable land is polluted, officials said in the report, shedding unexpected light on the scale of the problem--a legacy of China's three decades of breakneck economic growth and industrial expansion.

"The national soil situation overall does not offer cause for optimism," said the report. "In some areas, soil pollution is relatively severe. The condition of arable land is troubling, with the problem of pollution from industry and mining particularly worrisome."

While China's problems with air pollution are well-documented, environmentalists have warned about the effects of less-visible contamination of the country's land.

"Air pollution is definitely more visible and present, but soil is the last environmental media where pollutants end up," said Wu Yixiu, head of Greenpeace's East Asia toxins campaign. Heavy metal particles in the air and water seep into the land, then "get into the food and affect everybody," she added.

The report, based on a seven-year survey covering 2.4 million square miles, found that about 16% of the country's soil and 19% of its arable land was polluted to one degree or another. The vast majority of the pollution came from inorganic sources such as heavy metals, it said. China's total land area is 3.7 million square miles.

The most common inorganic pollutants found in China's soil were the heavy metals cadmium, nickel and arsenic, according to Thursday's report. Cadmium and arsenic, both known to cause chronic health problems, are byproducts of mining.

Nearly 3% of arable land in China was found to be either moderately or seriously polluted, the report said, without defining what those levels of contamination mean. Pollution was particularly severe in eastern China's Yangtze River Delta, the Pearl River Delta in the south and old industrial zones in the northeast, it said.

Pollution of farmland is of particular concern in China because of how little of it has. According to the most recent national land survey, China had 334 million acres of arable land at the end of 2012, roughly 37 million acres above the government's "red line" for the amount of farmland necessary to feed the country's population.

Already, some 8.24 million acres of arable land has become unfit for farming, China's Ministry of Land and Resources disclosed in December. Environmentalists say the majority of the remaining land is of poor or moderate quality, having been stripped of its productivity by decades of heavy fertilizer and pesticide use.

So much polluted soil means China will likely have to begin importing more food. "China will need to ease pressure on its natural resource base and import more of its food over the long-term," said Fred Gale, an economist with the U.S. Department of Agriculture's Economic Research Service. "Agriculture is impacted by industrial pollution but also creates a lot of pollution itself," he said, citing waste and ecological damage caused by China's growing taste for meat.

In April 2013, the discovery of unusually high quantities of cadmium in batches of rice grown in Hunan--the country's top rice-producing region, as well as a top-five producer of nonferrous metals like copper and lead--set off worries about farmland and sent prices for Hunan rice tumbling by as much as 14%.

Consuming cadmium in excess of the widely accepted standard of 0.4 milligrams per kilogram of rice over a long period can cause crippling pain the bones and liver and kidney damage. Several samples of the Hunan rice tested in 2013 showed levels of cadmium above that standard.

The cadmium disclosure came shortly after the Ministry of Environmental Protection rejected a request filed by a Beijing-based lawyer to release the results of the soil pollution survey. The ministry said at the time the data couldn't be released because it was a state secret.

Authorities have started to give more weight to the risks of environmental degradation.

In December, the Communist Party announced it would scrap its previous gross domestic product-driven performance evaluation system and replace it with one that would judge officials according to a wider variety of criteria, including environmental protection. Three years after an online campaign calling for more accurate information about air quality, most major cities in China now publish hourly data on air pollution levels. In July, the environmental ministry issued regulations requiring all Chinese provinces to establish an online platform for reporting pollution produced by major companies.