BEIJING (REUTERS) - Soil samples across China have revealed remnants of heavy metals dating back at least a century and traces of a pesticide banned in the 1980s, an environmental official said on Wednesday, revealing the extent of the country's pollution problems.

Street-level anger over air pollution that blanketed many northern cities this winter spilled over into online appeals for Beijing to clean water supplies as well.

The rotting corpses of thousands of pigs found last month in a river that supplies tap water to Shanghai drew even more attention to water safety.

Mr Zhuang Guotai, head of the ecological department of the Ministry of Environmental Protection, said a nationwide soil survey showed the countryside had paid a heavy price for an agricultural revolution that has seen grain production almost double in the last 30 years, despite a much reduced workforce.

"There is a cost behind the nine consecutive years of bumper grain harvests," he said at a conference in Beijing. "They rely on the heavy use of fertiliser, but the country needs to boost grain production so it is quite a difficult issue."

Mr Zhuang noted that as much as 65 per cent of the fertiliser in China's countryside was improperly used and left to pollute rivers and fields.

"All pollutants ultimately end up in the soil, and when we did the soil survey, we saw that even metal pollution from a hundred years ago was present, as well as the '666' pesticide banned in the 1980s."

Mr Zhuang said China aimed to release the results of the survey very soon, two months after access to the data was denied on the grounds that it was a "state secret".

He said the government always intended to disclose the survey results, which took four years to compile.

The disclosure of data is part of China's commitment to improve transparency and allay widespread public suspicions that the government has routinely covered up the extent of the damage done by more than three decades of breakneck economic growth.

China routinely vows resolve in cleaning up pollution in its cities, fields and waterways, but little is ever done, due mainly to lack of enforcement in the face of a corporate drive for profits.

At the beginning of this year, Beijing agreed to improve the way it monitored and disclosed air pollution in its major cities. Just weeks later, record-high smog readings in the Chinese capital forced the government to impose emergency restrictions on cars and factories.