Jiang Xicheng, chairman of the environmental protection arm of the Guangdong Lion's Club, a charity organization, broke the news on the Internet, alleging that farmers in a suburban area of the southern city of Guangzhou made fertilizer out of fermented garbage that contained used batteries, shattered glass and kitchen waste.

"I was shocked. The heavy metal content of the fertilizer must exceed safety levels," he said. "It poses a severe threat to human health."

Zhou Yongzhang, deputy director of the Department of Earth Sciences at Sun Yat-sen University, said the used batteries could severely contaminate vegetables with lead and mercury.

He added that if farmers rotate their soil often, the contamination could get worse.

Media reports on the case have aroused anger from netizens.

"We have big supervisory departments, but their ability to function is seriously in doubt," wrote a blogger with the screenname "ruyigongzi."

A farmer from Guangzhou surnamed Zeng said it is common practice for local farmers to use garbage as fertilizer to nourish vegetables planted in infertile fields.

So common is the practice that garbage collectors transport carts of waste specifically to sell to local farmers.

"(The tainted fertilizer) is cheap but efficient," Zeng said.

Zhu Lijia, a professor with the Chinese Academy of Governance, said government supervisors are to blame.

"The farmers do share some responsibility, but if the government was strictly supervising them, such things wouldn't have happened," he said.

The scale of contamination, as well as how much tainted produce has reached the market, is not yet clear.

Guangdong authorities said they are investigating the situation.

The Chinese public has become increasingly concerned over food safety after a slew of scares -- from melamine-tainted baby formula products to pork contaminated with clenbuterol -- exposed the vulnerability of the country's food sector in recent years.

Authorities handled 15,000 food safety cases and shut down 5,700 unlicensed businesses during inspections conducted across China in the first half of the year, according to the State Administration for Industry and Commerce.

The State Council issued a statement in July stating that it will make efforts to establish a better regulation mechanism, an improved legal and standard system and technical support for food safety and improve the food industry's overall performance in the area over the next five years.