(Reuters) - A nationwide investigation has shown that as much as 16 percent of China's soil contains higher-than-permitted levels of pollution, the environment ministry said on Thursday.

China is desperate to tackle the impact of rapid industrialization and urbanization on its food supplies, with the aim of maintaining self-sufficiency and reducing its dependence on grain imports amid soaring demand.

The Ministry of Environmental Protection said in a notice posted on its website (www.mep.gov.cn) that its long-awaited soil survey involved samples taken across 6.3 million square kilometers (sq km) of land, two-thirds of the country's total.

"The survey showed that it is hard to be optimistic about the state of soil nationwide," the ministry said.

"Some regions are suffering from relatively heavy pollution, the quality of soil in planting areas is worrying, and the problem of waste from industry and mining also stands out."

The ministry found that 82.8 percent of the contaminated samples contained toxic inorganic pollutants, including cadmium, mercury, arsenic, chromium and lead.

It blamed agricultural production and other "human activities" for the contamination, which it said had been accumulated over the long term.

The ministry said China is working on a series of measures to help resolve the problem of soil pollution and would also speed up the drafting of related legislation.

China announced last week its first pilot project to treat heavy metal in soil in Hunan province, the site of a scandal last year involving the contamination of rice by cadmium.

(Reporting by David Stanway; Editing by Tom Hogue)