BEIJING—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.

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