URUMQI - Plastic mulch, a technology that helped make China the world's top cotton producer over the past 30 years, may have outlived its usefulness, as local farmers battle mulch residue in soil.

In the country's cotton base, Xinjiang Uygur autonomous region, mulch films are buried deep in the soil, with some rotted to shreds that can't be picked up by hand.

"In the past when we turned up the land, the residue was there and very easy to clear. Now we can hardly find surface soil among the residue," said regional agricultural bureau chief Akbar Gopur.

The situation has emerged as a major environmental concern for the government, agrotechnicians, farmers and the general public. While the central government issued a policy document on January 19 vowing to preserve every inch of farmland, the regional government is striving to offer solutions to the soil residue problem.

When plastic mulch technology was adopted in the 1980s, domestic agrotechnicians hailed it as a "white innovation." Film-mulched cotton fields have higher soil temperatures and conserve more moisture than normal land due to the greenhouse effect, which improves seeding, growth and yield.

China currently accounts for about 30 percent of the world's cotton output, with only 15 percent of the world's cotton-growing land. In 2012, its cultivation area was 5.3 million hectares and the country had a total output of 7.62 million tons, while its unit yield was 85 percent higher than the world average, according to this month's Field Crops Research, an international scientific journal.

After 30 years of fanatical use of mulch films and inadequate attention to the problems of film residue, agrotechnicians said that the country's cotton fields have been hit hard by plastic pollution.

According to a sample survey conducted in 20 counties in Xinjiang by the regional agricultural bureau in 2011 and 2012, about 16.88 kg of plastic mulch has been left in every mu (about 0.067 hectare) of local cotton fields, almost five times the country's average.

In major producing areas in northwestern Xinjiang, the number even exceeds 40 kg. "With so much residue, the land seems like it has been film-mulched ten times in one planting season," said Akbar Gopur.

"This is like planting on scrap heaps. The aftereffects are reduced production and less income," said Qin Xiaohui, head of the resource and environmental protection station of the regional agriculture bureau.

Xiao Fulin, a farmer in Manas County, spent a lot of time getting rid of the residual mulch in his 40-mu field, but his efforts were in vain.

Scooping up a handful of soil, Xiao found four or five pieces of mulch film in his hand, with the longest measuring 30 cm.

"Heavy residues have blocked the water from penetrating into the soil, which is detrimental," said Xiao.

According to research by the Xinjiang Academy of Agricultural Sciences (XAAS), when the residues reach 52.5 kg per hectare, cotton production decreases by 15 percent.

"It's not clear whether or not mulching film will pollute underground water and soil in other ways. I have a hunch that it might be more harmful than it appears," said Dong Xinguang, an agricultural expert in Xinjiang.