Washington Scientists who studied the world's farmland with satellite maps found widespread damage to soil quality and said irrigation is draining underground water supplies faster than they can be replenished.
Either farmers switch to farming methods that improve soil conditions and use less water or they won't be able to feed the world's growing population, according to the report released Wednesday by the International Food Policy Research Institute.
"The basic story is that agriculture is being pretty successful at keeping the world in food. It's been somewhat less successful in nurturing the natural resources that underpin that production capacity," said Stanley Wood, the report's lead author.
About 16 percent of the world's farmland is free of fertility problems, or "constraints," such as chemical contamination, acidity, salinity or poor drainage, the report found.
In parts of Asia, as little as 6 percent of farmland is free of such problems. North America has the largest share of the best land at 29 percent.
Aluminum contamination is high enough on 17 percent of the farmland worldwide that it's toxic to plants, and salt deposits are a significant problem on irrigated land.
Nearly 4 million acres of farmland is lost to excessive salt every year, or about 1 percent of irrigated area worldwide, the report said.
Depletion of organic matter in soil also is widespread, reducing fertility and moisture retention and increasing emissions of carbon dioxide into the air, which is believed a factor in global warming, the report said.
Scientists need to find ways to increase food production without "major increases in the amount of new land under cultivation, which would further threaten forests and biodiversity, and without resorting to unsustainable farming practices," said Ian Johnson, a vice president of the World Bank and chairman of the Consultative Group on International Agricultural Research.
The world's population is expected to grow by 1.5 billion over the next 20 years.