WASHINGTON (AP) -- Much of the world's farmland is in such poor condition that farmers will have to find better ways to grow crops or else their production won't keep pace with the growing population, scientists say.
About 16 percent of the world's farmland is free of fertility problems, or "constraints," such as chemical contamination, acidity, salinity or poor drainage, according to a study by the International Food Policy Research Institute. The report was based on satellite maps.
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.
"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.
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.
The report also cited "an urgent need'' to use irrigation water more efficiently. Irrigation accounts for 70 percent of the fresh water withdrawn, and 30 to 60 percent is returned for downstream use, the report said.
"We must find ways to increase food production'' without putting significant amounts of new land under cultivation, said Ian Johnson, a vice president of the World Bank and chairman of the Consultative Group on International Agricultural Research.
Putting more land into production would require cutting down forests and otherwise damage the diversity of animal and plant life, he said. The world's population is expected to grow by 1.5 billion over the next 20 years.
Biotechnology could help boost production, if crops were genetically engineered to need less water and to grow in poorer soil, but that alone won't be enough, Wood said.
In many areas, the problem is that there isn't an economic incentive for farmers to change the way they farm.
In Kenya, the soil is so poor that corn yields are 20 percent or less of what they are in the American Midwest, partly because farmers can't afford to leave stalks and other plant debris in the soil to improve its fertility. They feed the plant material to animals or use it as fuel.
Chemical fertilizers aren't effective unless sufficient organic matter remains in the ground, Wood said.
Poor transportation systems also make it difficult for poor farmers to sell crops or obtain the chemicals they need.
The report is among a series of studies being done on the condition of various ecosystems, forests and marine areas.
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