The biggest grain-shipping port in the U.S. may be disrupted by the spread of the BP oil spill in the Gulf of Mexico, threatening to depress domestic prices by encouraging importers to buy from South America.

More than half of the grain inspected for export from the U.S., the world’s largest grower of corn and soybeans, is shipped from the mouth of the Mississippi River, according to the Port of New Orleans. Traffic has yet to be restricted through the main deepwater channel, the Southwest Pass, used by ships carrying commodities such as oil, coal and grain.

“Once demand for a commodity is lost, it will find another source or substitute and will then be lost for good,” an analyst said. “This may have a bigger impact on the cash market than futures.”

From Bloomberg News, April 30, 2010