* U.S. corn futures climb on bargain-buying, but gains limited
* Soybeans fall on supply pressures, wheat weighed down by Black Sea export competition
NEW YORK, Sept 19 (Reuters) - U.S. corn futures rose on Tuesday on bargain-buying after prices touched the lowest levels in nearly three years, but gains were limited by expectations of an ample harvest and weak export demand, traders said.
Soybeans ended lower after choppy trading, setting a one-month low on pressure from better-than-expected U.S. crop ratings and export competition from Brazil, while wheat futures sagged on export competition from Black Sea suppliers.
The most-active December corn contract on the Chicago Board of Trade (CBOT) settled up 1% at $4.76-1/4 per bushel, rebounding after a dip to $4.67-3/4, the lowest price on a continuous chart of the most-active contract since December 2020.
CBOT November soybeans settled down 1-1/4 cents at $13.15-1/2 a bushel, while December wheat ended down 7-1/4 cents at $5.84 a bushel.
Corn rallied after a three-session slide, but struggled to overcome a glut of global supply and weak demand for U.S. exports.
"Corn, it's going to be tough," said Ben Buie, a vice president at Crystal Valley, a farmer-owned cooperative in south-central Minnesota. "We have so many acres, and the demand just isn't there."
Buie said some were optimistic that lower-than-forecast crop yields could support prices for corn and soybeans.
"We're going to be definitely below last year (on harvested yields). Hard to say how much until the combines start rolling," he said, adding that in his stretch of the farm belt was not yet even 1% or 2% through the harvest.
Nationally, the U.S. Department of Agriculture (USDA) said the U.S. corn harvest was 9% complete by Sunday and the soybean harvest 5% complete, ahead of five-year averages.
The USDA rated 51% of the corn crop in good to excellent condition, down 1 percentage point from the prior week, but soybean ratings were unchanged with 52% of the crop seen as good to excellent, bucking trade expectations for a 1-point decline.
Late in the session, Brazilian crop supply agency Conab projected the South American country's 2023/24 soybean crop at 162.4 million metric tons, up 1.5% from the previous year.
CBOT wheat futures were pressured by reminders of ample supplies from the Black Sea. In Ukraine, a cargo vessel carrying grain left the Ukrainian port of Chornomorsk, a top government official said, in a test of Kyiv's ability to unblock its seaports for grain exports. The vessel was one of two that entered Chornomorsk last week.
Meanwhile, Russian wheat export prices continued to decline last week. (Reporting by Zachary Goelman in New York City; Additional reporting by Gus Trompiz in Paris, Naveen Thukral in Singapore and Peter Hobson in Canberra; Editing by Marguerita Choy and Grant McCool)
