* U.S. corn futures climb on bargain-buying, but gains limited

* Soybeans turn higher, rallying after early declines

* Wheat anchored by Black Sea export competition

(Updates with corn turning positive, pricing, adds quote, changes byline/dateline, previous PARIS/SINGAPORE)

New York, Sept 19 (Reuters) -

U.S. corn futures rose on Tuesday in choppy trading, lifted bargain buying after prices touched the lowest levels in nearly three years, but limited on the upside by expectations of an ample harvest and weak export demand, traders said.

Soybeans turned higher, shrugging off early pressure from the harvest and stabilizing U.S. crop ratings, while wheat futures sagged on expectations of continued export competition from Black Sea suppliers.

As of 11:20 a.m. CDT (1620 GMT), the most-active December corn contract the Chicago Board of Trade (CBOT) was up 4-3/4 cents 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 were up 2-1/4 cents at $13.19 a bushel while December wheat was down 1-1/4 cents at $5.90 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 CooCorn 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) late Monday

said that the U.S. corn harvest as 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.

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)