Soybeans climb back above $12 a bushel while wheat stumbles

* Soybeans bounced on short-covering and news of export sales and damaging South American rainfall

* A strong dollar and hefty Black Sea grain supplies weigh on wheat and corn futures

CHICAGO, March 20 (Reuters) -

Chicago Board of Trade (CBOT) soybean futures jumped 2% on Wednesday, climbing back above $12 a bushel as commodity funds appeared to cover short positions, brokers said, while news of fresh U.S. soybean export sales and excessive rains in Argentina lent support.

Wheat futures sank on reminders of plentiful supplies from Black Sea origins and strength in the dollar, which tends to make U.S. grain exports less competitive. The dollar firmed in early moves on Wednesday before turning lower after the CBOT close.

Corn futures closed mixed with the most-active May contract drifting lower.

Soybeans firmed after the U.S. Department of Agriculture confirmed private sales of 120,000 metric tons of U.S. soybeans to undisclosed destinations, the first such announcement under its daily reporting rules since Feb. 27.

CBOT May soybean futures settled up 24 cents at $12.09-1/2 per bushel. The November contract, representing the 2024 U.S. crop, rose 19 cents to finish at $12.00-1/4, its first close above $12 since Jan. 24.

"Beans were impressive today," said Sherman Newlin, analyst with Risk Management Commodities. "We did have that flash sale to unknown, so maybe there is something (else) in export sales that we will see tomorrow," Newlin said.

The market drew support from news of heavy rainfall in Argentina that

may damage the country's soy crop

.

Traders are also covering short positions, analysts said, ahead of the USDA's March 28 Prospective Plantings and quarterly stocks reports, which have a history of jolting markets. Commodity funds hold sizable net short positions in soybean, corn and wheat futures, priming markets for bouts of short-covering.

Wheat fell on Wednesday for the first time in three sessions while benchmark corn lost ground after rising on the previous session.

"We're trading back and forth and not doing a lot to build momentum," said Lane Akre, commodities analyst at ProFarmer.

CBOT wheat fell 7-1/2 cents to settle at $5.45 a bushel and corn slid 1/2 cent to $4.39 a bushel.

After the CBOT close, Egypt's state grains buyer

purchased

50,000 metric tons of Bulgarian wheat and 60,000 metric tons of Romanian wheat at an international tender that attracted

numerous offers

, primarily of Russian, Romanian, Bulgarian and French origin.

"The U.S. isn't competitive at all on beans and wheat in terms of exports, and that's a wet blanket on the market," said Austin Schroeder, commodities analyst at Brugler Marketing & Management.

A newly released Chinese customs report showing China slashing its imports of

U.S. corn and soybeans in favor of Brazilian crops

is also likely weighing on U.S. futures prices, Akre said.

(Reporting by Heather Schlitz in Chicago. Additional reporting by Naveen Thukral; Editing by Subhranshu Sahu, Savio D'Souza, David Gregorio and Richard Chang)