*

Wheat hits lowest price in 18 months

*

Ukraine, U.N. call for extension of Black Sea grain export deal

CHICAGO, March 8 (Reuters) - Chicago soybean futures were up on Wednesday after the U.S. government said the Argentine harvest will be nearly 20% smaller than previously estimated, pressured by a severe drought. Wheat fell to its lowest point in 18 months.

The U.S. Agriculture Department slashed its Argentine soy production estimate to 33 million tonnes from earlier predictions of 41 million tonnes, a deeper cut than analysts were expecting.

"If some of us are thinking the USDA may not get that aggressive ... no, they got a lot more aggressive than some of us thought," said Craig Turner of Daniels Trading.

At 12:03 p.m. CST (1803 GMT), the most-active soybean contract on the Chicago Board of Trade (CBOT) was up 2-1/2 cents a bushel at $15.18 after peaking at $15.33-3/4.

CBOT May wheat was down 7-1/2 cents at $6.90-1/2 per bushel after dipping to $6.84-1/4, the lowest price on a continuous chart of the most-active wheat contract since Sept. 14, 2021.

Corn fell 7 cents to $6.27-1/4 a bushel.

The wheat market is under pressure from Russian export competition and expectations that a wartime grain corridor from Ukraine will be extended beyond this month.

Ukraine's president and the United Nations Secretary-General called on Wednesday for the extension of a deal with Moscow that has allowed Ukraine to export grain via Black Sea ports during Russia's invasion.

Top U.N. trade official Rebeca Grynspan will meet senior Russian officials in Geneva next week to discuss extending the deal, a U.N. spokesperson said on Wednesday.

A Turkish diplomatic source said on Wednesday Ankara was "working very hard" to ensure the deal continues but that Russia's demands have not yet been met.

Russia signaled that obstacles to its own agricultural exports needed to be removed before it let Ukraine's exports continue. (Reporting by Cassandra Garrison, Naveen Thukral and Sybille de La Hamaide; Editing by Louise Heavens and Grant McCool)