GRAINS-Soy nears $10 a bushel ahead of Thanksgiving holiday

* Soy buoyed by slow farmer sales as U.S. harvest finishes

* Worries about La Nina and S.American crops lend support

* Wheat mixed, underpinned by short-covering

* Trade subdued ahead of U.S. Thanksgiving holiday

(Recasts; updates prices, adds quotes, changes byline, changes dateline from previous PARIS/SINGAPORE) CHICAGO, Nov 22 (Reuters) - Chicago soybean futures approached a two-week high near $10 a bushel on Wednesday, supported by firm cash markets as the U.S. harvest wound down and technical buying ahead of the Thanksgiving holiday, analysts said. Additional support stemmed from worries that the La Nina weather phenomenon, expected to emerge next month, could stress soy and corn crops in South America. The La Nina weather effect, characterized by unusually cold ocean temperatures in the equatorial Pacific Ocean, is linked with floods and droughts. As of 12:30 p.m. CST (1830 GMT), Chicago Board of Trade January soybean futures were up 7-3/4 cents at $9.96-3/4 per bushel after reaching $9.99-1/2, the contract's highest since Nov. 9. CBOT December corn was flat at $3.45 a bushel and December wheat was down 3/4 cent at $4.24 a bushel. U.S. markets will be closed on Thursday, with CBOT trade resuming Friday at 8:30 a.m. CST (1430 GMT). CBOT soybeans posted the biggest gains on Wednesday by percentage, reflecting slow farmer selling now that the harvest of a record-large U.S. crop is virtually complete. The U.S. Department of Agriculture said the soy harvest was 96 percent finished as of Nov. 19. "Producers shut the bin door, and (cash) basis levels are trying to firm. There is just a lack of country movement out here," said Don Roose, president of Iowa-based U.S. Commodities. Worries about dryness expanding in Argentina's crop belt lent support, although crop weather has been favorable in Brazil, the world's top soybean exporter and No. 2 corn supplier. Australia's Bureau of Meteorology issued an alert for a La Nina weather event starting next month, saying the chance it would take place was triple the normal likelihood. "It's a reminder that we are in a La Nina year, and we know that that increases the odds for dry (conditions) in Argentina," Roose said. Wheat futures were narrowly mixed, with front-month December losing ground to back months on spreads. A decline in U.S. wheat crop conditions and market chatter about radioactive pollution reported in Russia have encouraged some short-covering, but analysts said there was little impetus yet for a price rally in wheat. The meteorological service in Russia, the world's biggest wheat exporter, said on Tuesday it had measured pollution of a radioactive isotope at nearly 1,000 times normal levels in the Ural mountains. "With net short positions at a high level, such reports may prompt short-term-oriented market participants to reshuffle their investments," Commerzbank analysts said in a note.

CBOT prices as of 12:28 p.m. CST (1828 GMT):

Net Pct Volume

Last change change

CBOT wheat WZ7 423.50 -1.25 -0.3 51229 CBOT corn CZ7 345.25 0.25 0.1 138434 CBOT soybeans SF8 996.50 7.50 0.8 82848 CBOT soymeal SMZ7 324.40 6.10 1.9 43052 CBOT soyoil BOZ7 34.00 -0.17 -0.5 33823

NOTE: CBOT December wheat, December corn and January soybeans shown in cents per bushel, December soymeal in dollars per short ton and December soyoil in cents per lb.

(Additional reporting by Naveen Thukral in Singapore and Gus Trompiz in Paris; Editing by Mark Potter and Diane Craft)