* China may slow buying of U.S. soy
* Brighter prospects for showers in South America
* Markets in a holiday trading mode
By Sam Nelson
CHICAGO, Dec 23 (Reuters) - U.S. soybean futures ended lower on Monday, on concerns that China soon may be shifting its soy buying to South America from the United States, and on outlooks for better crop weather in South America.
Corn held firm for the third consecutive trading session on short-covering and weather issues in Argentina, while wheat was lower on plentiful global supplies and only routine export sales out of the United States.
Chicago Board of Trade January soybeans closed 10-1/2 cents per bushel lower at $13.28-1/2 per bushel, March corn was up 1 cent at $4.34-1/4 and wheat for March delivery was down 4 cents at $6.09-1/2.
Traders said volume was fairly light in all markets, which is typical of a holiday trading session.
"Weather is expected to improve in South America by the end of the week, but there is definitely a holiday feel across the board," said Sterling Smith, futures specialist for Citigroup.
Soybean and corn futures have been finding some support from hotter and drier-than-desired weather in Argentina and on pockets of dryness in Brazil.
Commodity Weather Group (CWG) said there are several opportunities for showers in southwest Argentina, but the broadest coverage is expected next week.
"The rains will be needed to prevent building concerns in early January for pollinating corn," said CWG meteorologist Joel Widenor.
High temperatures and dryness is a concern in Argentina and far southern Brazil from Wednesday into the weekend. However, "rains next week are projected to limit concern," Widenor said.
High temperatures and a lack of rain forecast for the coming days in Argentina's farm belt will hurt the 2013/14 soy and corn crop, the agriculture ministry said on Friday.
Argentina is the world's third-largest exporter of soybeans and corn, as well as the No. 1 supplier of soymeal and soyoil.
Ample supplies and slowing U.S. exports weighed on the wheat market, which slid 2.4 percent last week.
Cold weather is forecast for the U.S. winter wheat region this week, but temperatures are not expected to fall enough to damage the crop.
Investors in the corn market are concerned about slowing demand from China which has cancelled around half a million tonnes of U.S. corn cargoes.
The Chinese government's quarantine authority confirmed on Friday that it has rejected a total of 545,000 tonnes of corn from the United States because they contained an unapproved, genetically modified strain.
U.S. officials had earlier said they would urge China to act promptly to approve the variety, known as Agrisure Viptera and developed by Syngenta AG, and raised the issue with Beijing during annual trade talks between the two countries.