* Soybeans, corn tumble on improved weather

* Wheat extends fall after rally

CHICAGO, Jan 22 (Reuters) - Chicago soybean futures slid more than 4% on Friday - while corn futures dropped their daily limit - as rains in key South American growing areas offered relief amid tight global supplies, overshadowing strong weekly U.S. export data.

Wheat futures tumbled 4%.

The Chicago Board of Trade (CBOT) most-active soybean contract ended 58-1/2 cents lower at $13.11-3/4 per bushel, its biggest loss since Aug. 10, 2018.

For the week, CBOT's most-active soybean contract lost $1.05, dropping 7.4%, its biggest decline since July 4, 2014.

Corn fell 23-3/4 cents to end at $5.00-1/2 per bushel, falling 4.5%, its biggest drop since Aug. 12, 2019. CBOT wheat dropped 26-1/4 cents to end at $6.34-1/2 per bushel.

Robust soybean exports failed to lend support. The U.S. Department of Agriculture reported export sales of old-crop U.S. soybeans in the week ended Jan. 14 at 1.818 million tonnes, above trade forecasts. New-crop sales were 831,000 tonnes, also beating expectations.

The USDA also confirmed private sales of another 136,000 tonnes of U.S. soybeans to China.

"I think we have supportive fundamentals overall, but we’re lacking the news to feed it," said Arlan Suderman, chief commodities economist at StoneX. "That leaves us vulnerable to more significant correction."

Brazil's soybean harvest has been slow to start, which could impact U.S. markets.

"That’s keeping the Chinese business flowing our way for a few extra weeks," said Jack Scoville, market analyst at the Price Futures Group. "The fact that we're lower encourages more sales."

Argentine soybean and corn planting sped forward over the last week, helped by rain that moistened fields parched by months of dry weather, the Buenos Aires Grains Exchange said.

Corn futures declined, despite export sales of U.S. old-crop corn last week that topped expectations at 1.438 million tonnes.

Commodity funds hold a massive net long position in CBOT corn futures, leaving the market vulnerable to bouts of long liquidation.

Wheat continued lower on weak exports, as the USDA reported U.S. wheat export sales in the week ended Jan. 14 at 329,700 tonnes, near the low end of trade forecasts. (Reporting by Christopher Walljasper in Chicago Additional reporting by Naveen Thukral in Singapore and Sybille de La Hamaide in Paris Editing by Matthew Lewis)