* Russian export tax supports U.S. wheat

* Chicago soybeans ease on South American rain, palm oil slide

* Corn follows soybeans, supported as Ukraine debates export cap

CHICAGO, Jan 19 (Reuters) - Chicago soybean futures slid on Tuesday as rain across South America strengthened crop prospects and bolstered the global supply outlook, traders said.

Corn followed soybeans lower, but was supported by strong export demand and possible export limits in Ukraine.

Wheat futures fell slightly, but were bolstered by an export tax in top-producer Russia.

The most-active soybean contract on the Chicago Board of Trade (CBOT) fell 31 cents to $13.85-3/4 per bushel, losing 2.2%, its biggest drop since Oct. 12, 2020.

CBOT corn slipped 5-1/2 cents to $5.26 per bushel, while wheat shed 3-1/4 cents to end at $6.72-1/4 per bushel.

Rains across much of Brazil's growing regions bolstered parched crops, as the country slowly begins its soybean harvest, which could ease supply worries.

"Each week we go by, we’re picking up harvest in South America," said Don Roose, president of U.S. Commodities.

Falling palm oil futures further weighed on soybeans, while the U.S. Department of Agriculture's announcement of fresh export sales of 132,000 tonnes of soybeans to China for shipment in the 2021/2022 marketing year did little to strengthen the market.

Despite the drop, there is still upside potential in soybeans, according to Dan Anderson, broker at ED&F Man Capital.

"If you look at the rally, the dips have been shallow and we tend to remain fairly bullish," he said.

Corn fell after gaining early, as discussions in Ukraine about possible export limits offered support.

The USDA reported private U.S. corn sales totaling 128,000 tonnes to Japan and 100,000 tonnes to Israel, both for shipment in the 2020/21 marketing year.

Top importer China bought a record 11.3 million tonnes of imported corn last year, according to General Administration of Customs data.

Wheat, too, benefited from China's increased imports, with a record 8.38 million tonnes of wheat imported in 2020. (Reporting by Christopher Walljasper; additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by David Gregorio)