* Dryness in U.S., Black Sea, Argentina supports wheat prices

* Soybeans, corn rise as Brazil also dry for planting

* China buys 132,000 tonnes of U.S. soybeans, USDA says

CHICAGO, Oct 7 (Reuters) - U.S. wheat and soybean futures jumped to multi-year highs on Wednesday as unfavorable weather threatened production in exporting countries.

Dryness has raised concerns among traders about the prospects for wheat planting in the U.S. Plains and the Black Sea region and for soybean planting in Brazil.

Weather in the southern U.S. Plains is expected to remain mainly dry over the next six to 10 days, with temperatures also running above average, according to broker Country Hedging.

"Dry planting conditions in the U.S., Russia and Ukraine, as well as dry growing conditions in Argentina, weakened yield prospects around the globe," said Jacqueline Holland, market analyst for Farm Futures.

The most-active wheat contract on the Chicago Board of Trade (CBOT) was up 11-1/4 cents at $6.04 a bushel by 10:25 a.m. CDT (1525 GMT). It peaked at $6.11-1/2, the highest price since July 2015.

CBOT soybeans were up 8-1/2 cents at $10.52-1/2 a bushel after rising to $10.59-3/4, their highest since April 2018. Corn was 4-1/4 cents higher to $3.89-1/4 a bushel and reached its highest price since January.

Traders said delayed soybean plantings and harvests in Brazil could extend the export season for U.S. soy on the global market. The United States and Brazil compete for sales to countries like China, the world's top soybean importer.

The U.S. Department of Agriculture on Wednesday reported private sales of 132,000 tonnes of U.S. soybeans to China and 120,000 tonnes to unknown destinations for shipment in the 2020/21 marketing year.

U.S. agricultural exports to China hit $2.15 billion in August, the most ever for the month, led by strong soybean exports.

"The strong Chinese demand for U.S. beans has been driving prices, but yesterday the trade began to talk about the concerns about the South American crop in earnest," said Tomm Pfitzenmaier, analyst for Summit Commodity Brokerage in Iowa.

(Reporting by Tom Polansek in Chicago. Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Shailesh Kuber and Steve Orlofsky)