* Hot, dry weather raises concerns in Argentina

* Rains slow fieldwork in Brazil's Mato Grosso -Maxar

* USDA lowers ratings for Kansas, Texas wheat crops

CHICAGO, March 2 (Reuters) - U.S. grain and soybean futures rose on Tuesday in a turnaround from losses the previous session, as concerns increased about unfavorable weather in crop-growing regions of South America.

Traders are watching the effects of heat and dryness in Argentina and rains in Brazil amid worries about potential yield losses.

The countries compete with the United States for export sales to importers like China, and U.S. supplies have tightened as China has ramped up purchases.

"The heat in Argentina is still worrisome and there continue to be concerns about an extension of the Brazilian wet season causing more supply concerns," said Tomm Pfitzenmaier, analyst for Summit Commodity Brokerage in Iowa.

"All of these South American supply concerns are expected to eventually prompt more Chinese buying of U.S. corn in the spring/summer time period."

The most actively traded corn futures contract closed 6-3/4 cents higher at $5.45 a bushel at the Chicago Board of Trade (CBOT), after earlier dropping to its lowest price since Feb. 11. Last month, the most-active contract reached its highest price since June 2013 on concerns about tight supplies.

Most-active CBOT soybeans jumped 21-1/4 cents to $14.12-1/2 a bushel. Last week, it hit the highest price since June 2014 at $14.45-3/4 a bushel.

In Brazil, the world's biggest soy exporter, soybean harvesting and corn planting are furthest behind schedule in south-central Mato Grosso, according to weather firm Maxar. Widespread rains in Mato Grosso over the next week should keep fieldwork behind the normal pace, the firm said.

Argentina's major corn-growing areas, meanwhile, are increasingly likely to suffer hot, dry weather, said Tobin Gorey, director of agricultural strategy at the Commonwealth Bank of Australia.

In the CBOT wheat market, the most-active contract climbed 16 cents to $6.66-1/4 per bushel, after earlier setting its lowest price since Feb. 18.

The U.S. Department of Agriculture on Monday lowered condition ratings for wheat crops in Kansas, Texas and Oklahoma. (Reporting by Tom Polansek in Chicago, Naveen Thukral in Singapore and Sybille de La Hamaide in Paris; Editing by Subhranshu Sahu, David Evans and Jonathan Oatis)