CHICAGO, July 28 (Reuters) - U.S. corn futures hit a one-month low and soybean futures also tumbled on Tuesday as improving U.S. crop condition ratings and favorable weather outlooks supported expectations for large harvests this autumn, analysts said.

Wheat followed the weaker trend.

Chicago Board of Trade December corn settled down 4-1/2 cents at $3.30 per bushel after dipping to $3.29, the contract's lowest since June 29.

CBOT November soybeans ended down 12-1/4 cents at $8.87-1/2 a bushel and September wheat fell 4-1/4 cents to finish at $5.23-1/2 a bushel.

"Corn and soybean prices are under active selling pressure following Monday afternoon's USDA weekly crop progress report showing surprising strength in crop ratings this week," StoneX chief commodities economist Arlan Suderman wrote in a note to clients.

The U.S. Department of Agriculture late Monday rated 72% of the U.S. corn and soybean crops in good to excellent condition. Both figures were up from 69% the previous week and topped a range of trade expectations.

However, continued strong demand for U.S. exports limited losses.

"The soybean price is ... down in the wake of the crop progress report, though its fall has been slowed by positive demand data," Commerzbank said in a note.

The USDA on Monday confirmed U.S. soybean export sales to China, along with shipments to Mexico. It was the 10th straight day that the USDA announced soybean sales to either China or unknown destinations, often believed to be China.

Meanwhile, forecasts called for beneficial weather in most of the U.S. crop belt, including welcome rains in the southern Midwest later this week.

Wheat futures faced additional pressure from reminders of strong export competition from producers in the Black Sea region. Moscow-based consultancy IKAR raised its Russian wheat crop forecast on Monday.

Egypt's state grains buyer, the General Authority for Supply Commodities, on Tuesday bought 470,000 tonnes of wheat in an international purchase tender, including 350,000 tonnes of Russian-origin and 120,000 tonnes of Ukrainian grain. (Additional reporting by Colin Packham in Sydney and Sybille de La Hamaide in Paris; editing by Sherry Jacob-Phillips, Emelia Sithole-Matarise and Grant McCool)