Analysts attributed the weakness in futures to improved US crop weather and uncertainty about demand for US soybeans, given large carry-over supplies from the 2018 harvest and the ongoing trade dispute with China, the world's top soy importer.
"Whatever you do with the (US soybean) acres, you are still going to have some fairly large carry-out," said Chuck Shelby, analyst with Indiana-based Risk Management Commodities.
The declines in futures came after the US Department of Agriculture reported export sales of US soybeans in the latest week at more than 1 million tonnes, the largest weekly total in three months.
"We had some really good soybean export sales today," said Shelby.
The total included more than 600,000 tonnes earmarked for China, which purchased US soybeans last week in an apparent goodwill gesture ahead of the first meeting between US President Donald Trump and Chinese President Xi Jinping in seven months.
Nevertheless, Shelby added that the ongoing tariff situation with China could be adding to struggles in the soybean market.
At 11:53 a.m. (1653 GMT), Chicago Board of Trade August soybean futures were down 10-1/2 cents to $8.79-1/4 a bushel, after dipping to $8.74-1/2, the contract's lowest since June 12.
CBOT corn and wheat futures were modestly higher in choppy trade. Corn drew support from tightening supplies and uncertainty about US 2019 production prospects.
"The situation is totally different; the Chinese have not been a factor in that market," said Shelby. "Corn could really get a rationing-type scenario."
CBOT September corn was up 1/2 cent to $4.37-1/4 a bushel. CBOT September wheat was up 1-1/2 cents at $5.15-1/2 a bushel.
Improved US crop weather following excessive rains this spring hung over the market, limiting rallies.
"Temperatures over the next 10 days will be near to slightly above normal across most of the corn belt, which should accelerate crop growth, but limit heat stress," space technology company Maxar said in a daily weather note.
Traders await the US Department of Agriculture's next monthly supply/demand report on July 11, in which the government will update its forecasts of domestic and global grain inventories.