Wheat futures, too, posted contract lows as better-than-expected US winter crop conditions added to pressure from largely beneficial crop weather in key production areas around the globe.
Corn prices followed soybeans and wheat lower, although losses were tempered by worries over spring planting delays in the US Midwest due to wet soils and a rainy forecast into mid-May.
Grain markets have slumped to new lows in recent days even as government officials have expressed optimism for a breakthrough in US-China trade talks.
A trade deal could trigger accelerated US commodities purchases by China, which would help whittle down massive stockpiles of crops like soybeans, corn and wheat.
US Treasury Secretary Steven Mnuchin said on Tuesday that he hopes to make "substantial progress" with Chinese negotiators in the next two rounds of talks. He and other top US officials are in Beijing this week.
"There's nothing bullish out here. There's no development in the US-China trade talks and US export announcements remain very quiet," said Terry Reilly, senior commodities analyst with Futures International.
"I don't have anything bullish to say except that rain is preventing some of the producers from getting into their fields," he said.
Chicago Board of Trade July soybeans were down 7 cents at $8.53-3/4 a bushel at 11:48 a.m. CDT (1648 GMT), one of six contracts that posted new life-of-contract lows.
CBOT July wheat fell 5-3/4 cents to $4.29-1/2 a bushel. All contracts through May 2020 hit new contract lows.
Selling in wheat and soy accelerated as prices fell below recent lows.
July corn was 1-3/4 cents lower at $3.60 a bushel, retreating after three sessions of gains.
The US Department of Agriculture said 15 percent of the US corn crop has been planted, slightly ahead of market expectations, while soybeans were 3 percent seeded.
More rain is expected across the Midwest in the coming days, lengthening spring planting delays.
If delays stretch into late May, some farmers may switch acres from corn to later-planted soy.
Ample moisture has helped to boost winter wheat ratings, which pressured already slumping prices.
The USDA said 64 percent of the crop is in good-to-excellent condition, ahead of market forecasts.