Corn and wheat also edged lower in cautious trading in the run-up to closely watched US government supply-and-demand forecasts on Tuesday.
The soybean market remained underpinned by expectations that China could make purchases of US beans as part of a truce agreed by Washington and Beijing, but traders were still awaiting signs of possible sales.
US-China trade negotiations need to reach a successful end by March 1 or else new tariffs will be imposed, US Trade Representative Robert Lighthizer said on Sunday, stressing a "hard deadline" after a week of seeming confusion among President Donald Trump and his advisers.
Highlighting the impact of the trade standoff, China imported 5.38 million tonnes of soybeans in November, the lowest monthly amount in two years, customs data showed on Saturday.
China, the world's largest soybean importer, this year put extra tariffs on US soybeans, causing shipments of US supplies to dry up.
"Soybeans remain under the influence of diplomatic relations between the US and China," consultancy Agritel said in a note.
"Chinese demand has globally slowed down, imports in November were 38 percent down compared to November 2017."
Share prices fell, curbed by worries that the trade dispute will have wider repercussions on the global economy.
The most-active soybean contract on the Chicago Board Of Trade was down 0.3 percent at $9.14-1/4 a bushel by 1305 GMT.
CBOT wheat was down 0.7 percent at $5.27-1/2 a bushel. The contract closed up 3 percent on Friday after reaching its highest in almost three months at $5.32 a bushel.
Corn edged down 0.4 percent to $3.84 a bushel.
Wheat had rallied on Friday on the back of brisk export sales.
"We are, finally, seeing some signs that US wheat is benefiting from the more modest supplies elsewhere," said Tobin Gorey, director of agricultural strategy, Commonwealth Bank of Australia.
Traders have been waiting for signs that dwindling supplies in the Black Sea region, which includes top wheat exporter Russia, will spur more demand for US wheat.
Grain markets are awaiting direction from Tuesday's monthly supply-and-demand outlook from the US Department of Agriculture (USDA).