Talk of state restrictions on Russian and Ukrainian exports, meanwhile, supported wheat.
US grains markets were closed on Monday for a public holiday.
Chicago Board of Trade most-active soybeans were down 0.2 percent at $9.14-3/4 a bushel at 1222 GMT after rising on Friday.
Corn was down 0.5 percent at $3.79-3/4 a bushel. Wheat rose 0.1 percent to $5.18-3/4 a bushel.
Little has been reported about the trade war since confirmation on Thursday that China's vice premier will visit the United States on Jan. 30-31 for the next round of negotiations.
"Soybeans and corn are being weakened by lack of news about progress with the US-China trade war," said Matt Ammermann, commodity risk manager with INTL FCStone. "There was not really any significant news about progress in the US-China trade dispute over the weekend. The US government shutdown means that news of export sales is not being published by the USDA (US Department of Agriculture).
"Despite lot of market chatter about new US sales to China of soybeans and even wheat, the market is still awaiting actual confirmation. The market is now awaiting the next round of US-China trade negotiations in Washington at the end of January."
US corn is the cheapest in the world in fob (port price) terms but the market is also awaiting news about China, Ammermann added.
Rumours of large-scale US wheat and grain purchases by China have swirled in markets in the past week, but official confirmation is unavailable due to the partial US government shutdown which has hit the USDA.
"Commodity traders still wait for more accurate USDA export and production data," brokerage Allendale added.
Several reports about possible Black Sea export curbs have appeared.
Stronger controls on grain exports are being considered by Russia's state agriculture watchdog.
Ukraine's agriculture ministry urged exporters to comply with export limitation memorandum terms as most of the agreed volume of wheat for the 2018/19 season traders has already been shipped.
"Wheat is supported by reports of Russia and Ukraine in the headlines, chatter around export regulations in Russia and a reminder of the export memorandum in Ukraine reminding traders that exports will be slowing now via Black Sea which in turn could transfer world demand to the United States," Ammermann said.