Corn futures also slumped on disappointment over a lack of demand from China, traders said.
China bought US soybeans for the first time in six months after US President Donald Trump and his Chinese counterpart Xi Jinping met on Dec. 1 and set a 90-day negotiating window to resolve their trade differences.
Still, US cargoes face hefty tariffs that Beijing imposed as part of the conflict between the world's two largest economies.
In a sign of the impact of the dispute, China's soybean imports from the United States plunged to zero in November. This was the first time since the trade war started that China, the world's largest soybean buyer, imported no US supplies.
Instead, China has looked to South America to replace US cargoes. Brazil is set to harvest a massive crop in the coming months, fueling worries among US farmers that China will continue buying from South America.
"The problem for the bean market is that the US is rapidly losing its window for sales before South American beans become available," said Tomm Pfitzenmaier, analyst for Summit Commodity Brokerage in Iowa.
The Chicago Board of Trade's most-active soybean futures were flat at $8.97-3/4 a bushel by 10:50 a.m. CST (1650 GMT). The contract earlier dropped to its lowest price since Nov. 28.
Corn was down 3/4-cent at $3.77-3/4 a bushel, while CBOT wheat was 2-1/4 cents lower at $5.11-3/4 a bushel.
US farmers hope China will buy other farm products, such as corn and grain sorghum, along with soy.
"The trade is becoming tired of waiting for a Chinese corn purchase announcement," Pfitzenmaier said.
Trading volumes were thin ahead of Tuesday's Christmas holiday. CBOT markets will close early on Monday and remain shut on Tuesday.
Without major news, such as a sale to China, the markets will struggle to advance, said Arlan Suderman, chief commodities economist for US broker INTL FCStone.
"In the absence of that, I think we're just going to see a lot of choppiness and low volume trade," he said.