Corn futures also declined on the rainy Argentine weather and pressure from abundant supplies of the grain. Wheat futures edged higher on concerns about dry weather hurting the US winter crop.
"Near-term rains in Argentina are the focus today," said Rich Nelson, chief strategist for Allendale Inc.
"The market is also responding to some reports that USDA could raise corn yields next month. Those are two very bearish short-term factors," he said.
Rains that fell on Argentina's main farm belt over the weekend brought relief to parched soy and corn-growing land, meteorologists said on Monday, allowing the planting of crops to resume in most of the affected areas.
But forecasters noted that crops would need more precipitation following a very dry start to the planting season.
"The rains led to notable improvements in soil moisture, but more rainfall will still be needed to end the dryness in northern and central Argentina," Kyle Tapley, meteorologist with MDA Weather Services, said in a note to clients.
Chicago Board of Trade January soybeans fell 5-3/4 cents to $9.61-1/2 a bushel, the lowest since Sept. 13.
Soybeans slid despite news of large sales to China. The US Department of Agriculture on Monday reported 396,000 tonnes of sales to China after confirming on Friday sales of 257,000 tonnes to the country, the world's top importer.
CBOT March corn fell 1/2 cent to $3.47 a bushel after earlier matching its contract low of $3.46-1/2. May through September and March 2019 also matched contract lows and May 2019 futures notched a new low.
Speculation that USDA may raise its US corn production estimate next month weighed on prices after private analytics firm Informa Economics lifted its corn crop estimate on Friday.
CBOT March soft red winter wheat rose 2-1/4 cents to $4.20-1/2 a bushel, while K.C. March hard red winter wheat gained 1-3/4 cents to $4.19-1/4 a bushel.
A combination of delays to autumn planting and dry conditions in the southern US Plains have left the hard red winter crop vulnerable to freeze damage, crop experts said, which could further tighten supplies of high-protein wheat.