Corn and wheat futures followed the higher trend.
As of 12:46 p.m. CDT (1746 GMT), Chicago Board of Trade November soybean futures were up 17 cents at $9.66-1/2 per bushel after reaching $9.73-1/2, its highest since Aug. 10.
CBOT December corn was up 2-1/2 cents at $3.57-3/4 per bushel and December wheat was up 6 cents at $4.44-3/4 a bushel.
Soybeans led the way up, with the November contract forming a gap on its chart by opening above Friday's high, a bullish technical signal.
"There is a lot of short-covering. Funds came into the week short, and we went above ... resistance, and hit a lot of buy-stops on the way up," said Brian Hoops, analyst with Midwest Market Solutions.
Weather forecasts called for dry conditions across much of the Midwest over the next 10 days, potentially stressing the maturing US soybean crop.
"The dry weather will favor maturation of the corn crop but may stress soybean filling, especially in Iowa and Illinois, preventing soybean yields from reaching their full potential," MDA Weather Services said in a note to clients. Illinois and Iowa are the top two soy-producing states.
Additional support stemmed from stepped-up export demand from top global soy buyer China. The US Department of Agriculture on Tuesday said private exporters sold 136,000 tonnes of US soybeans to China, following a string of similar sales announcements since mid-August.
"Export hopes are helping to support soybeans today, with US soybeans looking competitive," Matt Ammermann, commodity risk manager at INTL FCStone, said. "Strong demand has been present from China for some time and if new Chinese demand comes in big style, it will be the US that will probably be the seller."
Uncertainty about the path of Hurricane Irma may have added light support. The storm could potentially bring heavy rain to the Southeast next week, MDA said.
"I don't know if that (means) anything in particular to beans, but it adds another level of uncertainty," said Terry Linn of Linn & Associates in Chicago.
CBOT corn futures followed soybeans higher, extending a rebound from contract lows set in nearby months last week. Cash sales of old-crop corn from farmers have slowed since the start of the month, helping to support futures, Hoops said.
"I think we've put the worst behind us as we've gotten through deliveries and a lot of old-crop sales being made in the late August-early September time frame," Hoops said.