The accident was reported at 7:13 a.m. just east of Birds Landing Road.
California Highway Patrol Officer Peter Van Eckhardt said no other vehicles were involved and no injuries were reported.
Van Eckhardt said the ground soybean was spread over several hundred yards of Highway 12.
"They say it's quite a section of roadway," he said.
At about 8 a.m. the two-lane highway was still closed, and the CHP did not yet have an estimate on when it would reopen.
Traffic is being diverted onto side roads, but motorists are advised to avoid the area.