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.
Crews worked to clear the spill, and the CHP reopened the highway at 9:11 a.m.