Soybean prices fell to their lowest point in nearly a decade, after a report forecast that Chinese tariffs will cut into exports and push up stocks of one of America’s most widely grown crops.

The U.S. Department of Agriculture on Thursday projected stocks of soybeans for the 2018-19 crop year at a would-be record of 580 million bushels, around 50% higher than its previous estimate and more than analysts expected.

Beijing...