The Chinese Ministry of Commerce is considering an anti-dumping probe against U.S. soybean exports. That would follow a new investigation of American sorghum exports launched last week and tighter inspection standards for soybeans launched in December.

There is a clear political motivation to the move. A $5 billion increase in U.S. soybean exports to China was a commercial centerpiece of President Trump’s visit to China in November, while the Trump administration is in the midst of the section 301 review of Chinese IP standards. American farmers can ill-afford to lose access, wit...