Soybean futures prices plunged, and Iowa lawmakers raised alarms last week as the Trump administration threatened additional tariffs on goods imported from China after the Chinese government countered with added tariffs on soybeans and other U.S. agricultural exports.

“We know what happens when the tariffs are increased. There is more retaliation from China,” said Iowa Sen. Charles Grassley in a conference call with ag reporters. “There is the potential in this for long-term damage that could hurt Iowa tremendously. We have to be careful on the trade front because we see agriculture being the first thing that gets retaliated against.”

Grassley, along with Iowa Sen. Joni Ernst and other Midwest lawmakers, vowed ...