For all his bluster about trade wars, President Trump seems willing to push China only so far: Witness the deal on Thursday to grant Chinese telecom giant ZTE a reprieve from harsh American penalties. The reason is likely to lead straight to Iowa soybean and corn farmers like Benjamin Schmidt.

Mr. Schmidt’s forebears have farmed the same land outside Iowa City for nearly 150 years. He and his father together till about 2,500 acres of the fertile prairie that stretches from Ohio through Nebraska. When I reached him last week, he was on his tractor, spreading fertilizer on this year’s corn crop.

Apart from the weather, hardly any issue looms larger for farmers than the prospect of retaliatory tariffs against American agriculture products. China has threatened a 25 percent tariff on soybeans and has already sharply curtailed purchases from the United States. This week Mexico imposed a 20 percent tariff on pork. The European Union and Canada have said they, too, will slap tariffs on a variety of American agricultural products.

“China is our most important export market for soybeans,” Mr. Schmidt said. “When your most important customer hits you with tariffs, there are going to be serious ramifications. My first reaction was this is going to hit us pretty hard.”