At the Kimberley family farm in rural Iowa the winter frost has lifted and the next few weeks will bring soybean planting season.

"One in every three rows of beans goes to China," said Grant Kimberley, watching a combine harvester spraying fertilizer across a vast field.

But Mr Kimberley, 42, whose ancestors have tilled the soil here since the 1860s, didn't look overly confident those exports would continue.

A potentially devastating trade war looms and US soybeans headed for China could face a 25 per cent tariff. The proposed tariff hangs like a sword of Damocles over Iowa, a state bigger than England known as the "breadbasket of America".

Forty per cent of China's soybeans, $14 billion worth a year, come from the US, and much of that comes from Iowa. The state is now awash with predictions of impending economic doom, and growing anger at Donald Trump for triggering the crisis in the first place by putting tariffs on Chinese steel.

Nowhere would a prolonged trade war bring more disappointment than at the 4,000-acre Kimberley farm, which has in some sense become a focal point for Sino-US trade relations.

In 2012 China's President Xi Jinping, who was vice president at the time, visited the Kimberleys - Grant, his father Rick, and mother Martha - on a tour of America. He sat in their living room and examined a jar of soybeans on the coffee table, explored the shiny metal storage facilities, and drove a giant computerised tractor.