American farmers hit by the U.S.-China trade battle are preparing to reshape the U.S. Farm Belt by planting more corn and less soybeans next year over a land mass potentially equal to the size of Connecticut.

For decades, corn was U.S. farmers’ crop of choice, its tall stalks carpeting the Midwestern landscape. Soybeans, shorter and bushier, began decades ago as a niche crop raised on less acreage but came to rival corn in recent years because of growing demand from China.

U.S....