Transportation challenges are mounting as farmers in the Midwest bring in another big corn and soybean harvest. While the Mississippi River system continues to experience low water levels, questions about diesel fuel supplies for trucking and whether a worker strike will shut down rail transport also linger.

On the lower Mississippi River as of last week, in areas south of St. Louis, barges were restricted to sit no more than nine feet below the surface due to low water levels, limiting their cargo capacity. In normal years, barges usually sit 11 to 12 feet below the surface. 

A 1-foot reduction in depth means about 5,000 fewer bushels of soybeans can be loaded for...