The economic historian John Gordon Steele points to another factor in what he calls "The High Price of Farm Productivity" (scroll down a bit): Henry Ford's introduction of the Fordson tractor, which by 1922 cost less then a team of good horses. By 1930 the only farmers using horses were the Amish.
In 1900, a third of U.S. cropland was used for fodder crops; by 1930 it was used mainly for making human food. I don't know which affect was more important, but surely they both were influential.
Here is John Gordon Steele's economic history of America, An Empire of Wealth.
