People Leave the Country: 1920's {corrected]

I knew the 1920's were not good for US farmers, but I was totally floored by the economic history piece on 1920's which included this graph: 20 million people left the farm in the 1920's.

[Correction: Note the different scales on the left (population) and right (employment), so the decline in population is about 1.5 million, not 20 million. I thank the author, Gene Smiley, for pointing out my mistake.}