I got the one in seven figure from Lester Brown, but it seems to stand up.
In 1900 US workforce was 24 million (http://www.bls.gov/opub/rtaw/pdf/intro.pdf )and the agricultural workforce 38 per cent of total (http://inventors.about.com/library/inventors/blfarm4.htm), i.e. 9.12 million. The population was 75,994,266 so each of the nearly ten million agricultural workers would have to grow enough food for seven people for the population not to starve.
Of the farmers' families, the total farm population in 1900 was nearly 30 million, so they must have had very small families, or more likely, lots of them had no families.