A 2004 study evaluated Department of Agriculture data for 43 garden crops from 1950 to 1999. The researchers found statistically reliable declines for six nutrients — protein, calcium, potassium, iron and vitamins B2 and C.
https://web.archive.org/web/20151224030742/http://saveoursoils.com/userfiles/downloads/1351255687-Changes%20in%20USDA%20food%20composition%20data%20for%2043%20garden%20crops,%201950-1999.pdf
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