A study found that there is a significant decline of the nutrient content in 43 different crops between 1950 and 1999
https://web.archive.org/web/20151224030742/http://saveoursoils.com/userfiles/downloads/1351255687-Changes%20in%20USDA%20food%20composition%20data%20for%2043%20garden%20crops,%201950-1999.pdfComments 20