Monday, September 21, 2015 - A 2004 study of USDA data for 43 garden crops from 1950 to 1999 found statistically reliable declines for six nutrients — protein, calcium, potassium, iron and vitamins B2 and C — but no change for seven others.
A 2009 review of this study by Donald R. Davis, the lead author of the 2004 study, concluded that it may not be as 
simple as soil depletion, as there is considerable evidence that such problems may be related to changes in cultivated varieties, with some high-yielding plants being less nutritious than historical varieties. Also mentioned as possibly affecting nutritive values were changes in farming methods, the extensive use of chemical fertilizers, and changes in food processing and preparation.
They also pointed out that modern fruits and vegetables were still nutritionally valuable and suggested the remedy was to eat more vegetables, fruits, whole grains, nuts and beans and less refined sugars, separated fats and oils, white flour and rice.
Davis said that more study was needed, especially of inverse relationships between yield and nutrient concentration.
COMMENT: It would seem that the author has not partaken of the knowledge of Dr. William Albrecht at the University of Missouri – it takes healthy plants to have healthy animals (and people), and it takes healthy soil to have healthy plants. Modern farming methods force yield at the expense of nutrition. Ag crops have probably declined in nutritive value more than the items in the above study. Feedstuffs Analysis Tables currently list the standard value of protein in corn grain as 7.5%. Years ago open-pollinated corn contained about 12% protein. If that’s progress – it’s progress in the wrong direction.
Learn more here
See the full study here