Last winter I read “Gardening When it Counts: Growing Food in Hard Times” by Steve Solomon (I highly recommend it as a gardening resource). In the section on mixing your own fertilizers, he states:
Seedmeals [used to make fertilizer] of all sorts once had quite a bit more phosphorus in them, but over the last 25 years their average phosphorus content has steadily decreased. Incidentally, a comparison of nutritional tables published by the USDA over the past 25 years (covered in theMarch 2001 issue of Life Extension Magazine) shows that the average nutritional content of vegetables has also declined about 25 to 33 percent across the board — all vegetables, all vitamins and minerals.
I’ve been curious about this claim, so did some digging. It turns out the truth is a bit more complex than the report in Life Extension Magazine (available online here). Overall it does seem there is a decline, but that decline has been offset some by the creation of new varieties, such as deep-yellow vegetables such as carrots, that have higher nutritional values. Further, refrigeration and global transportation have provided access to off-season and out-of-zone foods. For instance, while vitamin C from white potatoes has been dropping, vitamin C from citrus foods has tripled from 1909-19 to 2004.
Anyone know of studies that compre the nutritional values of the same fruits and vegetables grown industrially and in the home or small organic farm?
Gardening When it Counts: Growing Food in Hard Times by Steve Solomon. 2005. New Society Publishers, Gabriola Island, BC.
Nutrient Content of the U.S. Food Supply, 1909-2004: A Summary Report, USDA Home Economics Research Report 57, published February 2007. Accessed online on 8/11/12 at: http://www.cnpp.usda.gov/publications/foodsupply/foodsupply1909-2004report.pdf
Nutritive Value of Foods, USDA Home and Garden Bulletin HG72, published 2002. Accessed online on 8/11/12 at: http://www.nal.usda.gov/fnic/foodcomp/Data/HG72/hg72_2002.pdf
Vegetables without vitamins, Life Extension Magazine, March, 2001. Accessed online on 8/11/12 at: http://www.lef.org/magazine/mag2001/mar2001_report_vegetables.htm