Take selenium in tomatoes for example. If that selenium isn't in the soil, it won't be in the tomato.
I just have to comment on this point. There is selenium in all soil all over the planet. It has been found literally everywhere, it was how they discovered that a huge meteor hit the earth just before the extinction of the dinosaurs, a layer of selenium that encompasses the earth.
What this tells us is that there is selenium in the soil, it most likely needs the right microorganism(s) to release it from bondage so the plants can take it in.
It is a lot like taking vitamins, if your gut microbiota is not "right" then the odds are you will pass those vitamins through without any cellular benefit for your body.
Susan, the studies you are looking for
should start here,
nutrient density in vegetables
The next part of your research would be here,
increasing nutrient density in vegetables
and this is a list of research papers.
Journal Articles Status: Published Year Published: 2013 Citation: Meagy M. J., T. E. Eaton, and A.V. Barker. 2013. Nutrient Density in Lettuce Cultivars Grown with Organic or Conventional Fertilization with Elevated Calcium Concentrations. HortScience 48 (12):1502-1507.
Journal Articles Status: Awaiting Publication Year Published: 2014 Citation: Meagy M. J., T. E. Eaton, and A.V. Barker. Assessment Mineral Nutrient Density of Lettuce in Response to Cultivar Selection and Nutritional Regimes. Accepted to the HortScience on December 13, 2013. Awaiting for Production Checklist.
Journal Articles Status: Accepted Year Published: 2014 Citation: Meagy M. J., T. E. Eaton, and A.V. Barker. Zinc Accumulation in Lettuce Cultivars Grown with Organic or Chemical Based Nutritional Regimes. Accepted on August 8, 2013 to the Journal of Plant Nutrition, In Press, Awaiting Production Checklist.
Theses/Dissertations Status: Submitted Year Published: 2014 Citation: Meagy, Md J. Increasing Nutrient Density of Food Crops through Soil Fertility Management and Cultivar Selection. Doctoral Dissertation submitted to the Graduate School of the University of Massachusetts, Amherst, for degree to be awarded in February 2014.
Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Meagy, Md J., T. E. Eaton, and A. V. Barker. 2012. Nutrient Density in Lettuce Cultivars Grown with Organic or Chemical Fertilization with Elevated Calcium Concentrations. IPPSWR conference, Ventura, CA. September 2012.
Conference Papers and Presentations Status: Other Year Published: 2013 Citation: Meagy, Md J., T. E. Eaton, and A. V. Barker. 2013. Mineral Nutrient Density of Lettuce Grown with Organic and Conventional Soil Fertility Practices. Presented at the Annual Meeting of the American Society of Plant Biologists Conference, Northeast Section, UMass Amherst, MA. April 2013.
Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Jahanzad,E., A.V. Barker, M.Hashemi, T. Eaton, and A. Sadeghpour. 2013. Cover crop and nitrogen fertilizer influence on tuber yield and quality of potatoes. Abstracts Northeast Section American Society of Agronomy, Newark, Delaware. Presented at the Regional Meeting of the Northeast Section of the American Society of Agronomy, July 2013.
Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Jahanzad, E., A. V. Barker, M. Hashemi, T. Eaton, and A. Sadeghpour. 2013. Tuber Yield and Quality of Potatoes as Affected by Cover Crops and Nitrogen Fertilizer. ASA, CSSA, and SSSA conference, Tampa, FL. November 2013. Agronomy Abstracts, 2013.
Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Eaton, Touria E., A. V. Barker, Md J. Meagy, and E. Jahanzad. 2013. Mineral Nutrient Density of Cabbage in Response to Cultivar Selection and Nutritional Regimes. HortScience 48(9):S232. Presented at Annual Meetings of the American Society for Horticultural Science, Palm Desert, California, July 2013.
Have fun reading.
Redhawk
(or you could just ask me what to do and how it works)