This is a secret I've kept for over ten years now, but not because it's much of a secret or because I want to keep it. It is a bit of information that is difficult to convey with an appropriate sense of importance. Maybe someone else will pick up the ball and run with it, and forgive me my lack of evangelical zeal.
The U.S. Department of Agriculture has generated and accumulated data on the nutritional composition of grains and produce since well before WWII, and the data is all publicly available. If you compare the nutritional contents of various farm products from 1950, you will notice the major vitamins and minerals were present at about twice the concentration as they were for similar food items in the year 2000.
There's not much to argue about with this observation. Look a the data and you can easily see for yourself that the nutritional composition of commercially grown food is one-half now compared to what it was fifty years ago.
My explanation for this is simple, and it may even be accurate. Traditional farming practices were quickly abandoned after World War II when cheap nitrogen, phosphorus and potassium (NPK) fertilizers became available due to surplus chemical factory capacity after the national need to produce high explosives abated. Industry switched from making chemicals for bombs to making chemicals for fertilizers.
Although NPK fertilizers are very good for growing plants, they do nothing to replenish other chemical and elemental components of soil that are very good for human health. Every growing season the crops sucked iron, calcium, chromium, selenium, and magnesium out of the soil, and no thought was given to replacing these and other minerals.
Elemental minerals, however, are the essential building blocks for the more complex vitamin compounds. In the right conditional and with the right minerals present in the soil, plants produce an abundance of chemical compounds that are good for people to eat, but don't seem to be necessary for the plant's own survival. If the basic elements needed for producing a certain vitamin are lacking in the soil, the plant simply does not produce as much of that vitamin.
The plant itself does not seem to suffer from these mineral shortages.
There seems to be some vague sense which nags us that there's something not quite right about the food supply, when we stop long enough to think about it. The organic food movement is a symptom of this foreboding, but I think they threw out the baby with the bathwater when "organic" is defined to exclude NPK fertilizers.
Doing that makes organic foods much more expensive without making it more safe or more nutritious. Nitrogen, phosphorus and potassium are about as natural as it gets.