Directly from Dr. McDougall's book The Starch Solution, page 156: The sales pitch goes like this: "You must take supplements because of the poor condition of soil these days. The crops you eat were grown in soil that has been drained of its nutrients from years of overfarming. Now the foods that grow in them are also deficient in vitamins and minerals. Supplements will correct this problem by providing these missing nutrients."
(From Dr M:) This simply is not true. Plants synthesize vitamins; they are not found in the soil. If a plant is going to bear roots, seeds, flowers, and/or fruits fit for sale, then it is going to have to produce all of the organic chemicals that are necessary for its own survival. We call the plant-derived organic chemicals vital for human nutrition vitamins. Your risk of suffering from mineral deficiency caused by depleted soil is so incredibly small that a single case would make national headlines. That's because you eat foods grown in a variety of soil: corn grown in Nebraska, grapes from Chile, bananas from Panama, and so forth. In the unlikely chance that one food was low in a mineral due to depleted soil, your next food would likely contain an abundant supply. People take supplements to protect against unfounded fears of developing deficiencies, and false hopes of preventing and curing illnesses that those deficiencies have never been known to cause (for example heart disease and cancer).