Actually, if soil is constantly reused for the same crop, it gets depleted of nutrients. But, if crops are properly rotated, and good fertilizers (organic materials, compost, green manures etc) are used, the nutrient content and mineral content of the soil is not depleted but enhanced. Certain types of crops help soil to regain minerals which form from the growth process. At least according to my co operative extension's organic farming course.
But that is organic farming which is a whole different animal from commercial farming.