After all, soil does wear out over time; unless farmers replenish it with decaying plant matter, nutrients disappear from the soil.
Artificial fertilizers, pesticides, and herbicides also take their toll on the soil. Naturally occurring bacteria in the soil convert minerals into the form that plants can use, and those bacteria don't take kindly to poisons such as pesticides.
Herbicides, especially fungicides, can negatively affect the mineral content in soils. Some plants have a symbiotic relationship with fungi that helps them pull more minerals and nutrients out of the soil.When farmers use fungicides on their crops, those plants have a reduced mineral content. In fact, studies published by Dr. Linus Pauling have found that the mineral content of fruits and vegetables, from the time frame of 1940 to 1991, has decreased from 20 to 70 percent!
If you're concerned about this trend, consider taking a good multivitamin and mineral supplement, or try to buy organic foods from farms that practice sustainability. Or do both! Sustainable farms practice organic farming techniques and rely on letting fields lie fallow to keep the soil healthy. They also plant cover crops, such as clover, which return nutrients to the soil.