A recent study from the University of California at Davis found that organically grown tomatoes had higher amounts of two disease-fighting flavonoids than conventionally raised tomatoes. Plus, the levels of these two nutrients rose over the course of the 10-year project until they were 97 and 79 percent higher, respectively.
Why? Organic plants have to work harder than plants raised on chemical fertilizers to fight off infections and predators, and to extract nutrients from the soil. Because of this, they grow more slowly and have the time to produce more nutrients. There is also evidence that organic produce contains more antioxidants than conventionally-grown produce.
Need more incentive to buy organic? Consider the Bug Arms Race concept presented by Steven Hopp in Animal, Vegetable, Miracle: evolution is based on survival of the strongest, the fittest, the most adaptable. Repeated spraying of pesticides by farmers is weeding out only the weaker bugs, while the stronger bugs develop resistance to pesticides in much the same way that some germs in humans are developing resistance to antibiotic medicines. These chemicals are also killing micronutrients in the soil that are essential to plant health, fertility, and resistance to diseases.
When pesticides were first introduced in 1948, farmers used about 50 millions pounds of them and suffered a 7 percent crop loss. Contrast that with year 2000: farmers used more like a billion pounds of pesticides and lost 13 percent of their crops.
Note that 20 percent of chemical pesticides are listed by the EPA as carcinogenic in humans.
(sources: Good Housekeeping, January 2008, and Animal, Vegetable, Miracle by Barbara Kingsolver)