Pesticides, herbicides, artificial fertilizers, ripening agents, genetically modified grain, the use of radiation and other "conventional" methods improve crop yields. A far greater percentage of fruits or vegetables or beans survive until harvest, at which time the producer has more to sell, as measured by weight.
Conventionally grown foods are artificially coddled, protected and babied. Their natural defenses may atrophy, as they're supplanted by artificial defenses. Contrary to nature, the weakest plants and fruits survive until harvest just like the strongest do.
With organic foods, on the other hand, only the strong survive. An organic Apple for example, survived until harvest not because it was sprayed with man-made poison, but because it engaged in a battle against insects and plant disease using its natural defenses -- and emerged victorious.
Conventionally grown foods are weak, and they pass their weakness on to the person who eats them. Organic foods are strong, and this strength is passed on to the eater. New science is showing how this works.
Organic blueberries have been found to contain higher values of ORAC (Oxygen Radical Absorbance Capacity) than conventional, on average. (Higher ORAC means higher cancer-fighting ability.) Another study found that organic tomatoes are between 79 and 97 percent higher in the flavonoids quercetin and kaempferol -- both antioxidants -- compared with conventionally grown. And yet another study found not only higher rates of anti-oxidants in organic produce, but also more vitamins.
It turns out that the same biochemical factors that enable a plant to survive are the same ones that strengthen the human immune system and help us to survive. The phytochemicals so beneficial to human health are the same chemicals that enable plants to resist disease and insect attacks.
In other words, organic foods are generally stronger for two reasons: 1) weak plants and fruits die, and never make it to market; and 2) without man-made protection, plants must develop and strengthen their own defenses to survive, and these "defenses" benefit human health when you eat them.
Note that nutritional differences between conventional and organic foods is controversial, and that direct comparisons are very difficult due to variations in soil quality, weather, climate, and the skills of the farmers involved. For example, one meta-study found that looking at many comparative studies the organic produce was healthier, but that the overall differences weren't statistically significant.
More research is needed. In the meantime, common sense suggests that organic foods, which have to rely more on innate defenses rather than artificial ones, are stronger and probably healthier. And don't forget: Organic foods don't have traces of the toxic pesticides, herbicides, ripening agents and other chemicals that conventional foods are likely to have.