"In recent years, researchers have conducted several controlled studies to compare organic and conventional foods with respect to
nutritionalcomposition(Table2).Somestudieshaveconcludedthat organic production methods lead to increases in nutrients, partic- ularly organic acids and polyphenolic compounds, many of which are considered to have potential human health benefits as antiox- idants. However, other studies did not demonstrate differences in nutrients between organic and conventional production methods.
Two major hypotheses explaining the possible increases in or- ganic acids and polyphenolics in organic versus conventional foods have been proposed. One hypothesis considers the impacts of differ- ent fertilization practices on plant metabolism. In conventional agri- culture, synthetic fertilizers frequently make nitrogen more avail- able for the plants than do the organic fertilizers and may accelerate plant growth and development. Therefore, plant resources are allo- cated for growth purposes, resulting in a decrease in the production of plant secondary metabolites (compounds not essential to the life of the plant) such as organic acids, polyphenolics, chlorophyll, and amino acids.
The second hypothesis considers the responses of plants to stressful environments such as attacks from insects, weeds, and plant pathogens. It has been argued that organic production methods—which are limited in the use of insecticides, herbicides, and fungicides to control plant pests—may put greater stresses on plants and may require plants to devote greater resources toward the synthesis of their own chemical defense mechanisms. Increases in antioxidants such as plant polyphenolics have been attributed to their production in plant defense (Asami and others 2003), al- though the same mechanisms may result in the elevations of other plant secondary metabolites that may be of toxicological rather than nutritional significance."
http://ecochildsplay.com/2012/03/15/organic-vs-conventional-is-the-proof-is-in-the-nutrition/
Interesting stuff!