This meta-analysis, by Baranski et al, published in the British Journal of Nutrition, is based on a review of 343 peer-reviewed studies.  The researchers found statistically significant and meaningful differences in composition between organic and non-organic crops/crop-based foods. Most importantly, the concentrations of a range of antioxidants such as polyphenolics were found to be substantially higher in organic crops/crop-based foods.  Additionally, the frequency of occurrence of pesticide residues was found to be four times higher in conventional crops, which also contained significantly higher concentrations of the toxic metal cadmium. Significant differences were also detected for some other (e.g. minerals and vitamins) compounds.