A very quick post to direct you to the paper by Barański and colleagues  which is currently making a few headlines and sparking some debate (see here and see here) with their assertions: "the concentrations of a range of antioxidants such as polyphenolics were found to be substantially higher in organic crops/crop-based foods" and "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 Cd [cadmium]".
Lead by researchers based at Nafferton Farm affiliated to Newcastle University, the literature review and meta-analysis looked at over 300 studies looking at "the composition of crops and foods". According to the BBC website, Carlo Leifert who headed the review said: "This study demonstrates that choosing food produced according to organic standards can lead to increased intake of nutritionally desirable antioxidants and reduced exposure to toxic heavy metals".
I've had the pleasure of visiting Nafferton Farm once or twice and chatting to Carlo and his team about some of the work going on. It is a lovely part of the world and some great science is being done and reported in the peer-reviewed press . Appreciating that there are still some significant gaps in the science about organic vs. conventional farming methods and their impact on food in terms of yields and nutritional quality, I do think that this latest (and largest) review is something which should not be ignored...
 Barański M. et al. Higher antioxidant and lower cadmium concentrations and lower incidence of pesticide residues in organically grown crops: a systematic literature review and meta-analyses. Br J Nutr. 2014 Jun 26:1-18.
 Stergiadis S. et al. Improving the fatty acid profile of winter milk from housed cows with contrasting feeding regimes by oilseed supplementation. Food Chem. 2014 Dec 1;164:293-300.
Barański M, Srednicka-Tober D, Volakakis N, Seal C, Sanderson R, Stewart GB, Benbrook C, Biavati B, Markellou E, Giotis C, Gromadzka-Ostrowska J, Rembiałkowska E, Skwarło-Sońta K, Tahvonen R, Janovská D, Niggli U, Nicot P, & Leifert C (2014). Higher antioxidant and lower cadmium concentrations and lower incidence of pesticide residues in organically grown crops: a systematic literature review and meta-analyses. The British journal of nutrition, 1-18 PMID: 24968103