Here's my understanding:
o Organic plant foods are not sprayed with chemicals that might reduce phyto-nutrient quality.
o Organic plant foods are more often grown in soils that are more properly rotated and kept nutrient complete. This translates into higher quality phyto-nutrients.
o Organic foods, overall, are less processed, and processing generally reduces nutrient quality.
Have not read any studies regarding phytonutrients (don't know if there is any), but I am guessing same applies to them as to vitamins or minerals. The claim that depleted soil in conventional farming leads to lowered nutritional value is usually evidenced by the *slight* decrease in some vitamins from vegetables since 1950. However, it is not the method of farming that is the cause for (neglibly) lowered vitamin content, but rather the use of higher yielding varieties.
Conclusions:We suggest that any real declines are generally most easily explained by changes in cultivated varieties between 1950 and 1999, in which there may be trade-offs between yield and nutrient content
Note that the above study is often used as evidence to justify the "organic movement", yet conveniently disregarding the authors actual conclusions.
I am pretty sure there is not any respectable publication showing any benefits from organic farming nutrition vice.
It maybe that non-processed organic food has higher nutrition content than processed non-organic, but that is not really a fair comparison. You could get the same nutrients from buying non-processed non-organic food, for half the price probably.