Tom Philpott at Grist highlights an Iowa State study that showed greater productivity over 9 years from organic crop rotations as opposed to industrial ag cropping.
Although that's major news, and noteworthy, it doesn't prove what Tom wants it to prove. For organic ag to feed the world, replacing industrial ag, you don't need to prove that, acre for acre, organic is of equal productivity in a given year. You need to prove it over a course of years. In other words, organic ag rests on the idea of crop rotation. Corn one year, soybeans the next, maybe alfalfa years 3-5, then corn again. So over 10 years, the total production of the acre is 2 years worth of corn, 2 years worth of soybeans, and 6 years worth of something else. For industrial ag, it might be 5 years corn, 5 years soybeans, or even 7 years corn, 3 soybeans.