What Michael Pollan writes about in this book, essentially, is that it is healthier to eat locally, both for the individual and for the planet. This is basically his battle cry throughout the book. After defining what the "USDA Organic" label really entails, he goes into some shocking similarities between "USDA Organic" farms (both vegetables and animals) with those that are not. The basic reason is that "organic" farms are growing to such a size and servicing such large populations that their practices show little difference than large scale mass-consumer farms that are not technically organic.
Additionally, small local farmers tend to have more control over their "organic" practices because (as in the raising of chickens, for instance), they simply are not housing, killing, cleaning, and shipping the same numbers as many large-scale "organic" farms. He also asserts that "organic" meat from large farms could potentially be more dangerous because what this really means is the animals (while they may be fed differently) are not given antibiotics. However, they possibly live in the same overcrowded conditions as animals raised in CAFO's (concentrated animal feeding operations) and are just as susceptible to disease.
Finally, with the cost of fuel, refridgeration, and packaging for shipment, when a large-scale organic farm is providing for supermarkets around the country, they lose much of what could be considered "green" when you look at the entire picture. Pollan makes the basic arguments that buying locally cuts costs of shipment (naturally) because the consumer is does the bulk of the traveling. He factors in the amount of waste produced on a local farm over that of some of the bigger "organic" farms and asserts it is decidedly reduced at the local level. Finally, buying locally supports the local economy.