Corn in the U.S. is a tricky matter. It's no longer food, it's a commodity. We grow exponentially more than we can eat, and it becomes a source material for an unbelievable amount of other products. There is "corn" in just about every type of processed food (high-fructose corn syrup, citric acid, glucose, fructose, maltodextrin, lactic acid, sorbitol, mannitol, xantham gum, modified and unmodified starches, MSG, beverage-bound ethanol, etc. etc.). Most of our meat is actually fed corn - something that is completely unnatural for cows, and requires lots of drugs and antibiotics to treat the problems that arise from feeding them corn.

It no longer grows naturally (for the most part), because it can't. The reason it's so prodigious is due to the genetically-modified hybrid strain and the use of artificial fertilizers, made from and with fossil fuels. It's industrialized agriculture, and like just about anything else 'industrialized' it was made possible with the use of fossil fuels.

To get back on topic, carbon-neutrality is all but impossible to calculate since there are so many factors involved and so many things that happen upstream that are difficult to account for.

Bottom line is, if we can try to keep as much of these things in mind as we can, and then use that knowledge to choose better alternatives - choices made with our purchases (as that's where our power lies in this society) - we can do better.