Fossil Fuels/Brittleness It 'used' to be everyone at least rotated crops every year (corn, soy, wheat, corn) to put nitrogen back in the soil in a more natural process. Now, the rotations are more like corn, corn, corn, soy, corn. Since the soybeans aren't there to put nitrogen back in the soil, farmers use literal tons of oil produced nitrogen. When gas prices are high, my dad's not complaining about the 300 gallons of diesel a day per tractor he's using. He's talking about the increase in nitrogen prices. The price increases have made it so that farmers used to be able to break even at about 2.50 a bushel for corn. Now its something like 4.50. That's a huge jump when you're now getting 200+ bushels per acre and in the 1990's 170 bushels was a great year.
Diversity A few years ago (can't quite remember) there was a huge scare of European corn borer destroying all of one type of corn. It was the crop equivalent of the bird flu scare. It turned out to not happen, but the possibility is still there. Now that we're less diverse one 'superbug' or bad weather year could take out everything.