"When the fossil fuels run out, modern agriculture will collapse." We fertilize crops, especially with nitrogen, because the triple bond in atmospheric N≡N is

"When the fossil fuels run out, modern agriculture will collapse." We fertilize crops, especially with nitrogen, because the triple bond in atmospheric N≡N is a tough nut to break. We use chemical engineering (in this case, high pressure and heat, google Haber-Bosch process) to fix the nitrogen from the air on behalf of the plants. This requires energy, but any cheap energy would do, it doesn't need to be from fossil fuels.