The energy use by the agricultural sector of the economy has been widely discussed and debated in the peak oil community.  The amount of energy used directly at farms is not very large; typical claims for the fuel required to cover a field with a plow or other implement are in the range of one gallon of diesel per acre per pass.  Assuming seeding, harvesting and 3 other passes per year, the total comes to approximately 750 MJ per acre per year.  Nitrogen fertilizer applied at 200 pounds of nitrogen per acre would account for another 4600 MJ per acre¹.  Residues from many crops such as corn can supply over 20 GJ per acre and energy sources such as wood chips and fuel grasses are even more productive.  Farming operations such as dairies have already become net exporters of energy as electricity.  This suggests that even a mechanized farm can be self-sufficient in energy, and "fast crash" doom scenarios involving the collapse of farming are not very likely.