solarz
Brigadier
there is just not enough nitrogen fixation capability.
in a logistics disruption situation, the immediate cutoff of oil, fertilizer, electricity and water (which requires electrical pumps) means rapid starvation and thirst. then, long term, you have the diminished carrying capacity caused by loss of Haber Bosch process.
urban areas on the other hand are far more resilient, see Siege of Leningrad.
Now I'm not talking about feeding 7 billion people, just a family of 4-5.