Fertilizers are made from Amonia which in turn is made using the Haber-Bosch process which requires fossil fuels to provide the necessary energy and as reactants (see this related article).

There is also "natural" fertilizer made from organic mass left over from other activities which would otherwise go to waste, but that's insufficient for large scale intensive farming (composting is fine for your community garden or even for supplementing low intensity agriculture, but not for the intensive industrial farming growing things like hybrid corn).

Finally, the use of techniques like crop rotation which lets letting fields lie fallow so that natural nitrate fixation occurs and the soil recovers do not make the soil rich enough in nitrates to support hybrid corn growing because, as I mentioned, the plant density is too high to be supported by natural soil alone without further addition of fertilizers.