[pfdietz]

Nitrogen fertilizer is synthesized from hydrogen and nitrogen. The hydrogen is currently derived from fossil fuels, but there is no requirement that it be so.

Saying "ammonia is produced from fossil fuels, and so must always be" is like saying "cars run on fossil fuels, and so always must". A non sequitur.

Phosphates are derived from large phosphate deposits in various places, such as Florida. Phosphorus will ultimately have to be mined from lower concentration deposits, perhaps ultimately from average crustal rocks, where it appears at about 0.1% concentration. However, build up of mostly insoluble phosphates in soils will I think likely reduce the need for this fertilizer if erosion is kept in check.

[_DeadFred_]

This can't be replaced at a volume that can feed the world nor in a way the world can afford. Lots of things can be done in alternative ways if you remove half the requirements (in this case volume and affordability).

[pfdietz]

It can be replaced at volume that can feed the world. After all, the total energy involved is small compared to what would have to be produced to power the entirety of industrial civilization. Agriculture uses < 2% of total US energy consumption.