The fertilizer that is derived from fossil fuels is ammonia (and nitrate made from ammonia by partial oxidation). More precisely, it's made from hydrogen derived from steam reforming of methane. But any source of hydrogen would do, including electrolysis with renewable electricity. There is nothing essential about fossil fuels for industrial agriculture.

It should also be noted that the total energy consumed in growing food in the US (ignoring sunlight absorbed by the plants) is less than the energy used to COOK food in the US. It's roughly 1% of the total energy used by the US.