Before the world wars and invention of artificial fertiliser we didn't have monocultures. On the scale of agriculture artificial fertiliser and factory farming is extremely new not something we have been doing for hundreds of years.
The issue is that when the crop is harvested you are taking nutrients out of the field into the crop then taking that crop off the soil. These nutrients need replaced or you will deplete the field. A hundred years ago this would come back in the form of rotations of crops and animals that leave dung to fertilise the land today its done with artificial fertiliser which provides lots of nitrogen but is missing all of the trace nutrients that where removed.
Better artificial fertiliser could help but there are other problems with artificial fertiliser like run off where rain remove the fertiliser from the field and dumps it into wetlands and rivers which are poisoned by the massive amounts of nitrogen.
The old way was sustainable but required lots of manual labour the new way is not sustainable but is easily automated. The real sustainable solution is to automate the old way of farming.
Ammonia production can be designed to run when energy is cheap, free, or negative in price, this is unlike other energy-intensive industrial feedstocks such as aluminium smelting.
It isn't currently economically efficient to run factories this way, but (again, unlike smelting bauxite) it is physically plausible to start-and-stop Haber process plants on a scale of hours.
Nitrogen fertilizers cause their own cascade of second-order effects when they leach into the water supply, but this can be solved with well-understood chemistry which renders the nitrate largely insoluble in water but available to soil microorganisms, who are happy to share it with crop roots.
My point is that fossil fuels aren't a feedstock for the most important fertilizer, so the use of fossil fuels in their manufacture is instrumental rather than inherent.