The production of concentrated nitrogen compounds from thin air is useful enough that we'll almost certainly keep doing it en masse in an electric-only future.

Mining for phosphorous and potassium fertilizers, likewise, but situationally a little different because these aren't very mobile in the groundwater column like nitrogen is, and they don't offgas back into the air like nitrogen compounds do. Quite possibly we'll be mining manure lagoons more, for CH4 and for closing the loop better on P and K.

Ag will continue at industrial scale for cereal grains, because half the population is not going back to the fields.

Within that framework, there's a lot of difference between outcomes in terms of how green we make our farms, what we grow, how we grow it. Herbicide and insecticide practices do not have to be what they are, as we witness massive overuse of things like neonics, glyphosate, and aminopyralid mostly because there's little financial reason to constrain use. We could stand to dramatically reduce the amount of cereal grain we consume, from a diet perspective, but the logistical difficulties of alternatives like more fresh fruits & vegetables will tend to increase carbon emissions. Eating less grain-fed meat and more high-protein legumes is basically a win-win from diet and climate perspectives. Returning to a less industrialized industry where livestock are raised on farms instead of on "feeding operations" seems like a fair tradeoff against something like subsidized corn-ethanol production. Attempting to encourage long-term soil stability with reduced tillage and is another goal that we might tangle with that would reduce yields; We have plenty of yield to spare in the US, so this is an option.

> Eventually, you won't have a choice when fertiliser produced from oil runs out, becomes cost prohibitive, or is made illegal due to greenhouse gas problems

You mean, aside from the process of making ammonia using green hydrogen that doesn't use fossil fuel at all? A process that can be sustained indefinitely, using renewable energy?

The single big concern is nitrous oxide emission from bacteria in the soil, but that can be reduced by nitrification inhibitors, some of which can be produced naturally by plant roots (and likely engineered into crop plants.)

Fertilizer is mainly made from natural gas, not oil. Accordingly it should last much longer. Worse case scenario when we run out is we switch to less efficient production, for instance splitting water using nuclear power.

Any plan that relies on depopulation isn't going to work and any attempt to force it to work would require crimes against humanity.