I wonder, though. How all-encompassing a nuclear engagement are we postulating?
With a finite number of warheads launched, and with every warhead — especially if we assume mainly tactical nukes in the 1-100kt yield range being used — affecting a finite area
The greatest effect of the war — nuclear and otherwise — on agriculture I rather suspect would be that with a buckling infrastructure, the supply of petrol for tractors becomes patchy, or dries up altogether. This would force farmers to either revert to muscle-driven agriculture (animal-drawn, or in the extreme, human drawn), or else find out how to cook their own fuel snappish.
Either way, yields would certainly plummet, both for grain and other produce as well as for hay to keep livestock fed, but I have hard time envisioning that it would disappear entirely.
So, I'm only referring to Poland here, because we read little on the rest of the world. It seems clear that there was a strategic exchange and supply chains collapsed. Poland, on the other hand, must have gotten it pretty bad, since nukes were used tactically and operationally to break each other's attacks and counterattacks. This means along the front, but also in-depth, as maneuver columns, bridges and cities or infrastructure were hit.
So, soil contamination is definitely an issue, at least in the weeks after the exchange. But since that's the collapse period, clean-ups aren't likely to have happened. Then comes the winter. The snow covers the irradiated earth and once Rasputitsa (further to the East), spring floods and rains hit in March, most of Poland and the Western USSR will be washed over by toxic, stinking and radioactive mud. That leaves back huge pockets of radiation, but some areas will also clean up on their own. Maybe, by early summer 1999, survivors start recovering. The war seems to have boiled down by then, too. Maybe displaced person come back, too, in some quantities. So, they start rebuilding. We see Krakow and Silesia mostly do their own things. During the winter of 1999/2000, NATO starts drafting plans for Operation Reset, moving stuff into position during the time rivers are frozen and destroyed bridges are less of a problem. Once preparations are set, NATO executes the operation and fails, as we know so well, by mid-April.
By the way, I'd be curious to know, why FL advanced Operation Reset into April 2000 and thus changed the date of demise for the 5th Infantry Division from July 18th to April 18th 2000.
Addendum: There seems to be some form of military supply chain left, at least allowing for a massing of fuel, ammo, food, other supplies as well as personnel in advance of major advances, such as Operation Reset. So, maybe units in cantonment horde their fuel rations most of the year, unless in defensive operations. Then, once they start mustering for Operation Reset, they levy additional troops and supplies from their cantonment area, i. e. the population. All of this cannot be salvaged and recycled goods. Some of it must have trickled in from the rest of the world.