At least in my experience you plant cover crops there and plow them under at the end. My relatives used field peas. And the four-field system is considered a strict upgrade over the three-field system last I heard, basically just 3 productive fields and 1 planted in cover crops.

https://www.sare.org/Learning-Center/Books/Building-Soils-fo...

We can do better than that:

- Someone figured out a way to grow comfrey (and other nitrogen fixers) in a separate field with low maintenance to supply mulch material for the main crop, without having to rely on animal manure.

- There are people who built their animal barn at the top of a hill and dug swales to transport runoff from manure downward. They plant edibles all along the swale. Once established, low maintenance.

- I saw another practice that involved growing grasses, and using grazers and micrograzers to increase fertility in depleted soil. It’s part of an overal system where fences and chicken trailers are used to move grazing around.

Some of these practices may not be practical at scale, but that is not the point — these practices work well for decentralized food system