FakDendor

I'm a plant and microbial engineer, and I can answer Burnie's question about how certain crops can regenerate nutrients in the soil.

Living things are primarily made up of 4 elements (yes there are more we can talk in the comments): carbon, oxygen, hydrogen, and nitrogen. Plants can obtain their carbon, oxygen, and hydrogen from air and water. Nitrogen is the odd one out: as Michael Kaine points out in interstellar, "Our atmosphere is 80% nitrogen...we don't even breathe nitrogen". Nitrogen is actually a limiting factor, despite its abundance. Plants can get nitrogen from the soil, but even there they are restricted to nitrogen in certain forms (such as nitrates). This is why most fertilizers contain ample nitrogen in usable forms.

Certain bacteria can do the heavy lifting to convert nitrogen from its atmospheric state (N2 gas) to forms useful to plants. There's a catch: it has to be done in the absence of oxygen. Kind of difficult, being as the rest of the atmosphere (~20%) is basically oxygen. So certain plants and certain bacteria have learned to form a symbiosis: the plant hosts create special nodules free of oxygen that are colonized by nitrogen fixing bacteria. These bacteria fix nitrogen for the plant in exchange for food and shelter, giving both the plant host and the bacteria colonizer an advantage.

Growing the plants to a certain stage and then turning them over without removing the biomass from the soil lets the rest of the plant degrade back into base components, releasing the stored nitrogen. Most of our crops do not fix nitrogen, which is why good agricultural practices like cover cropping regenerate soil nutrition.

FYI my work as a plant and microbial bioengineer focuses on how to improve these relationships and establish novel ones through biotechnology. I'm happy to answer any questions!