Context: Most plants pull nitrogen from the soil, and over time the soil becomes depleted and your crops grow more slowly. Bacteria in legumes convert atmospheric nitrogen to plant-usable forms, so historically you've needed to cycle nitrogen-"fixing" crops into your fields to 'restore' the nitrogen balance in the soil. Alternatively, you can use fertilizers, which supply N directly.

This is potentially interesting because the corn acts as its own fertilizer, fixing nitrogen from the atmosphere.