In a pasture for instance, grass can grow because the plant incorporate enough organic matter in the soil to be consumed by microorganisms that will in return fixate the nitrogen from the air into nitrates that can be consumed by the plant. Then you have some equilibrium-ish (it depends on the seasons and the precipitation so it's not an actual equilibrium) amount of nitrogen and organic compounds in your soil.

When you plow the soil, you accelerate decomposition of organic matter that was previously sitting there (because you bring excess oxygen). In the short term, it favors the fixation of nitrogen by the microorganism of the soil (which is why fallow works) but the following years you have less nitrogen fixation than you'd have had otherwise (because there's less organic mater to provide energy to the microorganisms).

Enters the nitrogen fertilizer: with them you don't need microorganisms to provide the nitrogen for your plants, and as such you don't care about the organic matter load of your soil. That's what I call “engineered soil” in opposition to the soils that are driven by the microorganisms who balance the carbon/nitrogen content of the soil.

Of course that doesn't mean that the whole content of the soil is man-made, but coupled with other fertilization methods (which bring nutrients that were naturally almost absent from the soil before), it helped transformed regions which used to be margins with very low yields, into agricultural powerhouses (For instance, Brittany, the region I'm from in France, went from being one of the poorest due to low soil fertility, to the agricultural leader of the country).