On the East coast, there is a big problem with high nitrates in the air as it forms nitric acid with water, hence the name, acid rain. For most of history, nitrogen was not available to plants through manmade sources. A common way to get nitrogen into the soil was planting nitrogen fixing plants. Alfalfa, clover, and soybeans collect large amounts of nitrogen in their roots from the air, and it becomes available to new plants when the old crop is tilled under. Another added benefit is that the farmers can harvest the plants for crops or animal feed.
Animal manures are also a good source of nitrogen.
Generally, nitrogen is added to the soil either naturally or by manmade fertilizers. Please refer to the diagram of the nitrogen cycle in this article. Nitrogen is added to the soil from various sources. The materials are changed in the soil by micro-organisms to forms that can be used by the plant. This is why nitrate fertilizers are among the best because they require little change to be available to the plant.