It occurs naturally in the environment in soil, water, and air. Nitrogen is present in many compounds, including ammonia and nitrate. Ammonia is present in both human and animal waste. It can enter the soil from septic systems, animal enclosures, or manure storage facilities. Microorganisms then convert it to nitrate.
Agriculture is a significant contributor to nitrogen levels in the soil.
To improve crop yields, fertilizers that contain nitrogen are applied to crops. This nitrogen is also converted to nitrate in the soil. When more nitrate accumulates than the plants growing in the soil can use, water from irrigation and rain can carry it down through the soil into groundwater by a process known as leaching. How fast leaching occurs depends on the type of soil. However, this process does take time.