Nitrogen not removed through crop harvest can reach groundwater as nitrate. Nitrogen sources . Animal manure, human wastes, compost, organic wastes, legume and green manure crops are organic sources of nitrogen. A few nitrogen fertilizers are in the nitrate form, but more commonly fertilizers are in the ammonium form, initially. Nitrate is soluble in water and easily moves with water through the soil profile.
Ammonium is soluble in water, but is tightly absorbed on exchange sites in the soil.
Ammonium nitrogen, regardless of initial source, is rapidly converted to nitrate by soil bacteria at soil temperatures above 50 degrees Fahrenheit. When nitrogen is added to the soil, either from organic or inorganic sources, it becomes a part of the nitrogen cycle. When total nitrogen inputs exceed the amount used by plants, nitrate accumulates in the soil and may be leached to groundwater. From the standpoint of groundwater quality, leaching is the only concern. Other loss mechanisms contribute to low nitrogen efficiencies, but not directly to groundwater contamination.