The basic elements that occur in nature move through the
environment in a series of naturally occurring chemical and
biological states, a process commonly referred to as a cycle. The
cycle describes the state, chemical form, and relative abundance
of the element at each point along its route through the
environment. There is usually a state, chemical form, and
location in the cycle in which nature safely stores the bulk of
the element. In the case of the nitrogen cycle, the bulk is
stored as nitrogen gas in the atmosphere. Pollution occurs when,
through man's interference, an element occurs at some point in
the cycle in an inappropriate form or amount, thus disrupting the
environmental balance.
Nitrogen and phosphorus, elements essential to plant growth, move through the environment in such cycles. Fertilizers and animal wastes both contain both nitrogen and phosphorus. When these elements are applied to crop and pasture lands in amounts in excess of plant needs, they can adversely affect water quality.
Phosphorus, the less mobile of these two nutrients, is quickly bound to soil particles or taken up by plants. Because about 85% of phosphorus is bound to soil and organic particles, eroding sediments and organic materials borne by runoff are the chief source of phosphorus in water.
In contrast, nitrogen from fertilizer and animal waste is soluble in water as nitrate, and not held by soil particles. Nitrate ions, which are not taken up by plants or converted to gaseous forms by microbial action, can leach downward through the soil into the groundwater or move laterally with surface and subsurface flow to contaminate surface waters.