Nature provides safe storage for nutrients in biological cycles
[Diagram 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.