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.