Excessive growth of vegetation in water bodies results in a reduction in the transmission of light, production of toxins, and low oxygen levels in water, with detrimental effects on fish and other aquatic life. The role of agricultural phosphorus in this process of water quality degradation, or eutrophication, is addressed in this lesson. Phosphorus is an essential nutrient for the growth of all vegetation, including aquatic vegetation. Soil naturally contains much phosphorus that, if delivered to a water body by erosion or runoff, contributes to eutrophication. In addition to the naturally occurring soil phosphorus, additional phosphorus is applied to land to improve crop production as chemical fertilizer or manure. Phosphorus application may be excessive where manure is applied for convenient disposal, resulting in risk of increased P delivery to a water body. The lesson also addresses phosphorus dynamics in water bodies and how various factors affect phosphorus availability to aquatic vegetation.