Which nutrient is most commonly associated with water pollution due to agricultural runoff?

The nutrient most commonly associated with water pollution due to agricultural runoff is phosphorus. Phosphorus can contribute significantly to the eutrophication of water bodies, a process where excessive nutrients lead to algal blooms that reduce oxygen levels in the water and harm aquatic life. This nutrient typically enters waterways through runoff from fertilizer applications, livestock manure, and other agricultural practices.

While nitrogen, which is the chosen answer here, can also contribute to water pollution and is a concern in agricultural runoff, phosphorus is generally the primary target for water quality management efforts due to its direct impact on freshwater ecosystems. Potassium and calcium are less directly associated with eutrophication and water quality issues compared to phosphorus and nitrogen. However, their contribution to water pollution is minimal when compared to the significant effects of phosphorus.