Nitrogen, in the form of nitrate, is easily soluble. If more nitrogen is applied to fields than can be used by crops, then this excess or residual is available to leave the field in runoff, in tile drainage, and with leachate. Phosphate is only moderately soluble. It tends to adhere to soil particles. If soils have been overfertilized, they become saturated with phosphorus, and dissolved phosphorus losses in runoff will increase.
Erosion can transport considerable amounts of sediment-adsorbed phosphate to surface waters.
EPA reports that nutrient pollution is the leading cause of water quality impairment in lakes and is the third leading cause in rivers. Harmful algal blooms in coastal waters have been attributed to nutrients from manmade sources, including fertilizers. Whether nitrogen actually contaminates surface or ground water depends on the amounts of nitrogen applied to agricultural land, the leaching characteristics of the soil, and on precipitation. ERS Farm Resource Regions with the greatest potential for nitrate contamination of ground water include parts of the Heartland, Mississippi Portal, and Southern Seaboard, based on an index of groundwater vulnerability that considers factors such as soil type and depth to ground water. Regions with the greatest potential for nitrate contamination of surface water are the Heartland, and Mississippi Portal.