P abundance in Iowa's surface waters produces aquatic plant overgrowth, primarily of algae. Plants and algae use much of the oxygen in water as they die and decompose, resulting in oxygen shortages that are detrimental to many aquatic organisms. However, setting standards that protect receiving waters downstream is alsoa consideration. Each transport mechanism is affected by rainfall and each is difficult to manage consistently. P movement is also heavily influenced by soil properties and land management practices such as cropping and tillage.
Soil erosion is selective, with finer and less dense particles preferentially moved.
P concentrations in sediment are higher than those found just below the soil surface because fine particles have more surface area per unit of mass, and particles with higher organic matter have a lower density. Also, if subsoils are low in P, water moving through them adsorbs DP, resulting in low levels of DP relative to surface runoff water. However, if subsoils are high in P or become saturated with P, levels of DP in subsurface drainage are greater than normal. The thin layer of soil at the surface, called the mixing zone , interacts with rainfall and runoff and is the layer from which DP is extracted and PP is eroded. The concentrations of DP in water and PP in sediment are directly related to the levels of P present in the mixing zone.