How does phosphorus get lost from the field?

The loss of P in surface runoff occurs in sediment-bound and dissolved forms. Sediment P includes P associated with soil particles and organic material eroded during flow events and constitutes most of P transported in surface runoff from most cultivated land. Surface runoff from grass, forest, or noncultivated soils carries little sediment, and is, therefore, generally dominated by dissolved P. The dissolved form of P comes from the release of P from manure, soil, and plant material. This release occurs when rainfall or irrigation water interacts with a thin layer of surface soil (1 to 2 inches) and plant material before leaving the field as surface runoff. Most dissolved P is immediately available for biological uptake. Sediment P is not readily available but can be a long-term source of P for aquatic biota.

In most watersheds, P export occurs mainly in surface runoff rather than subsurface flow, due to fixation of P by P-deficient subsoils. Exceptions occur in sandy, acid organic, or peaty soils, with low P fixation or holding capacities and in soils where the preferential flow of water can occur rapidly through macropores and earthworm holes.

Increases in P loss in surface runoff have been measured after manure application. Phosphorus losses are influenced by the rate, method, and time of application; form of P applied; amount and time of rainfall after application; and vegetative cover. Incorporation of manure into the soil profile either by tillage or subsurface placement reduces the potential for P loss in surface runoff. It was demonstrated that the incorporation of dairy manure by chisel plowing reduced total P loss in surface runoff from corn 13 fold compared to no till areas receiving surface applications. The P loss in surface runoff was decreased by a reduction in surface runoff volume and a lower surface soil P content caused by incorporation of manure and tillage.