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Phosphorus Index
Phosphorus(P), a nutrient that is widely used in agriculture, is a concern because excessive P in surface waters can lead to eutrophication which is harmful to aquatic life. The P Index is an assessment tool developed by the NRCS "to provide field staff, water shed planners, and land users with a tool to assess the various landforms and management practices for potential risk of phosphorus movement to water bodies." The index is used by agriculture extension agents, watershed planners, and farmers, to help manage nutrient applications with minimal impacts to offsite resources(NRCS, 1994).

SOIL EROSION AND P-LOSSES TO THE ENVIRONMENT
Since P is bound to soil particles, the movement of soil to surface waters is the major pathway that agricultural P is lost to the environment (Wiederholt and Johnson, 2005). Therefore, factors that impact soil erosion will also affect soil P losses. Additionally, in soils that have been traditionally overloaded with phosphorus and exceed crop removal rates phosphorus levels become high unlike nitrogen that is more easily lost by leaching and volatilization, phosphorus is less mobile and can remain in the soil for many years. This buildup allows enough phosphorus to be transported by water that can be harmful for aquatic ecosystems (Brady et. al., 2008).

CALCULATING THE P INDEX
Different states have their own methods of calculating the P Index. However, most methods for estimating the P Index take into account characteristics of the land (slope grade, length, and shape), soil chemical and physical properties, land management practices, phosphorus forms, timing, rate, and application methods (NRCS, 1994). Because P-losses are affected by soil erosion, some states use the revised universal soil loss equation program(RUSLE2) to better estimate P loss that may occur. The P index is not a measure P concentration such as the soil test P. It is an index of the potential for P to be lost to the environment, because P losses are not the same for all soils.

SOIL, LANDSCAPE, and VEGETATION
Erosion and P-losses are impacted by slope grade, length, and shape. According to Toy et al., (2002), (NOT IN REFERENCE LIST) erosion increases with slope grade and length. Further, erosion accelerates on concave soil profiles where as convex soil profiles tend to result in the deposition of upslope eroded soil to the lower portion of the slope (Troeh et. al. 2004). Soil chemical and physical characteristics such as pore space and pH are also used to estimate the P Index. Bulk density, pore volume, and other physical properties of the soil influence the amount of P runoff (Davis, 2005). Soil pH affects the solubility of the different forms of phosphorus in the soil. The type of vegetation is considered because it can reduce P losses by altering runoff and infiltration rates that are critical in determing loss pathways(Kinley et. al,2007).

MANAGEMENT OF LAND
Management practices that are considered in estimating the P index include whether or not soil testing is performed regularly(Haden et. al., 2007). Soil testing aids in making phosphorus application decisions. Soil management, the amount of crop residue, forms of nutrients, and application methods can have a greater influence on the amount of P loss than soil test P in some cases(Brady et. al,2007).

Tillage is also considered when calculating the P Index. For sloping lands with no vegetation and loose soil, the land is more vulnerable to P loss from erosion. Other management practices that help with drainage such as field tile which is placed underground for water drainage can be a possible risk for P loss. In shallow, phosphorus-rich soils with numerous macrospores, application of liquid manure can result in phosphorus movement through the soil profile and into ground water that enters water bodies such as streams and lakes (Brady et. al.,2007). Another potential pathway is field tile that allows phosphorus to enter by water moving through the soil profile could easily impact the recieving water body.(Kinley et. al,2007). Although no-till management may be beneifical for reduced erioson, repeated broadcasted phosphorus fertilizer or manure application can lead to a buildup of P on the top layer of crop residue. This additional P increases the potential of P losses from surface water runoff (Sharpley, 2002).

FORMS
The P index depends on the form of fertilizer applied to the soil because the type affects the solubility of P. Some forms are very soluble while others are fixed making them unavailable for plant uptake. For example, a substantial fraction of the P in manure is soluble and readily available for plant uptake (Hayden, 2007). Generally when farmers use manure fertilizer they also over apply phosphorus to achieve a specific nitrogen rate because the nitrogen to phosphorus ratio in manure is low (Hayden, 2007). Applying manure year after year could potentially cause excessive phosphorus amount in the soil, more than plants can remove each year (DeLaune et al., 2004). Another factor that is taken into account when calculating the P-index is the amount of residue that is left behind and its content. Removing the entire crop will remove more phosphorus from the soil than leaving crop residue behind. Obtaining an analysis of the form of fertilizer will help better manage nutrient applications. Some forms of dairy manure may contain CaCo¬3 which would increase pH making the P less available and less likely to be leached from the soil (Hayden et al. 2007).

TIMING OF FERTILIZER APPLICATION
Timing the application of P to the land is an important factor considered in P index measurements. Applying P in times when high amounts of rainfall are likely will increase P loss risks (Foth and Ellis, 1996).

APPLICATION
P index estimates also consider application methods such as surface broadcasting, banding, or injecting. Broadcasting is the easiest method and widely used, but has a greater chance of loss from surface runoff. Banding P places the nutrient next to the crop or vegetation for fast uptake. Although banding may reduce P loss factors such as weather, or available equipment may prevent the nutrient application. The injection method which is used to apply manures is more likely to reduce runoff losses to surface waters(Foth and Ellis, 1996). When applying fertilizer or manure broadcasting or incorportation affects P risk losses. Broadcasting fetilizer over the surface of the land allows the phosphorus to be concentrate and could be moved easily by surface water that potentially enter nearby water sources(Mueller et. al.,1984). When using this method of application the closeness to nearby water should be taken into account to prevent water contamination. Incorporation of fertilizer can reduce the risks of water contamination by mixing the P with the soil, and prevent the P to be lost by surface runoff (Allen et. al.,2007).