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Nonpoint sources of P leaving the agricultural landscape can cause eutrophication of surface water and the associated growth of undesirable aquatic plants. USDA is developing an indexing procedure for identifying soils, landforms, and management practices that could have unfavorable impacts on water bodies because of P movement. This indexing procedure uses the characteristic of the field site, including soil erosion rates, runoff, available P soil test levels, and fertilizer and organic P application rates and methods to assess the degree of vulnerability of P movement from the site. A weighting procedure has been developed to include the various contributions each site characteristic may have: A summation of the products of the weighted site characteristic yields a site vulnerability rating. An assessment tool using the Phosphorus Index will assist field staff working with landusers to identify sensitive areas and recommend management alternatives to reduce risk of P losses. Problem The movement of P from the agricultural landscape is a concern because of the potential problem of surface water eutrophication and subsequent growth of undesirable aquatic plants. Landowners, producers, and field staffs striving for natural resource management must assess the soil, hydrology, and management of individual fields in order to identify those areas that have a greater‐than‐desired impact on the movement of P. By targeting these areas, landusers and resource planners can develop the necessary management practices that could reduce the potential for P movement. A field‐level P index is needed to make this assessment and assist in the development of management practices to lessen the potential for P movement. Background Models and equations have been used to assess a number of natural resource concerns, such as soil erosion by water and wind, nitrate leaching, and pesticide movement. A few simulation programs have attempted to calculate the potential delivery of P to the edge of the field. No assessment model, however, provides the user with a simple tool to measure the relative potential of P movement from the site based on readily available field parameter values. Project Description A two dimensional matrix has been developed that relates field site characteristics to values of categories based on the severity of the P movement problem (Table ). The site characteristics have been assigned a weighting factor based on the potential for that characteristic to affect P movement. There are eight site characteristics in version 1.0 of the Phosphorus Index. The eight characteristics with their weighting factor are: soil erosion (1.5) irrigation erosion (13) runoff (0.5) soil P test (1.0) P fertilizer application rate (0.75) P fertilizer application method (0.5) organic P source application rate (1.0) organic P source application method (1.0) Value categories are defined as measured levels of each site characteristic. The higher the value the greater is the potential for phosphorus to leave the site. The five value categories are: None Low Medium High Very high Applied Questions How does the Phosphorus Index work? To make an assessment using the Phosphorus Index, select a rating value for each site characteristic in the field based on available field data. Use the rating categories None, Low, Medium, High, and Very high. Multiply the site weight factor by the rating value to get the weighted value for that site characteristic. Proceed to rate and multiply each characteristic of the index. Sum the weighted values for all eight characteristics and compare the total with a site vulnerability chart (Table ). The vulnerability is a relative rating of the potential for P to move from the field site. How can the Phosphorus Index be used in planning management practices to reduce the potential for phosphorus movement.? Upon completion of the Phosphorus Index assessment, the matrix can be analyzed for each individual site characteristic. Those site characteristics that rate High or Very high should be examined closely in order to develop management strategies that can lower the rating; that is, move the rating to the left on the matrix to a Low or Medium. Such analysis of the matrix for each site characteristic will formulate a management plan for that particular site. Each site will require its own analysis based on its particular site characteristics. Phosphorus Index, version 1.0, with the site characteristics and weighting factor in the left column and the values of the categories in each row. Summation of the weighted rating value is used to determine the site vulnerability. Phosphorus loss rating (Value) Site characteristic (weight) None (0) Low (1) Medium (21 High (4) Very high (8) Soil erosion (1.5) Not applicable 15 tons/acre Irrigation erosion (1.5) Not applicable Tailwater recovery or QS QS > 10 for erosion QS > 10 for erodible QS > 6 for very 150 Po 5 lb/acre (0.75) P fertilizer application method None applied Placed with planter deeper Incorporated immediately Incorporated >3 months Surface applied (0.5) than 2 in. before crop before crop or surface >3 months before applied 90 P 2 O 5 lb/acre rate (1.0) Organic P source application None Injected deeper than 2 in. Incorporated immediately Incorporated > 3 months Surface applied to method (1.0) before crop before crop or surface pasture, or >3 applied < 3 months before months before crop crop <jats:tit
Lemunyon et al. (Fri,) studied this question.