The Pennsylvania Phosphorus Index: Version 2

Gives descriptions of factors and how to determine them in developing a nitrogen-based nutrient management plan that will then be evaluated using the Phosphorus Index provided.
The Pennsylvania Phosphorus Index: Version 2 - Articles

Water Quality and Phosphorus

Increased attention has focused on phosphorus (P) management due to water quality concerns resulting from eutrophication. In freshwater systems, elevated P inputs can lead to accelerated eutrophication and degraded water quality. Despite water quality concerns, adequate levels of soil P must be maintained to promote optimal crop production. Therefore, management options for P must be flexible in order to address agronomic concerns while providing water quality protection.

Phosphorus Index Application in Pennsylvania

Currently, Pennsylvania's Nutrient Management Program (Act 38), Concentrated Animal Feeding Operation (CAFO) program, and USDA-NRCS 590 Practice Standard require P-based plans developed using the P Index described in this fact sheet.

The Phosphorus Index

The P Index is a field evaluation tool that was developed to identify areas that have a high risk of P loss to bodies of surface water. This tool combines indicators of P source and of P transport. The P source indicators used in the Pennsylvania P Index are the Mehlich 3 soil test P; fertilizer application rates and methods; and manure application rates, methods, and P source coefficients (PSC). The transport indicators used are erosion, runoff potential, subsurface drainage, distance to a body of water, and evaluation of management practices that impact how P is potentially lost from a field.

In order to use the P Index, one must develop an N-based nutrient management plan for a crop management unit (CMU) and then evaluate this plan using The Pennsylvania Phosphorus Index Version 2 Worksheet.

A brief description of each factor associated with the P Index and the information needed for each factor's determination follows.

Instructions for Completing the Phosphorus Index

The P Index is made up of two major parts. Part A is a screening tool that determines if the more detailed Part B analysis is required.

Part A

The following four criteria must be evaluated in Part A of the P Index for each CMU in the nutrient management plan:

  1. The more detailed Part B analysis is required for all CMUs located within special protection watersheds.
  2. Part A is only appropriate for CMUs on operations with relatively stable management systems. Generally, in these systems, current, planned, and historic practices are similar. If there is a significant change in farm management as defined by the following Act 38 criteria, P Index Part B must be used. The following Act 38 criteria define when a farm is undergoing a significant change in management:
    • Net increase of greater than 10 percent in animal equivalent units (AEUs) per acre
    • Change in crop management resulting in a farmwide reduction of more than 20 percent of nitrogen necessary for realistically expected crop yields
    • Alternative organic sources replace all or some of the nutrient sources listed in the plan
    • Additional lands are brought into the operation (purchased or rented)
  3. If a CMU has a soil test P level greater than 200 ppm P (Mehlich 3), then P Index Part B must be used.
  4. If a CMU is closer than 150 feet to a receiving body of water, as measured in the direction of water flow from the lower edge of the CMU, then P Index Part B must be used.

If none of these criteria apply, the N- based plan for the CMU is acceptable as written.

Part B

Part B is a more detailed analysis of source and transport factors used to evaluate the risk of P loss.

Source Factors

Soil test rating--taken directly from the soil test report (Mehlich 3 P, in ppm).

Fertilizer rating (fertilizer P rates, methods, and timing of application)--taken directly from the N-based nutrient management plan.

Manure rating (manure P rates, methods, timing of application and PSC values)--taken directly from the N-based nutrient management plan. Phosphorus source coefficient values vary with the type of manure or biosolids applied. Analytical results from a water-soluble P test or Table 1 values can be used to determine an appropriate PSC value.

The source factor is determined by adding these factors:

Source Factor = Soil Test Rating + Fertilizer Rating + Manure Rating

Table 1. Phosphorus source coefficients.
Swine Manure1.0
Dairy-Bedded Pack0.8
BPR Solids0.8
All Biosolids (Except BPR)0.4

Transport Factors

Erosion--the calculated soil erosion (tons per acre) using the Revised Universal Soil Loss Equation (RUSLE). This can usually be taken directly from the farm conservation plan.

Enter the factor in the P Index that corresponds to each of the following:

  • Runoff potential--based on the soil type and drainage class. Can be determined using soils maps for the operation and tables provided by USDA-NRCS. These tables can be found for each county on the Pennsylvania Nutrient Management Program Web site.
  • Subsurface drainage--based on whether or not there is artificial drainage in the field or if the field is less than 100 feet from receiving water and has rapidly permeable soils. "Random" drainage is characterized by a field having a single tile drainage line or a few tile drainage lines, and "patterned" drainage is characterized by a field being almost or totally drained by a full patterned drainage system.
  • Contributing distance--the distance from the lower edge of the field to a stream or other receiving body of water. Choose the distance category in the P Index representing the distance of the majority of the lower edge of the field in the direction of water flow. Fields within 100 feet of receiving water with a 35-foot vegetated buffer should use a contributing distance factor of 6. Fields within 100 feet of receiving water with a 50-foot buffer designed according to USDA-NRCS standards should also use a contributing distance factor of 6, combined with an additional riparian buffer credit. See the "Modified connectivity" section below for more details.
  • Modified connectivity--accounts for management practices that modify P transport.
    • If the field is within 100 feet of receiving water and a 50-foot riparian buffer (designed to meet the sediment and dissolved-nutrient control criteria of USDA-NRCS Practice Standards 390, 391, or 393) is present, use the modified connectivity factor 0.85 to reduce the transport factor value.
    • If a field is more than 100 feet from receiving water, but a direct connection such as a pipe or ditch from the field to the receiving water is present, use the modified connectivity factor 1.1 to increase the transport factor value.

The transport factor is determined by first adding the erosion rate, the run-off potential, the subsurface drainage, and the contributing distance factors together to get the transport sum. Then, multiply the transport sum by the modified connectivity and divide by 24.

Transport Factor = (Transport Sum x Modified Connectivity) / 24

P Index Value

The P Index value is calculated by multiplying the source factor by the transport factor and then multiplying the product by 2.

P Index Value = (Source Factor x Transport Factor) x 2

Interpretations and management guidance for the P Index value are given in Table 2. These interpretations give general guidance for management based on the P Index. It is not only important to look at the final index rating, but, if the rating is "High" or "Very High," it is also important to go back to the P Index and determine why. Often management changes other than "limited" or "no manure application" can lower the P Index to an acceptable level that allows for manure application and provides water quality protection. Examples may include establishing best management practices (BMPs) to reduce soil erosion or changing the timing or method of manure or fertilizer application. Also, if the P Index rating is in the "Medium" category, be aware that continued N-based manure application rates will likely increase the P Index rating and may result in "High" or "Very High" P Index ratings in the future.

Table 2. Phosphorus index management guidance.
ValueRatingManagement Guidance
0 to 59LowNutrients can be applied to meet the nitrogen crop requirement. Low potential for P loss. Maintenance of current farming practices is recommended to minimize the risk of adverse impacts on surface waters.
60 to 79MediumNutrients can be applied to meet the nitrogen crop requirement. Medium potential for P loss. The chance for adverse impacts on surface waters exists. An assessment of current farm nutrient management and conservation practices is recommended to minimize the risk of future P loss
80 to 99HighNutrients can be applied to meet the phosphorus crop removal. High potential for P loss and adverse impacts on surface waters. Soil and water conservation measures and P-based management plans are needed to minimize the risk of P loss.
100 or higherVery HighNo phosphorus can be applied. Very high potential for P loss and greater adverse impacts on surface waters. Conservation measures and a P-based management plan must be implemented to minimize the P loss.

Information Resources for the Phosphorus Index

The Pennsylvania Phosphorus Index Version 2 Worksheet

The Pennsylvania Nutrient Management Program provides a comprehensive source for information about Pennsylvania's Nutrient Management Program and associated technical guidance and educational information.

Weld, J. L., R. L. Parsons, D. B. Beegle, A. N. Sharpley, W. J. Gburek, and W. R. Clouser. 2002. Evaluation of phosphorus-based nutrient management strategies in Pennsylvania. Journal of Soil and Water Conservation 57(6): 448-54.

Sharpley, A., and D. Beegle. 2001. Managing Phosphorus for Agriculture and the Environment. University Park: The Pennsylvania State University.

Sharpley, A. N., T. Daniel, T. Sims, J. Lemunyon, R. Stevens, and R. Parry. 2003. Agricultural Phosphorus and Eutrophication. U.S. Department of Agriculture, Agricultural Research Service, ARS-149.

Pennsylvania P Index Version 2 Developers

The following scientists developed the Pennsylvania P Index Version 2, and those with listed contact information serve as resources for additional information:

Douglas Beegle
Professor of Agronomy Farm Nutrient Management
Department of Plant Science
The Pennsylvania State University

Ray Bryant
Research Leader and Soil Scientist

William Gburek
Retired Hydrologist

Peter Kleinman
Soil Scientist Soil and manure testing

Andrew Sharpley
Soil management to minimize watershed P export
Department of Crop, Soil, and Environmental Sciences University of Arkansas

Jennifer Weld
Project Associate
Phosphorus Index implementation and plan development
Department of Plant Science
The Pennsylvania State University

Additional Contacts for Information

William Clouser
Nutrient Management Program Supervisor
State Conservation Commission

Herschel Elliott
Department of Agricultural and Biological Engineering
The Pennsylvania State University

Mark Goodson
USDA-NRCS in Pennsylvania

John Spargo
Director, Agricultural Analytical Services Laboratory
The Pennsylvania State University

Prepared by Jennifer Weld, project associate; Douglas Beegle, professor of agronomy, Penn State; Ray Bryant, research leader and soil scientist; William Gburek, hydrologist; Peter Kleinman, soil scientist, USDA-ARS PSWMRU; and Andrew Sharpley, professor, University of Arkansas.

Publication prepared with funding provided by the Pennsylvania State Conservation Commission.


Jennifer Weld