Nebraska 1998 P Index

In the Nebraska 1998 P Index,factors are assigned risk values of  0, 1, 2, 4, 8 with larger numbers indicating greater risk.  Factor scores are weighted for importance by multiplying by values given for the factor.  Weight values are 0.5, 1.0, or 1.5.  Source factors include Bray-P1 or Olsen P, and the rate, method and time of applying manure and fertilizer P.  Five categories for P application method and time and five levels for application rate result in total factor risk values for P application, after weighting, ranging from 0 to 8 for fertilizer P, and 0 to 16 for manure P.  The transport factors are runoff, erosion and distance from point of application to concentrated water flow.  Runoff risk has five levels based on soil permeability.  Erosion risks due to precipitation, furrow irrigation and sprinkler irrigation are estimated separately and have factor risk values, after weighting, ranging from 0 to 12 for precipitation and furrow irrigation, and 0 to 4 for sprinkler irrigation.  Distance to concentrated flow is considered as a function of the distance from the point of P application to concentrated water flow and management practices.  The PI rating is the total of the weighted risk values for all source and transport factors.  Interpretation is based on four classes of risk.  The Nebraska 1998 P index is easy to use, and calculations can be easily done using a paper form, although a spreadsheet version is available to ease calculations when comparing multiple management scenarios.  An accompanying technical document provides tables and other guidelines for estimation of factor scores, some of which account for the great climatic transition in moving from southeastern to northwestern Nebraska.  (Note:  the Nebraska 1998 P Index is no longer available, the 2011 version can be accessed here, with instructions in this pdf file.)