By Dan Sullivan, Oregon State University

I have been asked a lot of questions about the Natural Resources Conservation Service (NRCS) Phosphorus (P) Index lately.   Although I do not work for NRCS, and I cannot speak for them, I am willing to tell you what I have learned so far.   So, this “short story” is not so short.  It takes a while to explain the interaction of science and public policy…

What are NRCS nutrient management plans? Nutrient management plans, as defined by NRCS, include: 1) a nutrient budget for N, P and K, 2) an inventory of soil, manure and organic byproduct testing data, 3) a plan for the form source, amount, timing and method of nutrient application on a field by field basis, 4) location of field setbacks (buffer areas) adjacent to sensitive areas such as water bodies. 

When does NRCS get involved in nutrient management planning?  NRCS provides nutrient management planning guidance to 1) farms that receive federal cost-share assistance (to build or maintain conservation practices), and to 2) livestock farms (animal feeding operations) that are regulated by USEPA and/or a state agency.

What is a Phosphorus Index? The P Index is a field-scale phosphorus risk assessment tool used by NRCS in nutrient management plan development.  It evaluates risk of P movement from a field to a nearby water body. 

The Oregon/Washington Phosphorus Index was first adopted by OR/WA NRCS in the late 1990s.  It has been used in nutrient management planning since that time.   The original P Index (1990s) scored a list of factors that contribute to risk (rate of erosion, soil test P value etc), then tallied the overall score and rated overall relative risk (low to very high).   The thought process behind the original OR/WA P Index is described in detail in OSU Extension Publication EM 8848-E:  Agricultural phosphorus management using the Oregon/Washington Phosphorus Indexes, available online.

What events have spurred renewed attention to phosphorus and its role in nutrient management planning?  The NRCS updated its national Code 590 (Nutrient Management) Specification in 2011.   The revised 590 Standard requires assessment of all state P Indexes to ensure that they correctly rank fields by potential for P delivery to surface water.   The Oregon/Washington P Index is currently under revision, and the new Index will be piloted by conservation planners in 2013 & 2014. 

When will nutrient management plans be affected by changes to the NRCS Oregon/Washington P Index?    The process of constructing and evaluating a new P Index for OR/WA is ongoing.  Oregon and Washington NRCS convened three advisory meetings with technical experts from the land-grant universities and other agencies in spring 2013.    The main role of the university representatives (OSU, WSU) was to advise NRCS on the use and misuse of fertilizer/nutrient management guides, soil testing, and manure testing as tools within the OR/WA P Index.  NRCS also consulted with a national group of technical experts, the SERA-17 committee: (   A draft OR/WA P Index was prepared by staff at the NRCS West National Technology Support Center in Portland in June, 2013.  It is currently under preliminary evaluation by a few experienced conservation planners in OR and WA.  

How will the new P Index be different than the old P Index?    The old (1990s) version of the OR/WA P Index used a different “scorecard” for east and west of the Cascades.  The new OR/WA P Index will be a single tool.  The new Index incorporates algorithms that have been developed in conjunction with field research projects over the last 20 years.  The new Index will independently assess risk of P loss from soil erosion (particulate P) and from runoff (dissolved P).    The new P Index will have greater emphasis on the form of P present in organic amendments (whether P is water-soluble or not).   The new P Index will assign lower risk to manure or biosolids treated with alum (aluminum sulfate) to reduce P solubility.  The new P Index will use more site-specific data that can be extracted from NRCS soil databases.  It is likely that new P Index will assign greater risk and recommend little or no P application in fields that have all of these factors present: high soil test P, high runoff/soil erosion potential, and proximity to P-sensitive water bodies. 

Why should biosolids managers pay attention to the Phosphorus Index and other aspects of NRCS nutrient management plans?  

In general, it is good to know what might be coming down the road in the future.   Here are some things to think about:

Both farmers and biosolids managers need to be aware of long-term implications of NRCS nutrient management plans.  At some dryland wheat farms east of the Cascades, farmers have signed up for NRCS conservation incentive programs and found it more complicated to use biosolids on the farm as a result.

USEPA uses NRCS specifications and standards to inform its regulatory oversight and enforcement activities for animal feeding operations.  It is possible that USEPA or state agencies will require use of the NRCS P Index in conjunction with land application of biosolids at some time in the future. 

Why is reliable soil test data important for phosphorus management?   Agronomic soil test phosphorus (Bray P1 or Olsen method) is probably the single most important factor that determines the NRCS P Index rating (low to high risk) for a field.   Reliable soil test data, collected over time is needed to assess the rate of soil test P change.   Biosolids managers can prepare for the future by maintaining a quality database of soil test P values for their land application sites. 

What avenues are there for improving the quality of soil test data?   Begin by reviewing the procedures used for soil sampling and testing at your land application sites.  Recommendations for improving the consistency of soil sampling procedures are given in PNW Extension publication PNW 570-E, Monitoring Soil Nutrients Using a Management Unit Approach (available online).  Consider using a Certified Professional Soil Scientist (CPSS) or Certified Crop Advisor (CCA) to collect soil samples.  

Reliable soil analyses can be achieved by using a laboratory certified under the North American Proficiency Testing-Performance Assessment Program (    NRCS often requires farmers to have soil tests performed at laboratories certified by NAPT-PAP.