Checking Ambition with Reality: The Pros and Cons of Different Approaches to Site Assessment


This talk is intended to spark discussion over options related to site assessment for nutrient management. It will include a brief presentation followed by open discussion.

What did we do?

The revision of the USDA-NRCS national standard for nutrient management in 2011 was driven, in part, by inconsistencies in state phosphorus (P) indices, rekindling debates over standardizing indices at regional or national scales. Reasonable arguments exist for maintaining the status quo, which allows for state specific site assessment approaches, as well as for regional and national P Indices, which would take advantage of expertise, resources and technologies that may not exist locally. In addition, a diversity of site assessment approaches have now been proposed that differ from the original P Index. Understanding the benefits and limitations provided with these approaches is key to advancing site assessment for P management.

All site assessment tools are intended to identify critical source areas of P loss that should be targeted for improved management. The original P Index provided an elegant reduction of key factors affecting P loss from agricultural fields by categorizing factors into “transport” and “source.” More than a decade after the wholesale implementation of state P Indices in 47 US States, critiques of this approach range from inconsistency in their rating of P loss vulnerability, to differences in their recommendations, to poor or “clunky” links to site management.

What have we learned?

A variety of alternative approaches to site assessment have been proposed, most relying upon simulation models that produce an array of off-site metrics, most importantly, runoff P loads. These alternatives have been strongly advocated by their developers and by others interested in quantifying the effects of changing management, but have not yet replaced the original P Indices. Strong rhetoric has been employed in favor, and in opposition, to site assessment approaches. In general, supporters of the P Index argue that it is more of an educational tool, that should be “directionally correct” to affect change in management. Supporters of the modified fate-and-transport models argue that they too can be packaged to be educational and that they have the added benefit of projecting off site benefits. Concern exists over the ability of all site assessment tools to accurately quantify P loss or P loss potential.

Inconsistencies in site assessment approaches at geo-political boundaries (typically state lines but also physiographic and watershed divides) have led to proposals for regional or national approaches to P site assessment. Legitimate tension exists between the representation of unique, local conditions (physiographic or regulatory) and consistency to ensure fairness and accuracy. Past proposals to develop a national P Indexing framework from which local P Indices could be developed were intended to overcome this tension, but were unsuccessful due to local opposition and their top-down nature. Real conflicts are inevitable when state regulations are impinged, even in the name of regional consistency

Future Plans

While the P Index is decided strategic in its approach, a new crop of site assessment tools is emerging to address the day-to-day decision support tools of farmers. These tools employ short-term weather forecasting to identify the potential for runoff to occur following manure application, and range in their sensitivity from field scale to large watershed scale. In general, it is seen that these tools are complementary to the strategic site assessment tools, but, undoubtedly, opportunity exists for a merger of strategic and tactical approaches.


Peter Kleinman, Research Leader, USDA-ARS Pasture Systems and Watershed Management Research Unit

D. Beegle, Pennsylvania State University; D. Osmond, North Carolina State University; J. Lory, University of Missouri; P. Vadas, USDA-Agricultural Research Service; and A. Sharpley, University of Arkansas.

Additional information

This presentation is intended to underscore open discussion at the meeting on the subject of site assessment.


This presentation is the product of a national Conservation Innovation Grant aimed at promoting better coordination in nutrient management planning

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