Manure Management Planner

Background. In response to mounting water quality concerns, especially those associated with agricultural phosphorus and accelerated eutrophication, a joint federal initiative between the USDA Natural Resources Conservation Service (NRCS) and the US Environmental Protection Agency (EPA) was created to address manure management on concentrated animal feeding operations (CAFOs) (USDA & USEPA, 1999). NRCS began (early 90s) an evaluation of their nutrient management standard (590 standard), which at that time was nitrogen based, i.e., manure application rates were designed to meet the nitrogen needs of the crop with little consideration given to phosphorus. As most manures contain a combination of nitrogen and phosphorus that does not match plant needs, long-term manure application rates based on nitrogen resulted in elevating the level of phosphorus in the soil far above that required for adequate crop growth (Sims, 1996). Consequently, in many areas, soil phosphorus (from manure) accumulated to level that became more of an environmental than an agronomic concern (Sharpley et al., 1996).

Threshold Soil Test Phosphorus Levels: The Initial Approach. Because of the known relationship between the level of phosphorus in the soil and that contained in the runoff water and the absence of any federal guidelines, many states attempted to establish a threshold level of soil test phosphorus above which no more manure could be applied (Sharpley et al., 1996; Pote et al., 1996; Pote et al., 1999; Sims et al., 2002). However, scientists working in this area soon recognized that while the level of soil test phosphorus was certainly a contributing factor to phosphorus loss, other factors such as soil chemistry, rate and method of manure application, runoff, and erosion played an equal if not a more important role in determining phosphorus loss. Since movement of phosphorus to a surface water body is much more complex, it was understood that this single parameter approach should be abandoned in search for a more comprehensive, flexible, and inclusive method of manure management that incorporated the primary physical and chemical factors responsible for phosphorus loss.

Phosphorus Index. Because NRCS wanted a science based nutrient management policy, they organized an interdisciplinary and interagency group of nationally recognized scientists to provide scientific direction and input to the revisions of their 590 nutrient management standard (Sims and Leytem, 2002; Sims et al., 1997). NRCS was in dire need of a management tool that would allow field staff to assess the relative risk of phosphorus loss from individual fields, identify main sources (rate, erosion, etc.) influencing loss, and select best management practice (BMP) to reduce the risk, i.e., erosion control, reduced application rates, buffers, etc. To meet the management needs of NRCS, the tool had to be easily applicable to large sections of the landscape, require only obtainable site visit input parameters, and be easy to use by field technicians. This was recognized as a challenge and initially the group considered capitalizing on the wealth of information and expertise evolving from scientists developing computer models to predict edge-of-field phosphorus loss. Great scientific progress has been made over the last decades in understanding the runoff process by using computer-based research models such as EPIC (Erosion-Productivity-Impact-Calculator, Williams et al., 1984) to predicting edge-of-field phosphorus loss. While theses models can be useful in research to predict phosphorus loss in erosion and runoff, such models require a large database with many input parameters from each field. For example, more than 50 input parameters are required from each field to adequately run EPIC (Attachment II) to predict annual phosphorus loss and additional calibration and validation would likely be required. While this approach held promise for the future, the use of such data intensive research tools were determined to be too sophisticated and labor intensive to serve as a management tool on large landscapes requiring practical phosphorus management (Havlin et al., 2003) – see comments from national recognized scientists regarding this issue (Attachment III). So an alternative approach was pursued.

To meet NRCS’ management tool needs; the committee of scientists developed a prototype indexing procedure for identifying soil, landforms, and management practices affecting potential phosphorus movement. The indexing procedure uses field site characteristics, including soil erosion, runoff, soil test phosphorus, and application methods and rates to assess the degree of vulnerability of phosphorus movement from the site. A weighting procedure was developed to reflect the relative contributions each site characteristic might have, i.e., erosion = 1.5, runoff =0.5, rate and method of application = 0.5, etc. From this matrix, a numerical site vulnerability index was possible that would rank that specific field’s potential for phosphorus loss with other fields in the watershed. The prototype Phosphorus Index served as a special forum for a symposia sponsored by the American Society of Agronomy (1992) where the Index concept was presented (Lemunyon and Gilbert, 1993). States were encouraged to use this concept and framework as a backdrop for developing their own Index that better reflected local conditions and need for phosphorus management. NRCS then rewrote their 590 standard to include phosphorus-based planning standards that included the Phosphorus Index as an alternative. State and federal agencies in concert with the land-grant universities began to develop their own Phosphorus Index for use in the state.

The initial group of scientists asked to develop the concept of the Index saw an ongoing need to serve as a science resource to NRCS and EPA and formed (1993) a national research and information exchange committee entitled SERA17 Minimizing Phosphorus Loss from Agriculture. (Sims et al., 1997; Sims and Leytem, 2002). SERA17 is an official USDA committee within the Cooperative States Research, Education and Extension Service (CREES). This interdisciplinary/interagency committee rapidly grew in number and scientific credibility, numbering ~150 national and international scientists and serving as scientific resource to foster the Index approach and address other issues relating to phosphorus and manure management. For example, SERA17 was asked to provide written comments to EPA’s CAFO rules and to present these finding to the Senate and House aids. For a hardcopy list of the SERA17 membership see website.

Acceptance and Use. Many versions of the Phosphorus Index are now in use, demonstrating the robustness and flexibility of the indexing framework. NRCS is recommending at a national level that the Phosphorus Index be used in developing phosphorus-based nutrient management plans and meeting the state 590 Standard (Sharpley et al., 2001). To date, 44 states are using this approach to target phosphorus management (Sharpley et al., 2003). A listing of the states and contact personnel using the Index can be found in the website. In a recent study by Weld et al. (2002), although nutrient management plans based on the Index were more expensive to develop, plan writers and Pennsylvania farmers found the use of the Index to be the most flexible and practical approach.

While not the intention of the initial Index, some states are using their index in some form of a regulatory capacity, i.e., Delaware (Sims and Leytem, 2002) Maryland (Coal et al., 2002), and North Carolina (Havlin et al., 2002).

Phosphorus Index and Water Quality – Does it work? It is hard to deny the acceptance and success of the Phosphorus Index, but does it have a direct relationship to water quality? In a watershed study encompassing 16 years of water quality monitoring data from 30 watersheds, Sharpley (1995) showed a close correlation between the numerical value of the Phosphorus Index calculated separately on individual watersheds and actual phosphorus loss, concluding that the Index is directly related to water quality and can serve as a valuable tool for identifying phosphorus sources requiring management practices. In a similar follow up watershed study in Pennsylvania, Sharpley and coworkers (Sharpley et al., 2001) showed that the Phosphorus Index (PA) again paralleled actual phosphorus loss, i.e., the higher the index value the higher the loss. On a more regional scale, Birr and Mulla (2001) separated Minnesota into 60 large watersheds, calculated their average Phosphorus Indexes value and then compared these values to long-term water quality monitoring data, concluding that the Phosphorus Index value was strongly related to phosphorus water quality monitoring data. In another multiple watershed study that included land application of broiler litter, the calculated Index was again compared to measured phosphorus loss (Harmel et al., 2002). These workers concluded that the measured phosphorus losses increased as the Index value increased.

It is clear that the Index is directly related to phosphorus loss and that the Index can be used to identify those fields and factors most responsible for high risk and in need of best management practices (BMPs) to reduce that risk.

Text based on "Evolution of the Phosphorus Index" from Eucha-Spavinaw Phosphorus Index (ESPI).