Section III Phosphorus Indices, Best Management Practices, and Calibration Data © 2007 by Taylor & Francis Group, LLC 301 13 Phosphorus Indices Jennifer Weld The Pennsylvania State University, University Park, PA Andrew N. Sharpley U.S. Department of Agriculture-Agricultural Research Service, University Park, PA CONTENTS 13.1 History of Development 301 13.1.1 Background 301 13.1.2 Development 304 13.2 Index Framework 306 13.2.1 Parameters 306 13.2.2 Calculating a Phosphorus Index Value 322 13.3 Inclusion of Best Management Practices Factors 323 13.3.1 Examples of Index Site Assessment and Interpretation 323 13.4 Integration of P Indices into Existing Models or Nutrient Management Planning Software 325 13.5 Field Testing 326 13.6 Availability of P Indices 327 13.7 Conclusions 327 References 328 13.1 HISTORY OF DEVELOPMENT 13.1.1 BACKGROUND In response to mounting water-quality concerns, many states have developed guidelines for land application of phosphorus (P) and watershed management based on the potential for P loss in agricultural runoff (U.S. Department of Agriculture and U.S. Environmental Protection Agency 1999). These actions have been spurred, in part, by a federal initiative in which the U.S. Department of Agriculture (USDA) and U.S. Environmental Protection Agency (USEPA) created a joint strategy to implement Comprehensive Nutrient Management Plans (CNMPs) on animal feeding operations with a national deadline of 2008. © 2007 by Taylor & Francis Group, LLC 302 Modeling Phosphorus in the Environment Under the USDA–USEPA joint strategy, USDA’s Natural Resources Conser- vation Service (NRCS) is charged with implementing a new nutrient management policy. As a result, the NRCS planning standard addressing nutrient management (590 standard), which was based on nitrogen (N), has been rewritten to include a P-based planning standard. In each state, NRCS state conservationists must decide which of three P-based approaches will be used in nutrient management planning policy. These approaches are agronomic soil test P (STP) recommendations, envi- ronmental STP thresholds, or a P Index to rank fields according to their vulnera- bility to potential P loss. States have already selected the P-based strategies for their 590 standard. The P Index has been selected for most states’ 590 standards (Figure 13.1). Reasons for widespread adoption of the P Index approach are that other P management options (i.e., agronomic and environmental STP) are inflexible, overly restrictive, and do not account for the critical role of transport mechanisms in determining a site’s P loss potential. Generally, most P exported from agricultural watersheds derives from only a small part of the landscape during a few relatively large storms, where hydrologically active areas of a watershed contributing surface runoff to stream flow are coincident with areas of high soil P or recent manure applications. Even in regions where subsurface flow pathways dominate P trans- port (e.g., some areas of the coastal plains), areas contributing P to drainage waters are localized to soils with high soil P saturation and hydrologic connectivity to the surface drainage network. FIGURE 13.1 Summary of P management strategies adopted by state USDA-NRCS agen- cies for revision of the 590 nutrient management practice standard. (From Sharpley, A.N. et al. 2003. Journal of Soil and Water Conservation 58: 137–152. With permission) Alaska and Hawaii adopted the P Index P Index Soil test crop response P Index and/or environmental P threshold or soil test crop response © 2007 by Taylor & Francis Group, LLC Phosphorus Indices 303 To be effective, risk assessment must consider critical source areas within a watershed that are most vulnerable to P loss in surface runoff. Critical source areas are dependent on the coincidence of transport (i.e., surface runoff, erosion, and subsurface flow) and source factors (i.e., soil, fertilizer, manure) as influenced by site management (Table 13.1). Transport factors mobilize P sources, creating path- ways of P loss from a field or watershed. Source and site management factors are typically well defined and reflect land-use patterns related to soil P status, mineral fertilizer and manure P inputs, and tillage. TABLE 13.1 Factors Influencing P Loss from Agricultural Watersheds and Their Impact on Surface Water Quality Factors Description Transport Erosion Total P loss strongly related to erosion. Surface runoff Can carry soluble P released from soil or other P sources. Subsurface flow In sandy, organic, or P-saturated soils, P can leach through the soil. Phosphorus can also move through the soil by preferential flow through macropores. The presence of artificial drainage can capture this subsurface flow and move it directly to surface water. Soil texture Influences relative amounts of surface and subsurface flow occurring. Irrigation runoff Improper irrigation management can induce surface runoff and erosion of P. Connectivity to stream The closer the field to the stream, the greater the chance of P reaching it. Channel effects Eroded material and associated P can be deposited or resuspended with a change in stream flow. Dissolved P can be sorbed or desorbed by stream channel sediments and bank material and be taken up by or mineralized from biota. Proximity of P-sensitive water Some watersheds are closer to P-sensitive waters than others (i.e., point of impact). Sensitivity P input Shallow lakes with large surface area tend to be more vulnerable to eutrophication. Source Management Soil P As soil P increases, P loss in surface and subsurface flow increases. Applied P The more total or soluble P (mineral fertilizer or manure), the greater the risk of P loss. Site Management Application method P loss increases in the order: subsurface injection; plowed under; and surface broadcast with no incorporation. Application timing The sooner it rains after P is applied, the greater the risk for P loss. Source: Adapted from A.N. Sharpley, J.L. Weld, D.B. Beegle, P.J.A. Kleinman, W.J. Gburek, P.A. Moore, and G. Mullins, Journal of Soil and Water Conservation 58, 137–152, 2003. With permission. © 2007 by Taylor & Francis Group, LLC 304 Modeling Phosphorus in the Environment 13.1.2 DEVELOPMENT The P Index was originally developed to identify the vulnerability of agricultural fields to P loss (Lemunyon and Gilbert 1993). The original Index accounted for and ranked transport and source factors controlling P loss in surface runoff from a given site. Each site factor affecting P loss was weighted, assuming that certain factors have a different effect on P loss than others. A P Index value, reflecting site vulner- ability to P loss, was determined by selecting the rating value for each site factor, multiplying that value by the appropriate weighting coefficient, and summing the weighted products of all factors. Since its inception, three major changes have been introduced to many revised versions of the P Index. First, source and transport factors are related in a multipli- cative rather than additive fashion to better represent actual site vulnerability to P loss (Gbuerk et al. 2000). For example, if surface runoff does not occur at a particular site, its vulnerability should be low regardless of the soil P content. In the original P Index, a site’s risk could be ranked as very high based on site-management factors alone, even though no surface runoff or erosion occurred (Lemunyon and Gilbert 1993). On the other hand, a site with a high potential for surface runoff, erosion, or subsurface flow but with low soil P has a low risk for P loss, unless P as mineral fertilizer or manure is applied. Second, an additional transport factor reflecting distance from the stream has been incorporated into the P Index. The contributing distance categories in revised P indices are based on hydrologic analysis that considers the probability, or risk, of occurrence of a rainfall event of a given magnitude resulting in sufficient runoff to potentially transport P offsite. The third major change in Index formulation has been the use of continuous, open-ended parameter scaling for erosion, STP, and P application rate, as either fertilizer or manure. This enables indices to better address the effect of the very high erosion and STP values of the original P Index on P loss potential and to avoid having to subjectively quantify these categories. Finally, the open-ended scaling of erosion, STP, and P rate avoided the unrealistic situation where a one- or two-unit increase in any of these parameters could change risk category and dramatically alter P Index rating and its interpretation. Though the P Index concept has been broadly adopted, its development from a concept into a field-assessment tool has followed several different trends throughout the United States. The variations reflect not only regional differences in P transport but also philosophical differences as to how P risk from a site should be assessed using a P Indexing approach. Current research and field evaluations play a significant role in the continued modification of P indices to best fit and address regional and state conditions. This has resulted in many states incorporating unique factors that extend beyond the scope of the three previously described P Index modifications adopted by many revised P indices. As mentioned already, computation of final P Index values can be additive, as originally proposed by Lemunyon and Gilbert (1993), or multiplicative, as proposed by Gburek et al. (2000) (Table 13.2 and Table 13.3). Seventeen of the reviewed indices use the multiplicative approach, whereas 20 use the additive approach. © 2007 by Taylor & Francis Group, LLC Phosphorus Indices 305 TABLE 13.2 The P Index Approach Using Pennsylvania’s Index Version 1 as an Example PART A: SCREENING TOOL Evaluation Category Soil Test P (Mehlich-3) > 200 mg P kg −1 If yes to either factor then proceed to Part B Contributing Distance < 150 ft PART B: SOURCE FACTORS Soil test (Mehlich-3P) Soil Test P (mg P kg −1 ) Soil Test P Rating = 0.20 a Soil Test P (mg P kg −1 ) Fertilizer P rate Fertilizer P (lb P 2 O 5 /acre) Manure P rate Manure P (lb P 2 O 5 /acre) P source application method 0.2 Placed or injected 2" or more deep 0.4 Incorporated <1 week 0.6 Incorporated > 1 week or not incorporated April to October 0.8 Incorporated >1 week or not incorporated November to March 1.0 Surface applied to frozen or snow covered soil Fertilizer Rating = Rate × Method Manure P availability 0.5 Treated manure/Biosolids 0.8 Dairy 1.0 Swine Manure Rating = Rate × Method × Availability Source Factor = Soil Test P Rating + Fertilizer Rating + Manure Rating PART C: TRANSPORT FACTORS Erosion Soil Loss (ton/acre/yr) Runoff potential 0 Very low 2 Low 4 Medium 6 High 8 Very high Subsurface drainage 0 None 1 Some 2 a Patterned Contributing distance 0 > 500 ft 2 500 to 350 ft 4 350 to 250 ft 6 150 to 250 ft 8 < 150 ft Transport Sum = Erosion + Runoff Potential + Sub-Surface Drainage + Contributing Distance Modified connectivity 0.7 Riparian buffer: applies to distance < 150 ft 1.0 Grassed waterway or none 1.1 Direct connection: applies to distance > 150 ft Transport Factor = Modified Connectivity × (Transport Sum/22) Phosphorus Index Value = 2 × Source Factor × Transport Factor a Or rapid permeability soil near a stream. Source: Adapted from J.L. Weld, D. Beegle, W.J. Gburek, P.J. Kleinman, and A.N. Sharpley. 2003. The Pennsylvania Phosphorus Index: Version 1, Extension PUBLICATIONS.CAT US 180 5m3/03ps 4591. With permission. © 2007 by Taylor & Francis Group, LLC 306 Modeling Phosphorus in the Environment In addition to using the multiplicative approach, the Pennsylvania P Index varies the transport factor from 0 to 1 (Table 13.2). The original P Index outlined an approach to identify sites with a high vulner- ability to P loss based on evaluation of a variety of P source and transport factors. Most states (44 of 47) (Table 13.4) have maintained this original approach and have examined site vulnerability to P loss. However, in response to questions regarding the relationship between P Index values and actual P loadings, several states have taken a different approach. This approach adopted by Arkansas, Georgia, and Iowa uses source, transport, and management factors common to most other indices, but instead of estimating a vulnerability to or potential for P loss, calculates an estimated P loss that is used either directly or converted to a relative PI Index value (Table 13.4). 13.2 INDEX FRAMEWORK 13.2.1 P ARAMETERS P indices include assessments of both source management and transport factors to facilitate the assessment and identification of critical source areas. These factors have been chosen because they determine P loss in most cases (Table 13.1). TABLE 13.3 General Interpretations and Management Guidance Using Pennsylvania’s P Index P Index Value Rating General Interpretation Management Guidance <59 Low If current farming practices are maintained, there is a low risk of adverse impacts on surface waters. N-based applications 60 –79 Medium Chance for adverse impacts on surface waters exists, and some remediation should be taken to minimize P loss. N-based applications 80 –100 High Adverse impact on surface waters. Conservation measures and P management plan are needed to minimize P loss. P application limited to crop removal of P > 100 Very high Adverse impact on surface waters. All necessary conservation measures and P management plan must be implemented to minimize P loss. No P applied Source: Adapted from J.L. Weld, D. Beegle, W.J. Gburek, P.J. Kleinman, and A.N. Sharpley. 2003. The Pennsylvania Phosphorus Index: Version 1, Extension PUBLICATIONS.CAT US 180 5m3/03ps 4591. With permission. © 2007 by Taylor & Francis Group, LLC Phosphorus Indices 307 TABLE 13.4 P Index Approaches and Modifications State Alabama Alaska Arizona Arkansas Colorado Delaware Florida Reference NRCS (2001a) NRCS (2001b) Walther et al. (2000) DeLaune et al. (2004a, 2004b) Sharkoff et al. (2000) Sims and Leytem (2002) NRCS (2000a) Source Factors Soil P test Mehlich-1 P Mississippi extract Mehlich-3 P Olsen-P (High pH) Bray P-1 (Low pH) Mehlich-3 P Olsen-P (High pH) Bray P-1 (Low pH) Mehlich-3 P FIV based on Mehlich-3 P FIV based on Mehlich-1 P Application rate lb P 2 O 5 /ac/year lb P 2 O 5 /ac/year lb P 2 O 5 /ac/year lb soluble P/ac/year lb P 2 O 5 /ac/year lb P 2 O 5 /ac/year lb P 2 O 5 /ac/year Waste water volume Application method Injection Incorporation Sprinkler application Surface applied Injection Incorporation Sprinkler application Surface applied Injection Incorporation Surface applied Incorporation Surface applied Injection Incorporation Surface applied Injection Incorporation Surface applied Irrigation Incorporation Surface applied Application timing Days to incorporation Season applied Cover at application Time to planting Season applied Season applied Time to incorporation Season applied Time to incorporation Time to incorporation Management Animal access to surface waters — Grazing and feeding management Organic P source availability Grazing intensity Polyacrylamides Cover crops Organic P source availability Organic P source availability (continued) © 2007 by Taylor & Francis Group, LLC 308 Modeling Phosphorus in the Environment TABLE 13.4 (CONTINUED) P Index Approaches and Modifications State Alabama Alaska Arizona Arkansas Colorado Delaware Florida Reference NRCS (2001a) NRCS (2001b) Walther et al. (2000) DeLaune et al. (2004a, 2004b) Sharkoff et al. (2000) Sims and Leytem (2002) NRCS (2000a) Transport Factors Erosion Water (RUSLE) Gully erosion Water (RUSLE/USLE) Wind (WEQ) Irrigation Water (RUSLE) Wind (WEQ) Irrigation (QS value) Water (RUSLE) — Water (RUSLE) Water (RUSLE) Irrigation Surface runoff class Hydrologic soil group Field slope Hydrologic soil group Field slope Average precipitation Soil permeability class Field slope Curve number Field slope Precipitation Soil permeability class Field slope Soil permeability class Field slope Hydrologic soil group Field slope Artificial drainage Subsurface drainage/ flooding Underground outlet systems ——Flooding frequency — Drainage Water table depth Leaching rating Leaching potential Soil properties Watershed Contributing distance Distance to water Existence of a discharge —— —Distance from field to water Existence of a discharge Connectivity Filter strip width Presence of a buffer Buffer width Filter strips Contour buffer strips Vegetated buffer width Wetlands Buffer strip Detention/treatment area © 2007 by Taylor & Francis Group, LLC Phosphorus Indices 309 Receiving water priority Distance to critical habitat water Value of water body — — — State watershed categories Value of water body Index Value Determination Additive Risk assessment Additive Risk assessment Additive Risk assessment Multiplicative Loss assessment Additive Risk assessment Multiplicative Risk assessment Multiplicative Risk assessment State Georgia Illinois Iowa Kansas Kentucky Louisiana Reference Cabrera et al. (2002) NRCS (2002a) NRCS (2004a) Davis et al. (2004b) NRCS (2001c) NRCS (2000b) Source Factors Soil P test Mehlich-1 P Bray P-1 Mehlich-3 Bray P-1 Mehlich-3 P Olsen P Bray P-1 Mehlich-3 P Olsen P Mehlich-3 P Strong Bray P Application rate lb P 2 O 5 /ac/year Percent of annual recommended rate lb P 2 O 5 /ac/year lb P 2 O 5 /ac/year — lb P 2 O 5 /ac/year Application method Injection Incorporation Sprinkler application Surface applied Injection Incorporation Surface applied Injection Incorporation Surface applied Injection Incorporation Surface applied Injection Incorporation Surface applied Injection Incorporation Surface applied Application timing Season applied Time to incorporation Incorporation before or after a runoff event Season applied Time to incorporation Time to planting Time to incorporation Season applied Cover at application Season applied Time to incorporation Management Solubility factor for P sources — Soil conservation practices Tillage ——Varied weighting factors for organic P sources (continued) © 2007 by Taylor & Francis Group, LLC [...]... lands These differences do not reflect a deficiency in the Arkansas P Index, but instead the result of a different development objective In spite of these studies, watershed-scale evaluations of the indices are urgently needed Such testing and validation should focus on determining the effect of P-based nutrient management using the P index on P export at a farm or watershed scale 13. 6 AVAILABILITY OF P INDICES... receiving water are accounted for in several indices (Table 13. 4) Ranking of receiving waters was designed to emphasize water bodies designated as © 2007 by Taylor & Francis Group, LLC 322 Modeling Phosphorus in the Environment high quality Often, however, assessments of water body quality and priority have not been designated by the state, making inclusion in a P Index difficult 13. 2.2 CALCULATING A PHOSPHORUS. .. 326 Modeling Phosphorus in the Environment AFOPro includes a P Index component for Florida, Kansas, South Carolina, and Tennessee The factors used and the recommendations given are based on statespecific guidelines and indices (Kloot et al 2002) Manure Management Planner includes several P indices based on state-specific guidelines and recommendations (Hess and Joern 2005) Additional information regarding... the P Index rating for the same field would be 52 Clearly, the P Index can identify several opportunities to decrease the overall P Index rating of a field, which in the scenarios given did not include a reduction in the amount of manure applied This provides the farmer with long-term flexibility of nutrient management and may slow down STP build-up as less manure P is applied 13. 4 INTEGRATION OF P INDICES... of the 50 states enacting CNMP strategies shows that 47 adopted the P Index approach, one adopted an agronomic STP (crop response) approach, and two adopted an environmental STP threshold approach (Figure 13. 1) The specific factors included in these indices, how ratings are calculated, and whether the output is a risk- or lossbased estimate are given in Table 13. 4 Such widespread adoption of the P Index... PHOSPHORUS INDEX VALUE The modifications to basic calculations of site vulnerability have been incorporated into many recent versions of the P Index, as illustrated by the Pennsylvania P Index (Table 13. 2 and Table 13. 3) (Weld et al 2003) Included as an initial step in the Pennsylvania P Index is a screening tool (Table 13. 2, Part A); if a field has an STP greater than 200 mg kg −1 Mehlich-3 P and is... calculations, the associated nutrient management recommendations have also been modified by some states This results in some states having P Indices that are more restrictive than others 13. 3 INCLUSION OF BEST MANAGEMENT PRACTICES FACTORS The inclusion of BMPs and other management practices in the P Index represents a significant developmental difference across the reviewed indices Of the various P indices, 21 include... estimating soil loss, which is a transport factor in most P indices Most states educate nutrient management planners to address these issues as a part of the planning process However, software developers must address these issues as well The inherent flexibility of the P Index can result in the need for careful consideration when creating software that will incorporate a P Index process 13. 5 FIELD TESTING... (Table 13. 5) Neither of the methods of application in Scenario 1 involved manure incorporation; thus, no-till requirements are met, and erosion or runoff potential is not increased As a result of a change in timing of manure application, the P Index rating for the field decreased from 101 to 86, with applied P limited to crop removal (Table 13. 5) In Scenario 2, the establishment of a riparian buffer at the. .. edge of the field decreases the transport factor from 0.55 to 0.39 (Table 13. 5) Although source management remains the same, overall P loss potential as reflected in the P Index value decreased (72) compared to the baseline scenario (101), and nutrient management would revert to being N-based However, the riparian buffer must be carefully installed and maintained to ensure long-term protection against P . Calculating a Phosphorus Index Value 322 13. 3 Inclusion of Best Management Practices Factors 323 13. 3.1 Examples of Index Site Assessment and Interpretation 323 13. 4 Integration of P Indices into. avoid having to subjectively quantify these categories. Finally, the open-ended scaling of erosion, STP, and P rate avoided the unrealistic situation where a one- or two-unit increase in any of these. lb/ac/yr N leaching index Soil management group ——— (continued) © 2007 by Taylor & Francis Group, LLC 312 Modeling Phosphorus in the Environment TABLE 13. 4 (CONTINUED) P Index Approaches