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a Dep. of Natural Resource Sci. and Landscape Architecture, Univ. of Maryland, College Park, MD 20742
b Dep. of Plant and Soil Sci., Univ. of Delaware, Newark, DE 19717
c USDA-ARS, Kimberly, ID 83341
* Corresponding author (fc26{at}umail.umd.edu)
Received for publication July 27, 2001.
ABSTRACT
In 1998, the Maryland legislature mandated nitrogen (N) and phosphorus (P) nutrient management planning for nearly all of Maryland's commercial agricultural operations. State regulations required that a phosphorus indexing tool (P Index) be used for determining the potential for P losses from agricultural land, even though a reliable P Index did not exist. The development and assessment of the P Index as a dependable tool for the evaluation of the potential for P losses was constrained by a very aggressive implementation schedule imposed by state regulations. The Maryland Phosphorus Site Index (PSI) was evaluated on 646 state-representative field sites beginning in the spring of 1999 and continuing through the spring of 2000. Of the representative fields, 69% were determined to have a "low" P loss rating, 19% were in the "medium" P loss rating category, 8% were determined to be a "high" risk for P loss, and 4% rated as "very high" P loss potential. Fifty-five percent of the fields evaluated had soil test phosphorus (STP) levels less than the 75 mg kg-1 Mehlich-1 P environmental threshold established by state regulations. The frequency distribution of PSI performance was evaluated for several subcategories of the statewide data set. The Maryland PSI will be deployed for use in constructing farm nutrient management plans well before its predictive capabilities can be objectively and rigorously validated. Field validation is essential. In the meantime, the Maryland PSI should function adequately as a tool to assist in the prioritization of field P loss risk potential.
Abbreviations: PSI, Phosphorus Site Index • STP, soil test phosphorus
IN THE SPRING of 1998, the Maryland General Assembly enacted into law the Water Quality Improvement Act of 1998, which mandated N and P nutrient management planning for nearly all of Maryland's commercial agricultural operations. Included under the provisions of the law were all agricultural operations with either at least $2500 annual gross revenue or eight animal units (1 animal unit = 454 kg live weight). During the crafting of the state regulations that implemented the Water Quality Improvement Act of 1998, a phosphorus indexing tool (P Index) was identified as the primary method to be used for determining the potential for P losses from agricultural land to waters of the state (Code of Maryland, 2000). At that time, a reliable P Index for use in assessing the potential for P losses from Maryland's diverse agricultural lands did not exist.
In 1993, the P Index was introduced as a screening tool for use by soil conservation field staff and watershed planners to rank the vulnerability of agricultural fields as sources of P loss in surface runoff water (Lemunyon and Gilbert, 1993; USDA Soil Conservation Service, 1994). This original P Index accounted for P transport through surface runoff and erosion processes. It also estimated the effects of various sources of P (soil P, fertilizer P, manure P) and the rate and method of application of these P sources on the potential for P losses with surface runoff water. The goal of the original P Index was to use readily accessible information to identify sites where the risk of P losses to surface water was expected to be higher than that from other sites. The authors of the original P Index considered modification and adaptation of the P Index to be imperative to accurately reflect local landscape characteristics and management practices (USDA Soil Conservation Service, 1994).
The original P Index of Lemunyon and Gilbert (1993) has been shown to adequately reflect P loss potential for several small (approximately 2 ha), uniform, well-defined watersheds (Sharpley, 1995). However, on larger, more diverse watersheds with multiple slopes, soil types, and farm management practices, the original P Index was found to inadequately represent P runoff (Gburek et al., 1996). The identification of multiple critical source areas within a given watershed has proven to be an essential component of an improved P Index for complex landscapes (Sharpley et al., 1998).
The original P Index addressed potential losses of P via surface runoff processes. On coastal plain soils, large inland flood plains, or other low-slope landscapes, P leaching to shallow ground water and/or subsurface lateral flow to constructed drainage ditches are important potential pathways for P loss from agricultural land (Sims et al., 1998). The original P Index had no capability to assess P losses by leaching to shallow ground water or subsurface drainage.
The overall objective of our studies was to develop a modified P Index and assess the functionality of the modified P Index as a dependable tool for evaluating the potential for P losses from Maryland's agricultural lands. Our objective was constrained by state regulations that necessitated that an improved P Index be developed that could be applied dependably on all Maryland farmland regardless of geographic, topographic, or hydrological setting. Objective development and testing was further constrained by the very aggressive implementation schedule imposed by state regulations that required state-approved, P Index–based, nutrient management plans be developed for practically all Maryland farmland within six years of the passage of the law. Therefore, our specific objective was to create a reliable, broadly applicable P Index that could be put into practice as expeditiously as possible.
MATERIALS AND METHODS
Development of the Maryland Phosphorus Site Index
The original P Index (Lemunyon and Gilbert, 1993; USDA Soil Conservation Service, 1994) was used as a starting template for the development of the Maryland P Site Index (PSI). Several workshops were convened for scientists, watershed managers, conservation planners, and Extension educators from the eastern United States to help mold a consensus interpretation of the available information and assess the appropriateness of site characteristics, scale factors, and P loss ratings. Ultimately, P loss assessment categorization was made by reference to the published scientific literature, unpublished research data, and consensus of scientists experienced in agricultural P management and watershed hydrology of our region. The final P loss ratings were divided into categories depicting the potential for P losses from the site and outlining management implications for each P loss rating category.
The structure of the resulting interim draft of the Maryland PSI was a two-part decision-making process that evaluated the independent effects of the quantity and forms of P present at the site (P source) and the potential for P transport from the field (P transport). The separation of P transport pathways and P source characteristics was encouraged by Sharpley et al. (1998) as a much more realistic means of assessing the potential for P movement from the field.
A multiphase evaluation of the interim Maryland PSI was conducted. In the first phase of evaluation, numerous contrived scenarios were tested to determine if the PSI accurately evaluated fictitious sites from which P losses were expected to be high as high-loss sites, and evaluated fictitious sites from which P losses were expected to be low as low-loss sites. Scenario testing attempted to cover the entire spectrum of real-world possibilities for each site characteristic.
The second phase of testing evaluated on-farm sites that had been identified by experienced nutrient management planners as probable "high P loss" and "low P loss" sites. Sites where it was agreed that the P loss potential was moderate also were included. The interim draft of the Maryland PSI was field-tested on 155 production farm fields across 12 Maryland counties. These field evaluations identified several aspects of the draft PSI that needed additional refinement as well as several site criteria that seemed to be accurately reflected in the overall P loss potential.
The third phase of testing attempted to more rigorously reflect the performance of the PSI across Maryland. Through excellent cooperation of county-based Maryland Cooperative Extension and Soil Conservation District personnel, between 25 and 30 farm fields were identified in each of Maryland's 23 counties that, when taken in aggregate for a given county, reflected the diversity and proportion of soil types, topography, farming systems, and nutrient management practices prevalent in that county. Cooperating farmers were generally eager to help find a reasonable solution to the nutrient management challenges they faced. The PSI evaluation data were collected from 646 field sites beginning in the spring of 1999 and continuing through the spring of 2000. Evaluation of this data set ultimately led to the development of the current Maryland PSI (Tables 1 and 2).
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Five site characteristics were assessed to account for the magnitude and qualities of the P source present at a given site and the influence of farm management on the potential for P loss from the field (Table 1, Part B): conventional soil test phosphorus (STP) level; fertilizer P application rate; fertilizer P placement and timing; organic-source P application rate, which included an organic source phosphorus availability coefficient (PAC) reflecting P solubility; and organic source P placement and timing. Each of the 12 site characteristics or management factors was evaluated for a specific location and assigned a numeric value. The sum of the site characteristics determined from Part A was multiplied by a scaling factor of 0.02, so that P transport potential was expressed on a relative scale with a range of 0 to 1, for most situations. Thus, the total site and transport value determined from Part A can be interpreted as the proportion of the P present at the site that was susceptible to being transported off the field by drainage water and potentially imposing negative effects on adjacent surface waters.
The sum of the management practice and source characteristic values determined from Part B was multiplied by the total site and transport value determined from Part A. The product was the final "P loss rating" for the site. This multiplicative operation assured that fields that had the highest P loss rating had both a high P transport potential (i.e., large Part A value) and a large source of potentially damaging P (i.e., large Part B value). If either the P transport potential (Part A) or the P source (Part B) was low, then the final P loss rating was relatively low. The ultimate P loss rating was subdivided into four interpretive categories: "low," "medium," "high," and "very high" (Table 2). These categories were used to interpret the management implications of the P loss rating determined for a specific site. The P loss rating is only a relative value and is not a numeric or quantitative prediction of P loss from the field.
RESULTS AND DISCUSSION
It is critical to understand that assessment of the Maryland PSI's capability to predict the potential for P loss from farm fields was based on scientifically based, research-supported professional judgement. Rigorous, objective validation of the predictive capabilities of the PSI across diverse landscapes is essential, but was well beyond the time limitations imposed by state regulation. Accelerated deployment of the PSI was mandated and we were charged to develop the best nutrient management tool possible, as expeditiously as possible.
The performance of the Maryland PSI is summarized as a frequency distribution of results from the statewide assessment of 646 farm fields (Fig. 1) . Of the representative fields, 69% were determined to have a "low" P loss rating, 19% were in the "medium" P loss rating category, 8% were determined to be a "high" risk for P loss, and 4% rated as "very high" P loss potential (Fig. 1). These PSI distribution subdivisions may serve to adequately focus P-loss reduction efforts to high-priority sites. Overall, P loss was not considered a management concern for 69% of the fields and a moratorium on P application would have been recommended on 4% of the fields evaluated (Fig. 1).
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Statewide frequency distribution for PSI performance for those sites with conventional STP below and above the environmental threshold (75 mg kg-1 Mehlich-1 P) are presented in Fig. 2a and 2b , respectively. For the lower STP fields, 80% were determined to have a "low" P loss rating, 13% "medium," 4% "high," and 3% remained in the "very high" P loss rating category (Fig. 2a). For fields with conventional STP in excess of the environmental threshold, 55% exhibited a "low" P-loss potential, 25% were in the "medium" P loss rating category, 15% were categorized as "high," and 6% were designated as "very high" risk for P loss from the field (Fig. 2b).
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When evaluating the statewide data set according to past nutrient application history (poultry manure, dairy or beef manure, and commercial fertilizer P), it was essential to recognize that nutrient application history and physiographic region may be inextricably confounded. The overwhelming majority of farm fields evaluated that had a history of poultry manure application were located in the Coastal Plain region. Most of the farm fields evaluated that had a history of dairy or beef cattle manure application were located in the Piedmont, Appalachian Plateau, or Mountain physiographic regions. There were numerous exceptions in our data set to both of these generalizations. The evaluated fields that had received only commercial fertilizer P in the past were relatively uniformly distributed across the different physiographic regions of the state.
The PSI frequency distribution data for fields that had a history of poultry manure application and STP greater than 75 mg kg-1 Mehlich-1 P exhibited a greater potential risk for P loss than poultry manure–amended fields with lower STP levels (Table 3). However, 40% of the poultry manure–amended fields evaluated that had STP greater than the environmental threshold were determined to have "low" P loss potential (Table 3). In these cases, although a large source of P was present (large Part B value), the P transport potential was relatively low.
The regulation-established STP threshold appeared to be inadequate for distinguishing between the P loss potential from farm fields that had a history of dairy or beef cattle manure application (Table 3). Nearly the same proportion of dairy or beef manure–amended fields showed elevated potential for P loss regardless of whether the STP was less than or greater than 75 mg kg-1 Mehlich-1 P (Table 3), because P transport potential (Part A) dominated the overall P loss rating in the upland regions, where cattle operations are more common.
Maryland farm fields that had received only commercial fertilizer P in past years demonstrated a low potential for P loss (Table 3). Certainly, those fields that had received only commercial fertilizer and had STP less than the environmental threshold had low risk for P loss (Table 3). Even when STP was greater than the environmental threshold, only 20% of the fertilizer-only fields evaluated showed elevated risk for P loss (Table 3). It appears that farm fields that do not have a history of manure application are not at high risk for P loss and should be low priority for PSI evaluation.
CONCLUSIONS
The Maryland PSI was developed from an extensive combination of established scientific principals, published data, and professional judgements to be a broadly applicable and relatively easily used agricultural nutrient management tool. Unfortunately, the Maryland PSI will be deployed for use in constructing farm nutrient management plans well before its predictive capabilities can be objectively and rigorously validated. Field validation is essential. In the meantime, the Maryland PSI should function adequately as a tool to assist in prioritizing P loss risk potential.
REFERENCES
This article has been cited by other articles:
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  A. L. Shober and J. T. Sims Integrating Phosphorus Source and Soil Properties into Risk Assessments for Phosphorus Loss Soil Sci. Soc. Am. J., March 12, 2007; 71(2): 551 - 560. [Abstract] [Full Text] [PDF]
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