A Review of the Pennsylvania Phosphorus Index ... - PennFuture

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A Review of the Pennsylvania Phosphorus Index: Version 2IntroductionFor many years the dogma in soil science was that it didn’t matter how much phosphorus (P)was put on soils because all in excess of crop needs would adhere to the soil. The only Ptransport mechanism of concern was erosion of soil. However, in the latter half of the 1990’s, itwas realized that at high soil concentrations, P could remobilize off the soil and move in runoffwater. The runoff of P from some crop lands to waterways became a concern as levels of soil Pbecame elevated due to high levels of manure application over extended periods especially fromconcentrated animal operations which have large amounts of manure to manage. The runoff ofP from agricultural lands contributes to other sources of P, resulting in eutrophication especiallyin lentic waters (i.e. non-running waters.) In response, the USDA added a P managementrequirement to its 590 Nutrient Management Standard, and states across the country followedup with plans to implement this standard.The Pennsylvania (PA) P-Index is divided into two parts, A and B (Beegle et al 2009.) Part A is asimple screening tool to determine whether nitrogen (N) based nutrient management plans aresufficient or if a farmer needs to complete Part B which calculates a P-Index to determinewhether nutrient management plans need to take into consideration the potential for P runoff.The Part A screening is based upon 4 criteria, an affirmative answer to any one of whichdetermines the need to complete Part B and, if appropriate, a P based nutrient managementplan. The criteria are as follows:1. Is the crop management unit in a Special Protection Watershed?2. Is there a significant farm management change as defined by Act 38?3. Is the soil test Mehlich-3 P greater than 200 ppm?4. Is the Contributing Distance from the crop management unit to receiving waters lessthan 150 ft.?In addition, the USDA’s Natural Resources Conservation Service requires P-based evaluationsfor farms receiving technical or financial assistance for manure-related issues (Kogelman et al2006.)Part B of the P-Index is divided into 2 factors: source and transport. The source factor accountsfor P availability from the soil, fertilizer and manure. The transport factor considers erosion,runoff potential, subsurface drainage, distance to waterways (known as contributing distance),and any management practices that modify P transport (known as modified connectivity.)From the source and transport factors is derived the P-Index. The P-Index is divided into 4rating categories: Low, Medium, High and Very High. Phosphorus Index ratings of Low orMedium do not require nutrient management plans to consider potential P runoff and nutrientmanagement plans can be nitrogen based. A High P-Index rating requires that P can only beapplied at crop removal rates. A Very High P-Index rating means no P can be applied.Page 1
The P-Index source factor uses a Mehlich-3 soil test to estimate the amount of P at risk forrunoff from the soil. Phosphorus availability from fertilizer depends on the amount of fertilizerand the manner in which it is applied, whether applied in spring or fall and whether and when itis incorporated into the soil. The P available from manure, like fertilizer, is dependent on whenapplied and whether incorporated but also includes a P source coefficient (PSC.) The PSC canbe taken from a chart provided by the state (e.g., in the P-Index Fact Sheet), known as the Bookvalue. Alternatively, the PSC can be determined by submitting the manure sample for a waterextractableP (WEP) analysis. The WEP test option was added to Version 2 of the PA P-Index.The Part A screening test was also modified in Version 2 with the addition of two conditions; theSpecial Protection Waters, and the significant change in farm management. The conversion toVersion 2 was described as a natural evolution of the P-Index, recommended by a group ofscientists at the USDA-ARS Pasture Systems and Watershed Management Research Unit atUniversity Park, PA and the Pennsylvania State University College of Agricultural Sciences(Beegle. pers. comm., Beegle et al 2009.) This group conducts an ongoing review of the P-Indexcalculation. While the use of a WEP analysis-based PSC likely lowers the P-Index, the additionalPart A conditions likely result in more farms calculating a P-Index. Dr. Beegle mentioned thatPA was considering a third version of the P-Index which might come out this year. A preview ofpotential recommended adjustments in the P-Index might be presaged in the SERA-17recommendations on the revisions to the 590 Nutrient Management Standard (Sharpley et al2011.)Assessments of the Affect of the Phosphorus Index on Pennsylvania AgricultureDuring the development of the PA P-Index, efforts were made to assess the impact onagriculture. In a statewide survey of over 220,000 soil samples collected in PA between 1996and 2001, Kogelmann et al (2002) found 4.8% exceeded 200 ppm P and 1.5% exceeded 300ppm P. Several counties located in the southeastern part of the state had a higher than averagepercentage of excessively high soil P samples. Lancaster, Schuylkill and Montgomery Countieshad the highest proportion above 200 ppm P (17.6%, 16.7% and 16.1%, respectively.) Additionalhotspots were noted in Wyoming, Wayne and Lackawanna Counties in the northeast and ErieCounty in the northwest (Kogelmann et al 2004.) Also of note, statewide, 48% of the soilsamples had soil P test values of 50 ppm P or more. The optimum range of soil P for crops is 30– 50 ppm P. Lancaster County had 81% of soil samples exceeding crop P needs. Seventeen outof the 67 counties statewide had greater than 60% of soil samples exceeded crop P needs. Thissuggests the possible widespread over fertilization of P. Not surprisingly, a positive relationshipwas found between animal density and high soil P concentrations.Besides high soil P concentrations and animal densities, Kogelmann et al (2002) also found thata greater percentage of the agricultural lands in the north and south-central parts of the statewere in close proximity to streams (within 150 feet), elevating the risk of P loss to streams inthose areas. Modeling of runoff in hilly areas indicated that nearly all the P runoff wasgenerated from about 10% of the watershed areas, within about 98 to 197 feet of the channel(Kogelmann et al 2004). These areas, known as variable source areas (VSA’s), are likely thePage 2
Page 1: A Review of thePennsylvaniaPhosphor
Page 5 and 6: 1:100 extraction ratio (i.e. 2.5 g
Page 7 and 8: Table 1. Conditions assumed in exam
Page 9 and 10: plotted (Figure 2). By lowering the
Page 11 and 12: and Mehlich-3 extractable soil P co
Page 13 and 14: overestimation is incorporated into
Page 15 and 16: 4. Require BMPs to reduce P loss fr

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