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

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asis for the 150 foot buffer condition in the screening portion of the P-Index. Kogelmann et al(2004) stated that the 150 foot buffer should encompass most runoff from VSAs. Perhaps thebenefits of extending the contributing distance to 200 feet should be examined as a modificationof the P-Index Part A screening test conditions.However, the above findings regarding the areas of elevated risk of P loss overestimates theacres likely to require P nutrient management. The initially calculated P-Index is not fixed andnutrient management planners can revisit the source and transport components of the P-Indexand determine what changes or additional practices may be needed to reduce the risk of P loss(Kogelmann el al 2006.) Kogelmann et al (2006) examined the affect of P-Indeximplementation in 2 counties of the state that were likely to be highly impacted, Lancaster andSnyder Counties. Based upon screening results, 45.9% and 19% of the farms fields examined inLancaster and Snyder Counties (respectively) were required to calculate their P-Indices. Ofthese, 31.2% and 12% of the fields in Lancaster and Snyder Counties had P-Index rankings thatrestricted P application rates based upon initial nutrient management plans. Kogelmann et al(2006) then focused on 4 farms they defined as highly impacted (i.e. more than one third of thefarm acreage with P restrictions) to examine what types of management measures could beimplemented to ease P restrictions. Two of the four highly impacted farms appeared to requireoff-farm manure transport to comply with P based nutrient management. If this ratio were toapply to all the farms that required P-Index calculation in Lancaster and Snyder Counties, thepercentages prohibited from applying P would drop to about 16.5% and 6%, respectively.Overall, based upon this example, more than 60% of the farms that were required to calculate aP-Index would have been able to adjust management practices or add BMPs to lessen or removeP restrictions. However, this percentage is still likely to overestimate the farms impacted as thefour farms examined were selected as worst case scenarios in counties with a higher thanaverage percentage of P problems. In addition, this example was based upon P-Indexcalculations using PSC Book values. If WEP-based PSC values were used, it is likely that thepercentage of farms with P restrictions would be even lower still.Phosphorus Source Coefficient: Lab AnalysisThere have been numerous publications regarding analytical tests to determine the availabilityof manure P and its contribution to P runoff (Kleinman et al 2007; Kleinman et al 2005; Elliott,H.A. 2006; Sharpley et al 2004; Studnicka et al 2011). It stands to reason that a specific test of amanure sample would be a better determinant of its available P relative to a generalized factor.The question then becomes how good are the analytical tests at accurately determining manureP availability? The PA P-Index uses a WEP analytical protocol described in Kleinman et al(2007.) This test essentially dissolves water soluble P out of manure samples by swirlingmeasured quantities of manure in certain volumes of water for set periods of time. The Pconcentration in the water extract is then analyzed to yield the water extractable P (or WEP.)Studnicka et al (2011) reviewed the Kleinmann et al (2007) extraction protocol and determinedthat pre-analysis manure handling could affect WEP results but reported that those effects wereminimized and extractions better predicted spring P runoff concentrations when the manure towater extraction ratio was 1:1000 (i.e. 0.25 g of manure in 250 ml of water). These contrast withPage 3
1:100 extraction ratio (i.e. 2.5 g of manure in 250 ml of water or an equivalent ratio) which is thecurrent standard procedure approved for use in calculating the PA P-Index (based uponKleinman et al. 2007). Studnicka et al (2011) found the 1:1000 water extraction ratio producedWEP estimates significantly higher than found with the 1:100 extraction ratio. Additionally,Studnicka et al (2007) found that WEP analysis of available manure P applied in the fall werenot well correlated with P runoff rates but were better predicted by the rate of available manureP applied. Further, they found that the runoff-to-rainfall ratio was the best predictor of runoff Pconcentrations across all seasons. In springtime, more rainfall infiltrates the soil relative to thefall and hence could help to explain why fall runoff P rates were higher than those during springapplication. However, as the runoff-to-rainfall ratios would not be known during the time ofmanure application, this relationship is not useable to determine allowable manure Papplication rates but rather would be valuable for modeling P runoff after the fact.Studnicka et al (2011) used a relatively small sample (0.25 g) relative to Kleinman et al (2007:0.5 – 100 g sample sizes evaluated). While Kleinmann et al (2007) found no relationshipbetween the weight of the sample and the relationship to dissolved P runoff, small sample sizescould be a problem with heterogeneous samples such as poultry litter (because of wood chips.)Phosphorus Source Coefficient: Influence on Allowable Manure PhosphorusTo determine the influence of the PSC value on allowable manure P application rates, anexample from the Pennsylvania Phosphorus Index: Version 2 Users Guide (Beegle et al 2009)was modified and the maximum allowable manure application rates were calculated at 4different PSC values (Figure 1). Phosphorus solubility coefficients used in the calculation rangedfrom 0.8, which is the book value for dairy manure, to 0.2, which resulted in manure applicationbased upon crop nitrogen needs (i.e. no P application limits.) The specific conditions in themodified example P-Index calculations are provided in Table 1. As the Mehlich-3 soil test Plevels were above 200 ppm (201 ppm in this example), the need to calculate a P-Index wastriggered 1 . Under the example conditions, the P-Index calculation for manure application basedupon crop nitrogen needs resulted in a Very High P-Index rating (129) which would not allowany phosphorus to be applied.The P-Index was recalculated to find the maximum allowable manure P application rates foreach of the PSC values. Based upon the PSC Book value for dairy manure (0.8), the maximumallowable P rate was equivalent to crop removal (50 lbs P2O5/A) with a P-Index ranking of High(P-Index =86). The maximum allowable P application rates for PSC values of 0.6, 0.4, and 0.2were 52, 79 and 117 lbs P2O5/A, respectively. The P-Index ratings for the three PSC valuesbelow the Book value were in the Medium category (79, 79, 72, respectively) allowing nutrientsto be applied at nitrogen crop requirements (or at least as high a P application rate as wouldmaintain a P-Index below 80.) Only at the PSC of 0.2 did the P application rate actually equalthe rate at the maximum allowable nitrogen application rate.1 Actually the example in Beegle et al 2009 shows a Mehlich-3 soil test concentration of 195 ppm whichwould not trigger the part B requirement although a P-Index was calculated for this farm field. For thepurpose of the exercise in this report, a soil test concentration of 201 ppm was assumed.Page 4
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A Review of the Pennsylvania Phosphorus Index ... - PennFuture

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