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

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Phosphorus Index Ranking and Allowable Phosphorus RunoffUltimately, the importance of the P-Index and the phosphorus management it engenders isrelated to how much P is leaving the fields. Consequently, one of the most important aspects ofthe P-Index is how much P runoff it is designed to allow. Sharpley et al (2001) showed that therelationship between soil test P and runoff shows a break point at a soil test P of 200 mgP/l(mg/l = ppm) based upon a split line model (Figure 3.) Above 200 ppm P, runoffconcentrations increase at a higher rate as soil P increases relative to the rate below the 200ppm threshold. Sharpley et al (2001) noted that this break point is similar to the soil Pthreshold proposed for the Pennsylvania PI by Beegle (1999.) Sharpley et al (2001) estimatedthat on average, dissolved P represents the major portion (64%) of total P transported in surfacerunoff for soils that have not received manure within 3 weeks of measurements. When manurewas applied, however, dissolved and total P in runoff was much higher and no longer correlatedwith soil P. This can be seen in Figure 3 by looking at the symbols representing treatments withmanure applications. These data points all show much higher P runoff relative to that predictedfrom soil only. This is one of the major rationales for using a P-Index instead of the simple soiltest P threshold to manage P application.Figure 3. Relationship between the concentration of dissolved P in subsurface runoffPage 9
and Mehlich-3 extractable soil P concentration (Sharpley et al 2001)When measured farm field runoff was correlated with the P-Index, the relationship betweencombined soil and manure P runoff and P-Index was much improved over using the soil Pconcentration alone (Figs 3, 4 and 5). These results support the value of the P-Index as anappropriate estimator of runoff over using soil P levels alone. At the thresholds where the P-Index ranking increases from Medium to High, and from High to Very High, the concentrationof total P in runoff was approximately 1.0 and 1.6 mg P/l, respectively (Figure 4). The total Ploss in runoff at the threshold where the P-Index ranking increased from medium to high, andfrom high to very high, was approximately 200 and 350 g P/ha (Figure 5.) The P concentrationand loss values in Figures 4 and 5, respectively, are the levels of P allowed to be lost from thefield edge which are designed into the PA P-Index.Figure 4. Relationship between the concentration of dissolved and total P in surface runoff andthe P-Index rating for sites in fields where no P had been applied within the last six months andwhere fertilizer or manure had been applied within three weeks of rainfall. (Sharpley et al 2001)Page 10
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Page 3 and 4: The P-Index source factor uses a Me
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Page 7 and 8: Table 1. Conditions assumed in exam
Page 9: plotted (Figure 2). By lowering the
Page 13 and 14: overestimation is incorporated into
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A Review of the Pennsylvania Phosphorus Index ... - PennFuture

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