Technical ArticleSoil Phosphorus Saturation Ratio Sets Comparable Manure Application Cutoffs Across States Differing in Agronomic Soil TestCrittenden, Stephen J.; Ketterings, Quirine M.; Knight, Julia; Czymmek, Karl J. Author Information Department of Animal Science, Cornell University, Ithaca, NY. Address for correspondence: Quirine M. Ketterings, PhD, 323 Morrison Hall, Department of Animal Science, Cornell University, Ithaca, NY 14853. E-mail: [email protected] Financial Disclosures/Conflicts of Interest: None reported. Received October 23, 2016. Accepted for publication January 4, 2017. Soil Science: January 2017 - Volume 182 - Issue 1 - p 36-44 doi: 10.1097/SS.0000000000000192 Buy Metrics Abstract Regionalization of phosphorus indices (PI) across state boundaries requires evaluation of compatibility of agronomic soil test P levels used as screening tools and/or cutoffs for land application of manure. Within the Chesapeake Bay watershed, both Morgan and Mehlich-3 (M3) soil tests are used. In New York, 40 mg PMorgan kg−1 is the upper limit for land application of manure for high P transport risk scenarios. In Pennsylvania, 200 mg PM3 kg−1 determines whether the PI must be used (screening tool). We evaluate three M3-derived soil P saturation (Psat) ratios ([PM3]/[AlM3], [PM3]/([AlM3] + [FeM3]), and [PM3]/[CaM3]) as a means to standardize P-based limits across state boundaries. Eighty-three agricultural soils (75 mineral and eight muck soils ranging in pH from 5.6 to 7.7) were analyzed for PMorgan and PM3. Soil Psat based on [PM3]/([AlM3] + [FeM3]) correlated better with PMorgan (R2 = 0.80) and PM3 (R2 = 0.86) than [PM3]/[AlM3] (R2 = 0.78 for PMorgan; 0.82 for PM3) or [PM3]/[CaM3]) (R2 = 0.20 for PMorgan and 0.73 for PM3). Averaged across mineral soils, 40 mg PMorgan and 200 mg PM3 kg−1 equated to 19% and 22% Psat ([PM3]/([AlM3] + [FeM3]). Twenty percent Psat equated to 48 and 187 mg PMorgan and PM3 kg−1, respectively. Similarly, PMorgan and PM3 equivalents were set using Psat for muck soils but equivalents were very different from those for mineral soils. We conclude that [PM3]/([AlM3] + [FeM3]) can be used to derive PMorgan and PM3 equivalents for PI assessment within the Chesapeake Bay watershed, but different equations are needed for mineral and muck soils. Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved.