Soil Phosphorus Saturation Ratio Sets Comparable Manure Application Cutoffs Across States Differing in Agronomic Soil Test

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.