Soil Phosphorus Saturation Ratio Sets Comparable Manure Application Cutoffs for New York and Pennsylvania.

Phosphorus Index (PI) assessment tools are widely used across the United States to rank agricultural fields based on their relative risk of P runoff and to set application cutoffs for manure and fertilizer P. Although PIs differ from state to state, soil test phosphorus (STP) is commonly included. However, states vary in analytical procedures (agronomic soil tests) used to determine STP and in critical values for manure and P fertilizer management. Within the Chesapeake Bay Watershed, evaluations are on-going to explore the potential to link and consolidate PIs across physiographic regions (such as the Appalachian Plateau) instead of state boundaries. This requires an evaluation of STP-based manure application cutoffs currently implemented in NY where the Morgan soil test (40 mg P kg^-1 manure application cutoff) is used and PA where the Mehlich-3 test (200 mg kg^-1 screening tool for PI) is used. Here we evaluate Mehlich-3 derived soil P saturation ratios ([P_M3]/[Al_M3], [P_M3]/([Al_M3]+[Fe_M3]), and [P_M3]/[Ca_M3]), as a means to standardize across the NY/PA state boundary. Samples were collected from 83 NY agricultural soils, including 75 mineral and 8 muck soils. Averaged across all mineral soils, 40 mg STP_Morgan kg^-1 equated to 19 % soil P saturation using [P_M3]/([Al_M3]+[Fe_M3]), while 200 mg STP_M3 kg^-1 equated to a soil P saturation of 22 %. A standardized 20 % soil P saturation based limitation across states equated to a critical value of 48 mg STP_Morgan kg^-1 and 187 mg STP_M3 kg^-1 for mineral soils, and 31 mg STP_Morgan kg^-1 and 117 mg STP_M3 kg^-1 for muck soils. We conclude that for mineral soils, the soil P saturation approach can be used to develop comparable critical STP values and we propose the use of [P_M3]/([Al_M3]+[Fe_M3]) to set Morgan and Melich-3 STP thresholds for a physiographic PI for the Appalachian Plateau. Additional analyses of muck soils are necessary to confirm the differences in STP equivalent cutoffs between muck and mineral soils.