Direct and Indirect Assessment of Pennsylvania and Arkansas Phosphorus Indices.

The phosphorus (P) Indexing approach to determine the risk of P loss from a site has been used as part of the NRCS 590 Nutrient Management Standard for the last 10 years.  During this time, the P Indexing concept has been modified by individual states to consider differences in physiography, farming systems, and local policies and agendas.  While a great deal of research has been conducted to derive and support components of P Indices, particularly source factor components, less attention has been given to field assessment of whether individual P Indices appropriately rank site vulnerability to P loss.  Given the diversity of state P Indices and varied recommendations that they produce, there are calls for greater testing and validation under 590 Standard.  Here we demonstrate how field scale manure management based on P Index recommendations can be evaluated directly or indirectly at field and watershed scales.  In a central PA watershed, application of swine slurry at more than 5 times the agronomic rate to a field ranked by the PA P Index as “low risk” resulted in only a 0.1% increase in annual P loss in the stream, compared with 7.8% increase when slurry was applied to a “high risk” field.  Monitoring of runoff from fields in another PA watershed confirmed excellent representation of P Index source factors but room for improvement in representing transport (runoff) factors.  Although limited stream P concentrations are available following implementation of the watershed specific Eucha-Spavinaw P Index (ESPI) in AR, nutrient management planning information demonstrates the impact on P-sourced poultry litter applications.  Since 2004, annual litter applications to watershed pastures have decreased from an average 5.6 to 2.5 tonnes ha^-1 (i.e., 84 to 37.5 kg P ha^-1), providing indirect evidence of the benefits of using ESPI on P management in the watershed.