Validation of the Arkansas Phosphorus Index.

The Arkansas phosphorus (P) index was originally developed in 2001 specifically for pastures fertilized with poultry litter.  This was the first P index developed in the U.S. that actually predicted P loads in runoff rather than giving an arbitrary P risk value.  The Arkansas Natural Resource Commission requested that the index be revised in 2006 to include liquid manures and biosolids.  Changes in methodology for soil sampling and water extractable P (WEP) in manure also made this revision necessary.  Over 1,000 rainfall simulations were conducted during a three period to determine the effects of soil test P and the amount of WEP applied from manure and biosolids applications on P loads in runoff.  The slopes obtained from multiple regression analyses of these data were then used to determine the weighting factors for soil test P and WEP applications from poultry litter, liquid manure and biosolids in the P source component of the index.  The AR P Index Advisory Panel, which consisted of over 40 individuals from 14 agencies or companies in Arkansas, met several dozen times over a three year period to populate those portions of the index which were based on best professional judgment, including the P transport component and the amount of credit given for nine approved best management practices (BMPs).  The revised P index still predicts P runoff, but also provides a relative PI value. The objective of this research was to validate the revised index. Validation studies were conducted on field-scale paddocks located in Logan, Madison, Pope, and Washington Counties in NW Arkansas which were equipped with flumes and automatic water samplers.  Some fields were unfertilized, while others were fertilized with manure or biosolids. The revised P index predicted total P loads in runoff fairly well, as long as credit was given for the use of approved BMPs, such as fencing.