Building on the basic approach, the P Index developed in NYS helps planners to calculate a Source score and a Transport score on a field by field basis. Since P dissolved in runoff water ("Dissolved P") oand P bound to soil in erosion ("Particulate P") are lost from fields due to different transport processes and cause different impacts on watercourses, both a Dissolved P Transport score and a Particulate P Transport score are calculated. The Source score is multiplied by the Dissolved P Transport score to determine the Dissolved P Index score. Similarly, the Source score is multiplied by the Particulate P Index score to determine the Particulate P Index score. The greater of the two scores is used to rate the field into one of four risk categories: Low, Medium, High, and Very High. Explore the approach, below, by following through the chart from left to right.
As the P Index score increases, the guidelines for managing manure and fertilizer become more restrictive.