The next step in assessing a field with the P Index is to determine the potential for particulate P loss, shown in the diagram, below.
Note: The Particulate P Transport score is a value between 0 and 1, so all raw scores greater than 1 are rounded back to 1.
Phosphorus is also transported from fields to watercourses in particulate form, bound to soil during erosion. Erosion often occurs due to high intensity, short duration rainfall events. Particulate P is also less available for algal growth than dissolved P, because it is bound to soil particles.
The assessment is partially based on the measurement for soil erosion developed by the Natural Resources Conservation Service known as the Revised Universal Soil Loss Equation (RUSLE). RUSLE estimates average annual soil loss over the crop rotation due to sheet and rill erosion from a field. This includes all erosion short of gullies that cannot be erased with tillage. Such gullies are considered concentrated flows (more on that, later). More information on RUSLE can be found at http://msa.ars.usda.gov/ms/oxford/nsl/rusle/overview.html
The Soil Erosion Factor within the Particulate P Transport Score is calculated as follows.
Soil Erosion Factor = 0.1 x Erosion Rate from RUSLE (tons/acre)
The flooding frequency factor is determined with the same method as for the Dissolved P Transport Score.
|Flooding Frequency||Flooding Factor|
|Rare to Never ( > 100 years)||
|Occasional (10 - 100 years)||
|Frequent ( < 10 years)||
The flow distance factor is also handled with the same method as for the Dissolved P Transport score.
Because RUSLE doesn’t measure erosion from larger gullies, the influence of gullies that cannot be erased by tillage should be included in the Particulate P Transport Score. Such gullies are called concentrated flows.
Concentrated Flow Factor: If present, add 0.2. If absent, 0.
Now all the factors are complete for the Dissolved P Transport score.