The P Index is calculated for pastures with the same equations as for cropland. Manure deposition rates are estimated according to the number, type, and time of animals present and an average excretion rate for each type of animal. Managed pasture that is maintained with a stocking density low enough to maintain sod cover generally has very low RUSLE2 soil loss and rainfall runoff potential, and consequently has low P Index values compared to most cropland. On the other hand, exercise-area type pastures where sod cover is not maintained can have a high RUSLE2 erosion rate and also can have comparatively high manure deposition rates. Pastures used for over-wintering, especially those on medium to heavy textured soils, can be exceptions to the rule that managed pastures have very low P Index values. When there is a heavy concentration of animals on frozen soil, the resulting manure deposition results in comparatively high estimated dissolved P runoff from snowmelt and rain on frozen soil.
Figure 6 shows the P Index for a rotationally grazed beef pasture on the example Loyal silt loam field. The P Index is lower than for any of the cropland scenarios in previous figures. Also shown is the P Index for a year-round exercise lot with 10 cows per acre 6 hours per day. It has a higher P Index value than any of the cropland examples. This kind of lot usually has bare areas subject to erosion and higher soil test P along with a high rate of manure deposition.
Figure – Annual P Index from eroded soil (Particulate P I) , dissolved P from soil (Soluble P I soil), and dissolved P from manure (Soluble P I manure) for a rotationally grazed pasture and a dairy cow exercise area on a Loyal silt loam field with 4% slope, and 2.5% organic matter. Soil test P was 47 ppm for the grazing pasture and 250 ppm for the exercise area. The P Index is shown by component: Particulate P I is P in eroded soil, Soluble P I soil is dissolved P from soil, Soluble P I manure is dissolved P from manure.
Something to note about the P Index calculations for pastures is that the current version of RUSLE2 used in SnapPlus does not contain the most updated RUSLE2 for calculating erosion from pastures and overwintering hay crops. The USDA Agricultural Research Service has created a version of RUSLE2 with new equations for pasture soil loss, but this version has not yet been released by NRCS. When it is released, the new method for accounting for soil loss from pastures will be incorporated into the next release of SnapPlus. Use of the new RUSLE2 will not substantially change the very low erosion rates calculated for rotationally grazed and other managed pasture, but should allow for more accurate erosion estimates for over-wintering and high-use areas.