Variation in Phosphorus Index Values Associated with Differing Methods for Estimating Slope and Transport Parameters.

The Arkansas phosphorus index (P-index) is used to assess the risk of offsite phosphorus transport and is an important constituent of nutrient management plans written for farms in Arkansas.  The transport component of the index is calculated for a specific field using soil characteristics associated with the dominant soil mapping unit within the field as well as the slope of the field.  We were interested in whether a geographic information system (GIS) could be used to correctly estimate the various parameters utilized in the transport component of the index and potentially improve on the methodologies currently used to estimate P index values.  A study performed in the Illinois River watershed located in northwestern Arkansas indicated the nationally available digital data sets could be used to accurately calculate the slopes and P indices for fields between 2-40 acres in size.  The use of GIS to determine field scale parameter estimates based on all the soil mapping units in a field rather than just the dominant soil mapping unit, appeared to provide improved index values when multiple soils with differing characteristics occurred in the same field.  This was especially evident when the various soil mapping units in a field had different flooding frequencies.