The P Index equations were developed in the UW-Madison Department of Soil Science, using field and laboratory research on the relationship between Wisconsin soil characteristics and management
and runoff P, as well as the extensive body of published runoff P research. Comparison with Wisconsin field runoff data shows that the edge-of-field P Index equations accurately reflect the major P loss processes from farm fields. Figure 2 shows the strong relationship between annual measured P loads in lb per acre per year and annual P Index values. The measured loads are from monitoring in farm fields throughout Wisconsin, including UW-Discovery Farms and UW-Platteville Pioneer Farm. Annual P Index values were calculated for each monitored crop year. Measured sediment and runoff were used in calculating the P Index in Figure 2, rather than the average annual values, to remove the effects of year-to-year rain and snowfall variability on the comparison.
Note that the P Index loads are actually less than measured loads. This is because of its current equations under-estimate the P content of eroded sediment. A method to more accurately estimate sediment P enrichment so that the relationship between the P Index and measured loads is very close to 1:1 has been identified and published, but the P Index has not been revised in order to maintain the equations as they were referenced in NR151.
The equations for the P Index have been tested with monitoring date for Wisconsin: Testing the Wisconsin P Index at the Field Scale