Phosphorus (P) based nutrient management is a central concept in national nutrient management policy (NRCS, 1999), and several options for developing nutrient management plans using a P strategy are included in the 2002 Wisconsin nutrient management standard (590) (NRCS, 2002). A phosphorus (P) index approach is one of the options (along with soil test P categories) for implementing a P-based nutrient management strategy. The P index considers site-specific P source and transport factors to predict the risk of P loss in runoff from individual fields. Ideally, it can be used to rank or prioritize fields based on their risk of P loss and to identify management options for reducing P loss. Initially, a draft P index was developed for Wisconsin (Murphy, 2000; Bundy, 2001) following the general concepts proposed by Lemunyon and Gilbert (1993) and modified by Gburek et al. (2000). This method assigned index values to specific site characteristics and management practices to arrive at an overall P index value for a production unit. While conceptually sound, this approach lacked the flexibility to include all of the parameters known to influence P losses in runoff. A modeling approach, similar to that used in the Iowa P index (NRCS, 2001; Mallarino et al., 2001) was adopted as a more comprehensive and quantitative method for assessing the risk of P losses in runoff. The current version of the Wisconsin P index, developed largely through the efforts of Dr. Wesley Jarrell, can be viewed at the following web site (http://wpindex.soils.wisc.edu).