Draft 5 3/2001 The Phosphorus Index: A Tool for Management of Agricultural Phosphorus in Vermont

The Phosphorus (P) Index is a tool developed to assess the potential for phosphorus runoff from individual fields based on soil and field characteristics and on management practices. The P Index is much more comprehensive than relying only on a soil test because it considers erosion and runoff potential and manure and fertilizer P management. Use of the P Index provides a means of identifying fields that have high P runoff potential and, therefore, require conservation practices and/or limitations of manure or fertilizer P application, as well as fields that have low or moderate potential for P runoff. The P Index provides a relative rating as to the risk of P runoff from individual fields, which can be used to prioritize fields for nutrient and soil management practices. It does not attempt to estimate actual quantity of P lost in runoff. The Phosphorus Index for Vermont (Jokela, 2000) is based on the original Phosphorus Index developed several years ago by a USDA-university working group (Lemunyon and Gilbert,1993; NRCS, 1994; Sims, 1996). It also incorporates a number of ideas from modified P Index versions from other states (McFarland et al., 1998; Klausner et al.,1997; Gburek et al, 1998; and Coale and Layton, 1999), as well as some from our own work in Vermont (Jokela et al., 1998; Magdoff et al., 1999). Ten site characteristics are included in the Vermont version of the P Index (Table 1); they are grouped into P Source and P Transport Potential categories. The P Index combines an estimate of phosphorus available for loss via runoff and erosion (P Source Potential) with that of transport mechanisms that could move the phosphorus from the field in runoff (P Transport Potential) (Table 2). Each site characteristic in the P Index is weighted to account for the relative importance in contributing to P runoff potential. The relative weighting is reflected in the different values for the maximum, or very hig