Phosphorus indices as risk-assessment tools in the USA and Europe - a review

The "phosphorus index" (PI) is a semiquantitative tool to assess the risk of P loss from fields to surface waters, which is based on simple arithmetic computations of source (soil test phosphorus (STP), P applications as manure and fertilizer, plant residues) and transport factors (erosion, surface runoff, subsurface drainage, connectivity). Work done since the 1990s in the U.S.A. and several European countries has shown that this approach is capable of delineating critical source areas for P export within a watershed. It is intended to adopt such a PI tool in Germany as well. However, there is no "standard" PI, and the variety of P indices is confusing, since each state and country has developed its own version to account for special regional conditions which are important for P loss. This paper reviews the factors of P loss which are taken into account in P indices and different modifications of P indices according to their components and structural approach. The literature concerning single source and transport factors of P loss is overwhelming, and a structured selection is given in this review. Most P indices can be classed into one of three groups: 1. additive approach, 2. multiplicative approach, and 3. multiplicative-additive approach. For Germany, it is suggested that specific factors of P loss are incorporated into the basic backbone of a Pennsylvaniastyle PI. Realization of the factors could be adopted from existing Scandinavian P indices or from other research results.