Phosphorus indexes in four Midwestern states: An evaluation of the differences and similarities

Phosphorus (P) indexes have been developed to assess the potential for agricultural P loss to surface waters, to plan and regulate P application, and for education. We compared the P indexes of four states through application to field scenarios varying in soil test P, P application, distance to surface water, and erosion rates. Importance of these factors to runoff P risk assessment scores varied considerably with P indexes. Increasing soil test P from 30 to 90 mg kg−1 (ppm) increased scores by zero percent to 300 percent. Scores increased by 0 to 64 percent for surface application as compared to injection of manure and by 0 to 29 percent by applying manure 30 m (100 ft) rather than 15 m (50 ft) from the edge of a water body. Increasing erosion loss of soil from 2.3 to 11.3 Mg ha−1 (1 to 5 t ac−1) increased scores by 30 to 244 percent. The differences and similarities were evaluated in consideration of research findings.