Technical Abstract: The last decade has seen an explosion of concern over agriculture's role in phosphorus (P) loadings to surface waters. The National P Research Project (NPRP), consisting of researchers from over 20 locations in the U.S., was developed to coordinate agricultural P research with the primary objective of developing a scientific basis for recommendations to manage P in a sustainable manner within agricultural operations. One key objective of the NPRP is to develop a database of soil and runoff properties that will enable comparison of mechanisms controlling P runoff from different soils. Research prior to the initiation of the NPRP pointed to the possible need for a unique "environmental P" test (as opposed to traditional soil tests developed for agronomic interpretation). However, results from the NPRP show that most agronomic tests are nearly as effective at predicting runoff P as are environmental P tests. Findings of the NPRP also show that shallow soil sampling (0-5 cm) is needed to accurately reflect runoff potential from soils with P stratification (e.g., no-till or pasture), whereas deeper samples (0-15 cm) are sufficient for well-mixed surface soils. Finally, the NPRP has provided important validation and calibration data for the P Index, and NPRP studies were used to develop such unique factors as P availability coefficients that allow P amendments to be weighted on the basis of their availability to runoff.