Nonpoint P sources from the agricultural landscape are a significant environmental problem for surface water bodies because of the promotion of eutrophication. Many states have developed P assessment tools to help differentiate land uses and their potential for P losses to surface water. Kansas has developed such a P index (PI), and the purposes of this paper are to report on the calibration of that index against data collected from four runoff studies and to explore the modification of the PI as a means to improve the predictive capability of the PI. The PI includes soil test P, rate and application method for P from fertilizers and manure, soil erosion, runoff class, distance froth surface water bodies, and irrigation erosion as inputs. As originally proposed, the PI was well correlated with soluble P (r = 0.93) and bioavailable P (r = 0.94) losses but was less correlated with total P (r = 0.79). By modification of the PI, the r values improved to 0.97 for bioavailable P, 0.95 for soluble P, and 0.89 for total P Of the 90 plots at four different sites, plots from Neosho and Franklin-1 and Franklin-2 sites were ranked as having very low and low vulnerability to P loss (82%) whereas plots in the Riley County site were ranked as high and very high vulnerability to P loss (18%) due to manure-applications. Therefore, for only the Riley site, P management strategies need to be modified to reduce P losses. Moreover, additional P applications are not warranted for this site. Using soil test P as a single factor to predict P losses in runoff for our sites produced results similar to using the modified PI.