The increased potential for phosphorus (P) surface runoff losses from agricultural soils resulting from land application of organic amendments is leading many states to develop P-based land application rules.  A study was conducted to determine the effect of long-term biosolids loading on the potential for P runoff losses.  A total of 44 soil samples from 13 fields collected periodically during the periods in which the fields received multiple biosolids applications (up to 20 years) and up to 10 years after biosolids applications ceased were used in this study. The cumulative biosolids applications to those fields at each of the various sampling times ranged from 0 to 1,000 Mg ha^-1.  The soil samples were analyzed for oxalate extractable (reactive) P, Al, and Fe; and Bray P1 soil test P and water-soluble P (WSP).  Water-soluble P increased linearly with biosolids loading and correlated well also with soil test P.  Phosphorous saturation index (PSI), calculated as the molar ratio of oxalate extractable P to Al + Fe, ranged from 5.4 to 61.  A plot of the relationship between WSP (dependent variable; mg P kg^-1) and PSI (independent variable) showed that WSP remained unchanged as PSI increased to a value of about 20 (inflection point), beyond which WSP increased linearly with further increase in PSI.  The results show that the ratio of reactive P to Al + Fe associated with biosolids used as a weighting factor is more reliable than cumulative biosolids P loading alone for estimating the biosolids loading beyond which the potential for P runoff losses increases significantly.  This factor should be considered in developing P-based land application rules.  In this paper, data relating biosolids P loading to potential for P runoff generated by runoff simulations on soil collected from the fields evaluated in this study will also be presented.