Phosphorus (P) in runoff from agricultural soils is an important contributor to surface water eutrophication. To quantify P losses from organic (poultry litter [PL]) and inorganic (triple superphosphate [TSP]) P amended soils, the relationship between runoff P and Olsen and Bray I soil test P (STP) in two geographic zones in Puerto Rico was evaluated, using simulated rainfall. The soils were an Ultisol-Oxisol complex with nativized pastures in Mayagüez and an Ultisol with grass cover (Brachiaria Decumbens) in Corozal. The dissolved P fraction (DP) corresponded to 80% of total P (TP) concentrations in Mayagüez, while in Corozal DP represented between 32 and 35% TP concentrations in runoff in TSP and PL amended soils, respectively. Higher topography and lesser groundcover observed in Corozal favored the lesser DP/TP ratio in this site. The TP and DP concentrations in runoff increased with STP following an exponential model. Organic residues in surface soil and plant material increased P losses and concentrations in PL amended soils. Runoff P fractions and concentrations were reduced with increases in runoff volume and antecedent soil moisture. Environmental critical STP levels in inorganic amended soils, calculated based on a 1 mg L^-1 DP threshold were between 176 and 184 mg kg^-1 Olsen P STP and 206 mg kg^-1 for Bray I P STP, respectively. In organic amended soils thresholds values were 88 and 111 mg kg^-1 Olsen and Bray I, respectively.