In Regan et al. (2010), a runoff dissolved phosphorus risk indicator (RDPRI), based on rainfall alone, was developed for five Irish tillage soils. Results showed that tilled soils, subjected to simulated rainfall only (3 cm hr-1 inclined at 10-degree slopes), may produce surface runoff phosphorus (P) concentrations in excess of 0.03 mg L-1 (the value above which eutrophication of rivers is likely to occur) if their Morgans phosphorus (Pm), water extractable phosphorus (WEP) and Mehlich-3 phosphorus (M3-P) concentrations exceed 9.5 mg L-1, 4.4 mg kg-1, and 67.2 mg kg-1, respectively. The present study developed a modified RDPRI using the same soils, subject to two overflow run-on rates (225 and 450 ml min-1). Results of the 95% confidence intervals suggested new critical soil test phosphorus (STP) thresholds for Pm, WEP, M3-P, Pcacl2 and Psatox of 7.83 mg L-1, 4.15 mg kg-1, 61 mg kg-1, 1.2 mg kg-1 and 17.1%, respectively. While these new values for Pm, WEP, and M3-P are lower than those determined by Regan et al. (2010), the confidence intervals of the two sets of values overlap by more than 25%, indicating no significant difference. The improved finding for Pm in this study is still in close agreement with the agronomic optimum (Pm = 6.1-10 mg L-1) used in Ireland for plant growth and crop yields, and fulfils the statutory requirements of the EU Good Agricultural Practice for the Protection of Waters, which prohibits fertiliser application to tillage soils with a Pm > 10 mg L-1.