Effects of overland flow on critical soil test phosphorus thresholds in tillage soils

In Regan et al. (Journal of Environmental Quality, 39, 18-192 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° 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 Morgan's phosphorus (P_m), 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 P_m, WEP, M3-P, Pcacl₂ and Psat _ox of 7.83 mg L^-1, 4.15 mg kg^-1, 61 mg kg ^-1, 1.2 mg kg^-1 and 17.1%, respectively. Whilst these new values for P_m, WEP and M3-P are lower than those determined by Regan et al. (Journal of Environmental Quality, 39, 185-192 2010), the confidence intervals of the two sets of values overlap by more than 25%, indicating no significant difference. The improved finding for P_m in this study is still in close agreement with the agronomic optimum (P_m = 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 P_m>10 mg L^-1.