Phosphorus (P) losses from irrigated cropland, transferred to surface waters via irrigation return flows, can impair regional water quality. Best management practices to reduce soil erosion on fields and sediment concentration in return flows do little to reduce soluble P concentrations, which can exceed total maximum daily load (TMDL) limits for total P. Laboratory and field tests were conducted to evaluate the effect of alum on soluble P concentration in return flow water from an irrigation tract in southern Idaho. The laboratory study used two water sources (tap and irrigation), three sediment concentrations (0, 100 and 1000 mg L^-1), two added P concentrations (0 and 1 mg ^-1L), and five alum concentrations (0, 5, 10, 20 and 40 mg L^-1). Field tests were conducted at sediment ponds on two irrigation drains with 20 to 500 mg L^-1 sediment and 0.09 to 0.19 mg L^-1 dissolved P in inflow water. Regression analysis of laboratory data showed a 53% reduction in soluble P concentration with 20 mg L^-1 alum, which was similar to field data. Applying 40 mg L^-1 alum to irrigation water reduced soluble P concentrations up to 98% in the field. Achieving 50% soluble P reduction would cost about $0.007 m^-3 ($8.25 per acre-ft) at the current alum cost of $0.33 kg^-1 ($0.15 lb^-1). Although alum effectively reduced soluble P in irrigation return flow, the cost of applying alum may be too high for routine use in many irrigation districts.