To rank and better understand the risk of P loss from potentially erodible soil materials in the Mollisol-dominated watershed of Rathbun Lake in southern Iowa, we sampled seven representative soil materials at four floodplain sites. We compared the samples by using a variety of characteristics and indices, including particle size distribution; total P, C, and N; P sorption indices; equilibrium P concentration; and degree of P saturation, as assessed by dithionite, oxalate, and Mehlich 3 extraction (M3) methods. None of the Mehlich 3 P values of samples in the present study were high enough to suggest a high risk of water impairment caused by P. Equilibrium P concentration (EPC) values ranged from 0.01 to 0.23 mg L−1. We found that EPC values were significantly correlated with Fe extractable by oxalate or the M3, as well as with total C and total N. The oxalate and M3 provided generally consistent degree of phosphorus saturation indices, leading us to propose three general risk categories for these soil materials: low, intermediate, and high. We conclude that poorly crystalline Fe oxides and organic matter are likely to exert considerable control over the release of P to stream water from materials eroded from these sites. Moreover, risk rankings based on degree of phosphorus saturation values may not be consistent with interpretations of P mobility that are derived from EPC and sorption indices.