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A P index was developed as a tool to rank agricultural fields on the basis of P loss vulnerability, helping to target remedial P management options within watersheds. We evaluated two approaches, a soil P threshold and components of a P index, by comparing site vulnerability estimates derived from these two approaches with measured runoff P losses in an agricultural watershed in Pennsylvania. Rainfall–surface runoff simulations (70 mm h⁻¹ for 30 min) were conducted on 57 sites representing the full range of soil P concentrations and management conditions found in the watershed. Each site was comprised of two, abutting 2‐m² runoff plots, serving as duplicate observations. For sites that had not received P additions for at least six months prior to the study, Mehlich‐3 P concentration was strongly associated with dissolved P concentrations (r² = 0.86) and losses (r² = 0.83) in surface runoff, as well as with total …