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Phosphorus (P) losses from agricultural soils are a major cause of fresh water quality impairment. The objective of this study was to validate, at field level, the Caribbean Phosphorus Index (CAPI). The CAPI is a tool developed for the identification of agricultural soils with high risk of P losses to surface waters. P runoff concentrations measured under natural rainfall condition on various fields of dairy and poultry farms in Puerto Rico were compared with risk levels determined using CAPI. Average Olsen soil test P (STP) values were 41 mg P kg-1 and 152 mg P kg-1 for the dairy and the poultry farms, respectively. Average total phosphorus (TP) and dissolved P (DP) concentration losses in runoff were 2.29 mg TP L-1, and 1.79 mg DP L-1 for the dairy farm, and 5.87 mg TP L-1 and 4.82 mg DP L-1 for the poultry farm. The CAPI ranked both dairy farm fields as fields having a Medium potential for P movement and poultry farm fields with Medium and High potential. The current CAPI version underestimated the impact of nutrients on the surrounding waters because runoff concentrations greater than 1 mg L-1 from fields less than 30 m from a surface water body should be ranked in the Very High category. A modification of the CAPI excluding the soil erosion factor from the CAPI matrix yielded results that were more attuned to the observed runoff concentration losses. This exclusion is justified from evidence indicating that soil erosion is minimal in fields exhibiting abundant (>80%) grass cover.