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Phosphorus applications in excess of crop need have affected the concentrations of P in surface soil and caused losses of P by runoff in Korea. The transport of P in a small agricultural watershed, the Goseong–Cheon watershed, Gong-Ju, Korea was studied. The area is of a mixed land use (11.8% paddy field, 9.9% upland, and 74.9% forest). Since in general ecosystems P is released from land and transported to water, P concentrations in surface soil, surface runoff, and the stream water of the watershed were monitored. The dissolved reactive P (DRP) concentration in surface runoff was closely related to soil test P (Lancaster P2O5) content (coefficient of determination, R2 = 0.87). Also, DRP concentration in runoff was highly related to the dissolved P concentration of stream water (R2 = 0.73). These results, when combined, indicate that soil P and near-stream surface runoff determined the export of P from the soil to the stream. Thus, plans to minimise losses of non-point source P can be more effective when they are implemented at the particular sources. If best P management practices are to be developed, the integration of areas of high soil P with the areas where surface runoff occur must be considered.