Kleinman, P. J. A., Sharpley, A. N., Budda, A. R., McDowell, R. W. and Allen, A. L. 2011. Soil controls of phosphorus in runoff: Management barriers and opportunities. Can. J. Soil Sci. 91: 329–338. The persistent problem of eutrophication, the biological enrichment of surface waters, has produced a vast literature on soil phosphorus (P) effects on runoff water quality. This paper considers the mechanisms controlling soil P transfers from agricultural soils to runoff waters, and the management of these transfers. Historical emphases on soil conservation and control of sediment delivery to surface waters have demonstrated that comprehensive strategies to mitigate sediment-bound P transfer can produce long-term water quality improvements at a watershed scale. Less responsive are dissolved P releases from soils that have historically received P applications in excess of crop requirements. While halting further P applications to such soils may prevent dissolved P losses from growing, the desorption of P from soils that is derived from historical inputs, termed here as “legacy P”, can persist for long periods of time. Articulating the role of legacy P in delaying the response of watersheds to remedial programs requires more work, delivering the difficult message that yesterday's sinks of P may be today's sources. Even legacy sources of P that occur in low concentration relative to agronomic requirement can support significant loads of P in runoff under the right hydrologic conditions. Strategies that take advantage of the capacity of soils to buffer dissolved P losses, such as periodic tillage to diminish severe vertical stratification of P in no-till soils, offer short-term solutions to mitigating P losses. In some cases, more aggressive strategies are required to mitigate both short-term and legacy P losses.