Agricultural, nonpoint pollution has increasingly become the focus of state and federal water quality mitigation efforts. However, this pollution is spatially dispersed and temporally uncertain, making regulatory efforts aimed at its abatement difficult. For these reasons, policymakers have concentrated on reducing the potential of agricultural, nonpoint sources to emit pollutants. Because the majority of the nonpoint pollution originates from a minority of U.S. cropland, these efforts have often been targeted using indices, such as the phosphorus index. This paper develops the concept of a phosphorus index to explicitly include heterogeneous productivity, which is necessary to efficiently target nonpoint pollution efforts. Such targeting can improve cost effectiveness and increase the scope of voluntary conservation programs designed to mitigate agricultural phosphorus pollution.