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Historically phosphorus (P) was applied to cropland based on expected crop response and to correct soil nutrient deficiencies. As these deficiencies have become less common and animal production has intensified in many regions, concerns about surplus cropland P becoming a water resource pollutant have replaced historical concerns. To understand spatial and temporal changes in P surpluses, P budgets were developed for Pennsylvania cropland at approximately ten-year intervals from 1939 to 2002. The P balance, the difference between inputs to crop land (manure and fertilizer) and outputs (harvested crops), estimates the change in soil P storage plus P losses through runoff, erosion and leaching. Although the state-level P balance decreased since 1949, in 20 counties the maximum excess P occurred since 1992; Nutrient budgets, such as those developed for P in this research, can contribute to more informed strategies to reduce or prevent cropland P surpluses and thus reduce the risk to water quality. [ABSTRACT FROM AUTHOR]