Phosphorus (P) availability differences among sources, especially those between commercial fertilizer and manure or other organic P sources, are of increasing interest to farmers, agronomists, and nutrient management planners. While the traditional interest in maintaining adequate P supplies for crop production continues, P availability effects on the risk of P loss in runoff and the environmental implications of these losses are receiving increasing attention. This is particularly true since soil test P and the characteristics of P in manures and organic materials are important factors in P-based nutrient management planning.