The Dubious Relationship Between P Use Efficiency and Loss Mitigation.

Research and field experience has generally taught us that practices leading to increased efficiency of N use by crops also contribute to reduced potential for N losses to water. The case for this same relationship for P is much less convincing. Situations with high P use efficiency, using traditional efficiency expressions such as partial nutrient balance, agronomic efficiency, or recovery efficiency, may result in high loss of P to water bodies, while situations with low P use efficiency, as defined by these expressions, may result in relatively small losses. A focus on increasing P use efficiency may be a successful approach to loss mitigation where past or current P input levels have been excessive, but in other cases, mitigation often requires a more holistic approach to P stewardship that encompasses P  timing and placement, conservation measures, and better utilization of all P sources to accomplish the water quality and productivity objectives of P use.  The challenge is that agronomically unimportant P losses that have a negligible impact on recovery efficiency or nutrient balance, may represent important losses to water.