P Efficiency and Effectiveness in Cropping Systems of the U.S.

Meeting the P needs of crop plants is an essential characteristic of all sustainable cropping systems. Doing so requires the P exported in crop harvest to be replaced or soil P supply eventually drops below levels required to meet crop demand. The finite supply of phosphate rock and the logistical and financial challenges of P recycling, place a high premium on efficiently using P inputs in a manner that is effective in meeting the objectives of the managers of cropping systems. One expression for P efficiency is partial nutrient balance (PNB) which is the ratio of crop P removal to the sum of fertilizer P applied and recoverable manure P. For U.S. cropland in the 2007 crop year, PNB is estimated at 92%, up considerably from the 79% estimated for 1987. However, great variability in PNB exists across the U.S. due to differences in cropping systems, soil properties, cropping history, and livestock density. Research and management opportunities exist for increasing P efficiency where PNB appears low and for increasing P effectiveness where PNB appears too high. Wise stewardship of P resources by the agriculture industry must involve both approaches. Regional examples will be discussed.