One of the most commonly used agricultural phosphorus (P) performance indicators is the nutrient balance, writes Heidi Peterson, Director, Phosphorus Program, International Plant Nutrition Institute. They can be applied at a variety of spatial and temporal scales to evaluate the efficiency of a system by quantifying the amount of a nutrient removed from a system relative to the amount applied. When analyzed together with data on soil fertility and crop productivity, results can be a long-term indicator of whether a system is being managed sustainably while minimizing excess nutrient losses.
Farm-scale P nutrient balances can be approached from two different perspectives: cropland P use or the whole farm system. To understand cropland P use efficiency (PUE), P inputs include fertilizer or manure applied to the crops, including all inorganic and organic sources. To adequately account for P inputs from manure or non-conventional sources, a chemical analysis should be completed. The P outputs harvested include the nutrients taken up by the crop and removed from the field.
Phosphorus Use Efficiency (PUE) = (P outputs) / (P inputs)
Cropland PUE <1 indicates that the P input is greater than the harvested crop needs, contributing to increased soil test P (STP) and potential for losses to the environment. Cropland PUE =1 indicates that inputs are equal to the harvested crop output. This could be considered sustainable for some time if the STP is above the optimum concentration; however, this is not the case if STP is below the critical concentration or in a region with low natural soil fertility. Cropland PUE >1 indicates that more nutrients are removed from the harvested crop than applied, suggesting that the harvested crop is mining soil P.
