An evaluation of whole farm nitrogen balances and related indices for efficient nitrogen use

Quantification of nitrogen (N) flows creates awareness among farmers, can help them to re-evaluate N management and may reduce nitrate loss to groundwater. Hence, whole-farm balances play a crucial role in legislation on N management in Netherlands. This paper reviews the strengths and weaknesses of whole-farm balances for assessment of the environmental impact of agriculture. The usefulness and reliability of a balance strongly depends on its completeness. The surplus per unit area indicates the environmental impact, provided that all relevant terms are included. However, the surplus per unit area, the surplus per unit output and the output per unit input, as derived from the balance, may not represent accurate indicators of the operational management skills of a farmer, as these estimates not only depend on the conversion of N within the farm, but also on the extent to which the farm relies on animal feed produced outside the farm and the extent to which processing of crops takes place outside the farm. Without additional information on the processes underlying the whole-farm level and N fluxes at spatial scales above the level of an individual farm, whole-farm balances do not reveal the nature and magnitude of losses, nor do they provide sufficient clues how to improve the efficiency of N use