An empirical model is developed to estimate the effects of alternative farming practices on potential nitrogen runoff and leaching in 128,591 National Resources Inventory sites across the Midwest and the Northern Plains of the United States. This model integrates the effects of soils, climate, crops, and management practices on nitrogen loss. The model is applied to evaluation of two policy scenarios. The first scenario reduces N fertilizer application rates by 25 percent through the soil N test. The second replaces continuous cropping practices with crop rotations. The results show that policy effects vary widely across the study region. This analysis emphasizes the importance of conducting policy analysis on a disaggregated scale.