Mapwindow GIS + MMP Tools.

Year to year variations in optimum fertilizer nitrogen (N) rate within the same field clearly indicates that weather drives soil and fertilizer N transformations and crop N availability in the field. Combine yield monitor data usually show at least a 2:1 ratio in grain yield within the same field, indicating that both soil and topography clearly affect yield as well. To better predict in-season optimum N rates in the field, we developed a model that couples soil surface and subsurface N mineralization algorithms and soil and fertilizer N transformation and loss processes with a simple crop N uptake model to determine optimum fertilizer N rates at a 10m x 10m grid scale. Processes considered in the model include soil and manure N mineralization, nitrification, denitrification, ammonia volatilization, nitrate leaching and crop N uptake, and the model is driven by temperature, soil moisture, pH and, optionally, terrain attributes. Readily available data including soil texture, pH and organic matter, daily max/min air temperature, precipitation/irrigation, fertilizer N source, placement and timing and crop planting/emergence date are used as model inputs. Modeled N uptake and yield vs actual yield data collected from combine yield monitors at a 10m x 10m grid will be presented in a spatial context to demonstrate the ability of this model to predict corn N uptake, grain yield and optimum in-season fertilizer N rates in a spatial context. Comparisons of model results with traditional fertilizer N recommendation strategies will also be made.