Development of a Variable Source N Fertilizer Management Strategy Using Enhanced Efficiency N Fertilizer Sources to Optimize Crop N Response and Economic Returns.

Use of slow-release N fertilizer and other enhanced efficiency N sources is one management option that may increase crop production and profitability and reduce possible negative environmental impacts on water and air resources.  Recent research in Missouri has been testing the concept of variable source N management (VSN) in which enhanced efficiency N fertilizer is applied to areas in a field which have higher probability of N loss and conventional N fertilizer is applied to low-risk field areas.  The objectives of this research were to assess spatial differences in soil water content and soil N availability across agricultural fields with claypan soils containing low-lying or depressional areas and to determine the spatial variability in crop response due to application of different enhanced efficiency N fertilizer sources.  A two-year field study was initiated in Northeast Missouri in 2007.  Treatments were arranged in a split plot design with four replications.  The field was separated into 3.0 x 228 m plots which passed through the low-lying and sideslope areas in the field.  Nitrogen fertilizer treatments of a control and 168 kg N ha^-1 of urea, polymer-coated urea, urea + NBPT urease inhibitor, and urea + nitrapyrin nitrification inhibitor were applied in the spring prior to planting of corn and incorporated immediately after application.  Soil samples were periodically collected at depths of 0 - 15 and 15 - 30 cm in a 9 m grid pattern to assess changes in soil water content and soil inorganic N.  Initial results for 2007 indicate significant spatial differences in grain yield and earleaf N in response to enhanced efficiency N fertilizer treatments across the field.  For example, PCU treatments out-yielded conventional urea treatments by as much as 5142 kg ha^-1 in low lying areas.  In those areas, net economic return from use of PCU versus urea reached up to $687 ha^-1.  These results suggest that a VSN management strategy may increase grain yields and economic returns in claypan soils.