Approaches to management zone definition for use of nitrification inhibitors

Site-specific use of nitrification inhibitors has been proposed as one means of reducing the cost of nitrification inhibitor use on fields that vary in their potential for leaching or denitrification. This study evaluated one approach for site-specific use of the nitrification inhibitor nitrapyrin (2-chloro-6-[trichloromethyl] pyridine) based on slope and surface texture. Treatments of uniform and variable N fertilizer and nitrapyrin were applied to field length strips in a center-pivot irrigated field planted to maize (Zea mays L.) from 1995 to 1998. Nitrapyrin application was controlled spatially according to an arbitrary leaching potential derived from slope and surface soil texture. Growing seasons were grouped into dry, wet, or average precipitation years. Alternative approaches for nitrapyrin management zones based on fuzzy cluster analysis of grain yield or soil apparent electrical conductivity (ECa) were compared with the slope-texture management zone approach. There were small effects of nitrapyrin application method (none, uniform or variable) on grain yield. Uniform nitrapyrin application increased yield in a wet year. Fuzzy cluster analysis of grain yield or soil ECa identified unique patterns of yield or soil ECa. Yield clusters corresponded to patterns of water availability or excess. There were small effects of N or nitrapyrin application method on soil residual NO3-N. Yield Cluster 3 soils had higher residual NO3-N levels all 4 yr. In a wet year (1996) NO3-N levels were lowest in yield Cluster 2 soils-in the other 3 yr of the study, there was no difference in NO3-N levels between yield Clusters 1 and 2. Soil ECa clusters corresponded closely to soil series. There were significant correlations between soil ECa and yield clusters. Fuzzy cluster analysis has the potential to define management zones for use of nitrification inhibitors from relatively easily obtained spatial yield or soil ECa, rather than expensive grid sampling of soil chemical and physical properties.