Spatial variability of corn response to nitrogen fertilizer: Implications for variable rate fertilization

Producers and agronomists have long ago recognized that corn yield and its response to nitrogen fertilizer varies within fields. New technologies allow varying the rate of nitrogen fertilizer within fields, but the adoption of this technology is limited due to a lack of recommendations developed for this management strategy. The objectives of this dissertation were to develop a methodology to estimate site-specific production functions useful to provide recommendations for variable rate nitrogen fertilization, to assess the economic value of this technology and to analyze the spatial variability of the Illinois Soil Nitrogen Test (ISNT). On-farm experiments were conducted in sixteen fields in Illinois using production agriculture equipment. Site-specific production functions for corn nitrogen fertilization were developed based on terrain attributes, the ISNT and nitrogen fertilizer as independent variables. The site-specific characteristics interacted with nitrogen fertilizer indicating that they affect corn response to nitrogen fertilizer application and that corn response varies within fields. The ISNT was the variable that most consistently interacted with nitrogen fertilizer. As the ISNT concentration in the soil increased corn response to nitrogen fertilizer decreased, supporting the claim that this soil test can be considered a surrogate for potential soil nitrogen mineralization. The specific catchment area, a terrain attribute related to landscape position, was the most consistent topographic characteristic that affected corn yield and response to nitrogen fertilizer. Prescription maps for variable rate nitrogen fertilization were developed based on the profit maximization of the site-specific production functions. The ex-post economic value of variable rate nitrogen fertilization would have covered the variable rate application fee in fields were the coefficient of variation of the site-specific economically optimal nitrogen rate exceeded 10%. The economic value of variable rate nitrogen fertilization increased with the variability of the site-specific economically optimal nitrogen rate. The ISNT showed a strong spatial structure (mean range of 150 m, low nugget/sill ratio) and relatively low variability. These characteristics indicate that the ISNT would be a suitable nitrogen test for variable rate application of nitrogen.