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Field-Scale Variability in Optimal Nitrogen Fertilizer Rate for Corn

^a Agron. Dep., Univ. of Missouri, Columbia, MO 65211
^b USDA-ARS, Cropping Syst. and Water Quality Res. Unit, Columbia, MO 65211
^c USDA-NRCS, Columbia, MO 65203. Contribution from the Missouri Agricultural Experiment Station and the USDA-ARS

View larger version (27K): [in a new window]   Fig. 1. Quadratic-plateau response functions describing yield response to N fertilizer for the eight experimental locations. Location abbreviations are CP = claypan soil region, DL = deep loess soil region, MD = Mississippi Delta soil region, 00 = 2000, 01 = 2001, and 02 = 2002. EONR = economically optimal N rate determined from the best-fitting quadratic-plateau response function for the whole field.

View larger version (14K): [in a new window]   Fig. 2. Cumulative distribution function for the coefficient of determination for yield response models in the 586 yield response cells modeled as responsive to N (25 cells were modeled as nonresponsive). Approximately two-thirds of the models fit the yield data with R2 of 0.90 or higher.

View larger version (27K): [in a new window]   Fig. 3. Coefficient of determination for yield response models was related to the size of the yield response. When yield response was small, we observed more response functions with low coefficient of determination.

View larger version (17K): [in a new window]   Fig. 4. Yield data and the fitted response function for a 20-m yield response cell at the Mississippi Delta location in 2001. Yield was modeled using a quadratic-plateau response function for this cell and for 571 of the 611 cells. This cell was chosen arbitrarily as an example based on having the median R2 for the 586 responsive cells.

View larger version (20K): [in a new window]   Fig. 5. Box-and-whiskers diagram of economically optimal N rate (EONR) distributions for the eight experimental fields. The upper and lower limits of each box signify the 25th and 75th percentiles for EONR, the horizontal line in the center of the box indicates the median, the "+" in each box indicates the mean, and the "whiskers" or arms represent the full range of EONR observed at an experimental location. The EONR ranged from 0 to 280 kg N ha–1 (the highest N rate used) for five of the eight locations. The span from the 25th to the 75th percentile was 69 kg N ha–1 at all locations except the deep loess location in 2000. Any uniform rate would miss the optimal N rate by a large margin for much of the field area. In the location abbreviations on the x axis, CP = claypan soil region, DL = deep loess soil region, MD = Mississippi Delta soil region, 00 = 2000, 01 = 2001, and 02 = 2002.

View larger version (57K): [in a new window]   Fig. 6. Class maps of economically optimal N rate (EONR) for four experimental fields. Each gray rectangle represents an area in the field about 20 by 40 m, which contained six N rate treatments ranging from 0 to 280 kg N ha–1. A quadratic-plateau function (see text for exceptions) was fitted to describe yield response to N rate in each 20- by 40-m area, and then this function was used to calculate EONR. The EONR for each area in the field is indicated by shade of gray, with darker shade signifying higher EONR. Missing rectangles in B are due to an irregular field boundary; in C, they are due to a drainage channel (west of center), a pivot road and rep break (center), and stand loss (near the southeast corner); and in D, they are due to the break between replications. Average yield at optimal N rate was 10.3, 11.6, 11.7, and 8.1 Mg ha–1 for the experiments shown in A, B, C, and D, respectively. Numbers on the boxes circumscribing the experimental areas are UTM coordinates in meters. Scale is identical for all fields in Fig. 6 and 8, and up is directly north in each of these figures.

View larger version (23K): [in a new window]   Fig. 7. Fitted semivariograms for economically optimal N rate (EONR) at the eight experimental locations. A restricted maximum-likelihood procedure was used to fit semivariogram spherical model parameters to the observed EONR data for each location. The length of each semivariogram is limited to the experimental length. In the location abbreviations in the legend, CP = claypan soil region, DL = deep loess soil region, MD = Mississippi Delta soil region, 00 = 2000, 01 = 2001, and 02 = 2002.

View larger version (48K): [in a new window]   Fig. 8. Class maps of economically optimal N rate (EONR) for four experimental fields. Each gray rectangle represents an area in the field about 20 by 40 m, which contained six N rate treatments ranging from 0 to 280 kg N ha–1. A quadratic-plateau function (see text for exceptions) was fitted to describe yield response to N rate in each 20- by 40-m area, and then this function was used to calculate EONR. The EONR for each area in the field is indicated by shade of gray, with darker shade signifying higher EONR. Missing rectangles in A, B, and C are due to the break between replications. Average yield at optimal N rate was 13.5, 12.4, 7.4, and 10.2 Mg ha–1 for the experiments shown in A, B, C, and D, respectively. Numbers on the boxes circumscribing the experimental areas are UTM coordinates in meters, and up is directly north. Scale is identical for all fields in Fig. 6 and 8. Location abbreviations: DL = deep loess soil area, MD = Mississippi Delta soil area, 01 = 2001, and 02 = 2002.