Spatial variability of the Illinois soil nitrogen test: Implications for soil sampling

The Illinois Soil Nitrogen Test (ISNT) is a recently developed test that has shown promise for corn (Lea mays L.) N fertilization. However, among-field and within-field spatial variability of this test is unknown and sampling recommendations have not been defined. The objectives of this study were to estimate the spatial variability and structure of the ISNT to provide sampling recommendations and to determine the short-term effect of N fertilizer on this test. Soil samples were collected on 14 production fields and analyzed for the ISNT. Data was analyzed with traditional statistics and with geostatistical techniques. In the short term, N fertilization did not have a significant effect on the ISNT (p > 0.3). The ISNT was normally distributed in 12 fields and showed a relatively low coefficient of variation (18% on average). Field mean ISNT ranged from 98 to 255 mg kg^-1 and tended to be higher in Mollisols than in Alfisols. On average across fields, at least 10 samples are required to determine the mean ISNT for a given field with a precision of 24 mg kg^-1. The geostatistical analysis revealed that the ISNT has strong spatial structure, as indicated by the bounded variograms, the relatively low nugget/sill ratio (<30% in 10 fields) and the mean range of 150 m. The spatial variability indicates that the ISNT can be mapped with a relatively sparse sampling grid and with a low number of samples, critical characteristics when assessing the economic implication for variable and uniform rate N fertilization.