In-Field Variability of Illinois Soil Nitrogen Test and Loss-on-Ignition for Nitrogen Management.

The Illinois soil N test (ISNT) with loss-on-ignition at 500°C (LOI₅₀₀) adjustment can effectively identify fields with low versus high soil N supply potential in New York. However, spatial and seasonal variability in ISNT-N and LOI₅₀₀ can influence soil N supply classifications. We aimed to determine (i) the influence of sample density on the ISNT-N/ISNT-N_Critical ratio and soil N supply classification, (ii) the implications of change in spatial and temporal variability on ISNT-N interpretations, (iii) the probability of obtaining an accurate interpretation as impacted by sampling intensity, and (iv) the influence of using LOI₅₀₀ equivalents [derived from loss-on-ignition at 360°C (LOI₃₆₀)] on ISNT-N interpretations. Two 4-ha silage corn (Zea mays L.) fields were sampled (150 samples per field, 0–20 cm depth; 64 in regular 25 by 25 or 23 by 27 m grids, the remainder in a pattern optimizing lag distance distribution) in July and November. Semivariograms were constructed to investigate spatial dependence. Soil nitrate showed the weakest spatial dependence. Increasing sampling intensity to 3.75 samples ha^-1 reduced the confidence interval for ISNT-N/ISNT-N_Critical while augmenting the probability (>95%) of obtaining an estimate within the true field mean (±6.4%), resulting in consistent classifications. The LOI₅₀₀ and LOI₃₆₀ results correlated well (r² = 0.78) using LOI₅₀₀ equivalents derived from LOI₃₆₀ produced identical classifications for 92% of the samples. We conclude that effective sampling protocols for ISNT-N should include sampling at 3.75 samples ha^-1 (or more if needed) for consistent ISNT-N interpretations, and LOI₅₀₀ equivalents can be used to derive ISNT-N_critical values.