Need for a Soil‐Based Approach in Managing Nitrogen Fertilizers for Profitable Corn Production

Nitrogen fertilization for corn ( Zea mays L.) production has relied extensively on yield‐based recommendations that were developed to represent regional averages, yet are routinely applied to individual fields, on the assumption that fertilizer N serves as the major supply for crop N uptake. Using data from 102 on‐farm N‐response studies, an evaluation was conducted of the Illinois proven‐yield (PY) method for accuracy and economic profitability on a site‐by‐site basis. As additional objectives, the Illinois soil N test (ISNT) was evaluated for detecting whether N fertilization was economical, and for quantifying crop response to N fertilization relative to soil and management factors. For 18% of the site‐years studied, N recommendations by the PY method were accurate to within 20 kg ha −1 , whereas 13% were underfertilized by 25 to 129 kg ha −1 (60 kg ha −1 on average) at a current cost of $5 to $170 ha −1 ($75 ha −1 on average), and 69% were overfertilized by 21 to 235 kg ha −1 (103 kg ha −1 on average) at a cost of $12 to $130 ha −1 ($57 ha −1 on average). The latter group included 30 site‐years that were completely nonresponsive to N fertilization, all but two of which were predicted by site‐average ISNT values assuming a critical test level of 230 mg kg −1 This level was exceeded for 19 of 69 responsive site‐years, mostly during 2001–2003 when corn followed soybean ( Glycine max L. Merr.) with high plant populations. A higher critical test level would have been required under such conditions, owing to more extensive residue inputs that would promote microbial N immobilization, and increased crop uptake of mineralized soil N. The ISNT was significantly related to crop N requirement, and was the most powerful predictor of error in PY recommendations ( P < 0.001).