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.sup.-1], whereas 13% were underfertilized by 25 to 129 kg [ha.sup.-1] (60 kg [ha.sup.-1] on average) at a current cost of $5 to $170 [ha.sup.-1] ($75 [ha.sup.-1] on average), and 69% were overfertilized by 21 to 235 kg [ha.sup.-1] (103 kg [ha.sup.-1] on average) at a cost of $12 to $130 [ha.sup.-1] ($57 [ha.sup.-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.sup.-1] . This level was exceeded fur 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).