Adjusting Corn Nitrogen Management By Including A Mineralizable-Nitrogen Test With The Preplant And Presidedress Nitrate Tests

The anaerobic potentially mineralizable N (PMN) test combined with the preplant (PPNT) and presidedress (PSNT) nitrate tests may improve corn (Zea mays L.) N fertilization predictions. Forty-nine corn N response experiments (mostly corn following soybean [Glycine max (L.) Merr.]) were conducted in the U.S. Midwest from 2014-2016 to evaluate the ability of the PPNT and PSNT to predict corn relative yield (RY) and N fertilizer over- and under-application rates when adjusted by PMN. Before planting and N fertilization, PPNT (0-30, 30-60, and 60-90 cm) and PMN (0-30 cm) samples were obtained. In-season soil samples were obtained at the V5 development stage for PSNT (0-30, 30-60 cm) in all N rate treatments and PMN (0-30 cm) in only the 0 and 180 kg N ha(-1) preplant N treatments. Increasing NO3-N sampling depths beyond 30 cm with or without PMN improved RY predictability marginally (R-2 increase up to 0.20) and reduced over- and under-application frequencies up to 14%. Including PMN (preplant only) with PPNT or PSNT improved RY predictability minimally (R-2 increase up to 0.10) only for coarse- and medium-textured soils, but N fertilizer over- and under-application frequencies were not substantially reduced (<= 12%). These marginal improvements in RY predictability and N fertilizer over- and under-application frequencies, regardless of the variables used (e.g., fertilization, sampling depth, soil texture, and growing degree-day categories), demonstrate that including PMN with soil NO3-N alone does not improve corn N fertilization need predictions enough to recommend their use.