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 (R² 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 (R² 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.