Effect of soil sample handling and storage on inorganic nitrogen determination: Implications for the presidedress nitrate test

The presidedress nitrate test (PSNT) is a nitrogen (N) fertilizer decision support system (DSS) that uses soil test nitrate (NO3−) to provide an N rate recommendation for corn. We studied the effect of soil sample storage and handling on soil test NO3−, ammonium (NH4+), and the N rate recommended by the PSNT. Soils from 35 agricultural fields located in southern Ontario, Canada, were collected in June and July and underwent two soil storage treatments and four soil handling treatments. Soils were either handled immediately or frozen (−15°C) for 3 months before handling. Soil handling included (1) immediate extraction in a moist state or oven‐drying for 24 h at (2) 35°C, (3) 65°C, or (4) 105°C. Samples were extracted with a 2.0 M KCl solution for inorganic N determination and immediate, fresh‐extracted samples served as baseline soil test values. Ammonium‐N was increased by freezing and drying at any temperature, while NO3− test values were significantly affected by freezing only. However, oven‐drying samples changed PSNT DSS N rate recommendations in 34%–43% of location‐years when handled immediately and 51%–68% of samples when initially frozen, depending on drying temperature. Most of these deviations were underapplications and relatively small when handled immediately (20 kg N ha−1) or frozen (37 kg N ha−1). We conclude that PSNT samples should not be frozen and that oven‐drying often alters PSNT‐based N rate recommendations. Core Ideas NH4+ but not NO3− test values were significantly affected by oven‐drying relative to fresh extraction. Oven‐drying altered pre‐sidedress nitrate test (PSNT) nitrogen (N) rate recommendations in 34%–43% of location‐years depending on drying temperature. Changes in PSNT N rate recommendation were overall small (average of 20 kg N ha−1). Freezing increased NO3− and NH4+ mg kg−1 values and reduced precision of PSNT‐based N rate recommendations.