Split Nitrogen Timing in a Wet Spring for Irrigated Corn
A central Nebraska producer has 160 acres of continuous corn following soybean, expected yield 240 bu/ac. In late March he applied 120 lb N/ac as anhydrous ammonia (82-0-0) pre-plant. Soil samples (0-24 in) taken at sidedress time show 38 lb NO3-N/ac. Since planting, rainfall has totaled 4.5 in above normal, and the field's sandy loam has 1.2% organic matter. Tissue sampling at V6 shows a lower-than-expected nitrate concentration. The producer wants to apply the remaining 60 lb N/ac now as 28% UAN through a pivot, but his fertigation system cannot exceed 30 lb N/ac per pass without runoff risk. Nitrogen is $0.62/lb and corn is $4.80/bu.
1. Calculate the total N applied to date, the remaining N need based on a mass-balance approach, and whether the producer's plan (60 lb N/ac now) is agronomically and economically sound.
Read the worked answer
Total applied to date = 120 lb N/ac (anhydrous).
Mass balance: Crop N requirement for 240 bu/ac at ~1.0 lb N/bu = 240 lb N/ac. Credit legume (soybean) ~40 lb N/ac. Credit soil residual nitrate = 38 lb NO3-N/ac (0-24 in).
Adjusted need = 240 − 40 − 38 = 162 lb N/ac.
Already applied = 120 lb N/ac, so remaining = 162 − 120 = 42 lb N/ac.
The plan to apply 60 lb N/ac exceeds the calculated need by ~18 lb N/ac. At $0.62/lb, that is ~$11/ac of extra N with limited yield response. More importantly, the sandy loam with 1.2% OM and 4.5 in excess rain has high leaching risk; the pre-plant anhydrous N may already be partially lost. A better plan: apply 30 lb N/ac now via fertigation, then reassess with a presidedress nitrate test or tissue test before the second 30 lb pass. This matches the 30 lb/pass limit and avoids over-application. Economic check: if 1 bu of corn requires ~1 lb N, 42 lb N/ac costs ~$26/ac and supports ~42 bu/ac, worth ~$202/ac - strongly positive, so the remaining N should be applied, but not more than needed.
2. The tissue test at V6 shows low nitrate. Explain why tissue nitrate alone may not reliably indicate N status in this situation and what complementary test would strengthen the decision.
Read the worked answer
Tissue nitrate at V6 can be misleading because:
- Nitrate concentration in tissue reflects recent uptake and dilution by growth, not total soil N supply.
- Excess rain may have leached nitrate below the root zone, so the plant is taking up what is available but the soil reserve is depleted - tissue may read low even if some N remains deeper.
- The sandy loam with low OM has limited mineralization, so tissue N depends heavily on fertilizer timing.
A complementary test is the presidedress nitrate test (PSNT), which measures soil NO3-N in the top 12 in at V4-V6. It integrates mineralization and residual N and is calibrated for corn. If PSNT is above the critical level (typically ~20-25 ppm NO3-N for corn after soybean), sidedress N can be reduced. Combining PSNT with tissue analysis and a mass balance gives a more reliable recommendation than tissue alone.