Variable-Rate Nitrogen with a Drone Imagery Decision Tool
A 500-acre corn and soybean farm in Iowa uses a variable-rate nitrogen prescription based on drone NDVI imagery collected at V8. The imagery shows a 40-acre low-NDVI zone in a field that received 150 lb N/ac pre-plant. The farm's decision-support tool recommends an additional 60 lb N/ac in that zone, costing $37/ac for the extra N. The field has a 2% slope and a grassed waterway. The farm has a 4R nutrient stewardship plan and wants to document the decision.
1. Evaluate whether the variable-rate N recommendation is justified and identify the data needed to support it.
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The NDVI low zone could reflect N deficiency, but also water stress, compaction, disease, or soil variability. Before applying 60 lb N/ac, the farm should:
- Ground-truth the zone with tissue sampling and a PSNT or soil nitrate test.
- Check yield history and soil maps for the zone.
- Confirm the pre-plant N rate and any leaching/denitrification losses.
- Consider an on-farm strip trial to validate the response.
If tissue N is low and soil nitrate is low, the 60 lb N/ac is justified. The cost is $37/ac; if 1 lb N produces ~1 bu and corn is $4.80/bu, 60 lb N could support ~60 bu/ac worth ~$288/ac, so the economics are favorable if the deficiency is real. Document the imagery date, ground-truth results, and the prescription in the 4R plan.
2. Explain how this decision fits the 4R principles and what records should be kept for the nutrient management plan.
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4R principles:
- Right source: UAN or urea-ammonium nitrate matched to need.
- Right rate: variable rate based on verified deficiency, not imagery alone.
- Right time: V8 sidedress is within the uptake window.
- Right place: apply in the low-NDVI zone, avoiding the waterway and steep areas.
Records: field ID, acreage, imagery date and source, ground-truth tissue/soil test results, recommended rate and product, application date and method, weather conditions, applicator, and calibration. Also keep the 4R plan, the P index or erosion control documentation, and the economic analysis. These records support compliance and continuous improvement.