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Do This at Planting to Make Your July NDVI Flight Actually Work

· 4 min read ·
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Do This at Planting to Make Your July NDVI Flight Actually Work

If you’re planting potatoes or onions this week in the Magic Valley, there’s a ten-minute setup step that will determine whether a July multispectral flight can produce a usable variable-rate nitrogen prescription — or just a pretty map.

It’s called an N-rich strip, and it has to go in at planting. You can’t add it later.

What an N-Rich Strip Is and Why It Matters

A Normalized Difference Red Edge (NDRE) prescription flight works by identifying which zones in your field are showing nitrogen stress. If you’ve heard the term NDVI, NDRE is a related spectral index that reads deeper into the canopy — more accurate for nitrogen assessment in dense crops like potatoes and onions. But “stress” is a relative term. To know that a field zone is behind, you need a reference point that shows what a nitrogen-adequate stand looks like in that field, under those soil conditions, in that year.

That reference is the N-rich strip: a narrow swath of ground where nitrogen was applied at 40 to 50 lbs per acre above your standard rate at planting. The strip gets everything the field gets, plus extra nitrogen it almost certainly doesn’t need.

By July, when an NDRE flight passes over, the N-rich strip will be visually dense, dark green, and fully saturated on the spectral index. Zones in the rest of the field that are reading below the strip’s NDRE value have a nitrogen response opportunity. Zones that match the strip do not. The prescription comes from that comparison — not from an absolute number.

Without the strip, you’re flying blind. An NDRE value of 0.25 in a potato field might mean nitrogen stress, or it might just reflect your soil type, your variety, or your irrigation regime. The strip gives the flight a field-specific benchmark that accounts for all of those variables.

How to Set One Up

The protocol is straightforward.

Strip dimensions: 10 feet wide, minimum 300 feet long. Longer is better — it needs to be visible from altitude and large enough to produce a clean spectral reading.

Nitrogen rate: 40 to 50 lbs of actual N per acre above your planned starter rate, applied in the same form and timing as your base application. This isn’t a special product — just more of what you’re already applying.

Placement: Put the strip in a representative productivity zone, not on a high corner or a known problem area. You want the strip to reflect what a well-managed part of your field looks like when it’s not nitrogen-limited. Avoid field edges, headlands, and any area with unusual soil type, drainage, or compaction.

Mark it. Record the GPS coordinates of both endpoints. You need to be able to locate the strip in July when the canopy is closed and you can’t see it from the ground. A flag at each end or a waypoint in your field records works.

One strip per field is sufficient for most operations. On fields with strong yield zone variation — visible from prior yield maps or previous NDVI imagery — a strip in each major productivity zone produces a more precise prescription.

The July Connection

The SE Idaho prescription flight window for potatoes opens roughly July 10 and runs through July 25. That’s the critical bulking period, when fertigated nitrogen can still be delivered through center-pivot systems and will have a measurable effect on yield.

An NDRE flight during that window, combined with your N-rich strip data from planting, produces a zone map: apply more nitrogen here, skip it there, cut rate in these patches. The variable-rate savings documented in the research literature — 10 to 25% reduction in applied nitrogen with no yield penalty — assume this kind of field-calibrated approach.

For onions, the decision window is similar: July 10 to 25, aligned with the onset of bulb enlargement. The same strip protocol applies.

The strip you put in at planting this week is what makes that July prescription scientifically defensible. A flight without it is just imagery.

What Happens If You Miss It

If you’re past planting and didn’t put in an N-rich strip this year, a July flight can still produce useful information — NDRE imagery will show you which zones are reading high or low, and a skilled agronomist can interpret the patterns in the context of your variety and soil type. But the prescription will carry more uncertainty, and it won’t be as easily defensible to a skeptical agronomist or lender.

Put the strip in this year, and next season’s flight will be significantly more reliable.

Key Takeaways

  • N-rich strips must be established at planting — they cannot be added after the crop is up.
  • Use 40 to 50 lbs of actual N per acre above your standard rate, in a 10-foot-wide strip at least 300 feet long.
  • Place the strip in a representative productivity zone and record the GPS coordinates of both endpoints.
  • The strip becomes the in-field calibration reference for your July NDRE prescription flight.
  • Without it, a July flight produces imagery but not a field-calibrated prescription.

Planning for the July Window

We’re scheduling NDRE flights for the July prescription window now. If you’re planting potatoes or onions this week and want to set up a strip-referenced prescription flight for mid-July, contact us to get on the schedule. The earlier we plan, the more useful the data.

Sources: University of Idaho Extension BUL 840 (Potato Production in Idaho); University of Idaho Extension CIS 896 (Onion Production in Idaho); Raun et al., Crop Science 45:1988–1994, 2005; Koch et al., Precision Agriculture 5:25–35, 2004; Agronomist growing season brief, Penrose Development internal (April 20, 2026).