Using NDRE for Variable-Rate Nitrogen Management
Why NDRE is the nitrogen index
Of all the satellite vegetation indexes, NDRE (Normalized Difference Red Edge) is the one built for nitrogen. It reads leaf chlorophyll — the molecule nitrogen builds — by looking through the red-edge band, which penetrates deep into the canopy. Where NDVI tells you how much leaf there is, NDRE tells you how much nitrogen is actually inside those leaves.
This matters because nitrogen is the input farmers spend the most on, and the one they waste the most of. A blanket application treats the whole field the same, but fields are never uniform. NDRE shows you exactly which zones are fed and which are hungry — the basis for a variable-rate prescription that puts nitrogen only where the crop can still use it.
How NDRE reveals nitrogen status
When nitrogen is abundant, chlorophyll is abundant, and the red-edge band sits in a certain position. As nitrogen depletes, chlorophyll falls, and the red-edge shifts — NDRE drops. Crucially, this happens inside the leaf while the canopy still looks green. NDVI cannot see it because it saturates over dense canopy; NDRE can.
NDRE = (NIR − RedEdge) / (NIR + RedEdge)On Sentinel-2: Band 8 (NIR) and Band 5 (red-edge, ~0.705 µm).
Reading an NDRE map for nitrogen
| NDRE range | Chlorophyll status | Nitrogen read | Action |
|---|---|---|---|
| 0.4 – 0.6 | High | Sufficient | No application needed |
| 0.25 – 0.4 | Adequate | Healthy | Monitor; maintain program |
| 0.15 – 0.25 | Declining | Early depletion | Candidate for side-dress / top-up |
| 0.05 – 0.15 | Low | Deficient | Targeted application recommended |
| Below 0.05 | Very low | Severe deficiency or senescence | Ground-truth before acting |
Do not chase absolute thresholds. The power of NDRE is spatial: map it across the field and look for zones reading below the field average. Those low patches are where a targeted top-up pays off, and where a blanket application was being wasted on the already-sufficient zones.
The variable-rate workflow
- Start after canopy closure. NDRE needs leaf layers to measure. Before the canopy closes, use NDVI. The nitrogen window opens mid-season — typically stem elongation through grain fill for cereals.
- Pull an NDRE map. A cloud-free Sentinel-2 composite gives you the spatial pattern.
- Identify the zones. Group pixels into management zones: sufficient (no action), declining (top-up candidate), deficient (priority). The variation within your field is your prescription map.
- Prescribe by zone, not by field. Apply nitrogen to the deficient and declining zones; skip or reduce the sufficient ones. This is variable-rate application.
- Re-check in 10–14 days. NDRE should rise in the treated zones. If it does not, the problem was not nitrogen — check water (NDMI) or root health.
Timing: when the window opens and closes
The NDRE nitrogen window is mid-to-late season, once the canopy is established and before the crop starts ripening. Apply too early and there is no canopy to read; apply too late and the crop has already set its yield. For winter cereals this is roughly stem elongation through boot stage; for corn, V6 through tasseling.
A sudden mid-season NDRE drop in a still-green canopy is the signal to act — it means nitrogen is being depleted faster than it is being taken up. If NDRE stays flat while NDVI is high, the crop is well-fed and an extra application is likely waste.
Pair NDRE with these indexes
NDRE tells you nitrogen status, not the whole story. Nitrogen uptake depends on water, so pair it with NDMI — a crop can be nitrogen-sufficient but water-stressed, and adding fertilizer to a dry field does nothing. Use NDVI to confirm the canopy is dense enough for NDRE to be meaningful. Together, NDVI (growth) + NDRE (nutrition) + NDMI (water) cover the three questions that drive a fertilizer decision.
Try it free: Get an NDRE map for your field and see your nitrogen zones — free, no signup. Works on any land worldwide. Check my field’s nitrogen →
Frequently asked questions
Can NDRE really detect nitrogen deficiency?
Yes — it is the index’s core purpose. A falling NDRE in a green, dense canopy indicates declining chlorophyll, most often caused by nitrogen depletion. Because NDRE sees inside the leaf via the red-edge band, it catches this before the canopy yellows, which is the window where a side-dress or foliar feed still pays off.
How accurate is NDRE for variable-rate fertilizer?
NDRE is highly effective for identifying where nitrogen is needed (spatial prescription). The absolute rate to apply still depends on your crop, soil, yield goal, and agronomic advice — NDRE gives you the map, your agronomist sets the rate. The biggest win is usually from skipping already-sufficient zones rather than over-feeding deficient ones.
When is it too late to apply nitrogen based on NDRE?
Once the crop transitions to ripening and chlorophyll degrades naturally, NDRE falls as part of senescence — not deficiency. Applying nitrogen then is wasted. For most cereals the useful window closes around grain fill. If NDVI and NDRE are both dropping over a mature crop, it is likely natural senescence, not a fixable deficiency.
Why use NDRE instead of NDVI for nitrogen?
NDVI saturates over dense canopy — past roughly 0.8 it cannot distinguish well-fed from nitrogen-stressed vegetation, because both look like a thick green wall. NDRE uses the red-edge band, which keeps responding to chlorophyll inside the leaf after NDVI has gone flat. For nitrogen questions over a closed canopy, NDRE is the only one that still talks to you.