What is NDRE? (Normalized Difference Red Edge Index)
What is NDRE?
NDRE, the Normalized Difference Red Edge Index, measures chlorophyll content and nitrogen status in crops. It is the precision-agriculture cousin of NDVI: more sensitive to what is happening inside the leaf once the canopy is established, and the index of choice for guiding variable-rate nitrogen applications. If NDVI tells you how much canopy there is, NDRE tells you how well-fed that canopy is.
How it’s calculated
NDVI uses the sharp jump between red absorption and near-infrared reflectance. NDRE uses a more subtle transition: the red-edge — the narrow band where reflectance rises steeply from red into near-infrared. The position and slope of this edge shift with chlorophyll concentration, making it an early indicator of nitrogen status.
NDRE = (NIR − RedEdge) / (NIR + RedEdge)On Sentinel-2 these are Band 8 (NIR) and Band 5 (Red Edge, ~0.705 µm). Because red-edge light penetrates deeper into the canopy than red light, NDRE samples more of the leaf layers — not just the sunlit top. This is why it stays informative after NDVI has saturated.
Typical value ranges
NDRE runs lower in absolute magnitude than NDVI — a dense canopy might read 0.3 in NDRE where it reads 0.8 in NDVI — but its changes carry more information about chlorophyll and nitrogen.
| NDRE range | Meaning | Typical for |
|---|---|---|
| 0.4 – 0.6 | High chlorophyll, sufficient nitrogen | Well-fertilized crop at peak growth |
| 0.2 – 0.4 | Moderate chlorophyll | Adequate nutrition, normal development |
| 0.1 – 0.2 | Declining chlorophyll | Early nitrogen stress, mid-to-late season depletion |
| 0.0 – 0.1 | Low chlorophyll / stressed | Nitrogen deficiency, senescence |
| Below 0 | Bare soil or water | No active canopy |
A better way to use NDRE: map it across your field and look for spatial variation. Patches that read lower than the field average are nitrogen-deficient zones — candidates for a targeted top-up rather than a blanket application.
When to use it
Use NDRE for mid-to-late season nitrogen monitoring, variable-rate fertilizer planning, and detecting chlorophyll stress over dense canopies where NDVI has saturated. Over a season, NDRE stays relatively stable in a well-fertilized crop, then declines as nitrogen is taken up. A sudden mid-season drop signals nitrogen depletion — the window where a side-dress or foliar feed still pays off.
NDRE dominates when the canopy is closed and the question becomes “is it well-fed?” For early-season growth tracking, NDVI or SAVI remain the better choice. As the crop ripens and chlorophyll degrades naturally, NDRE falls — do not confuse natural senescence with deficiency.
Comparison with other indices
NDRE and NDVI answer different questions: NDVI tracks canopy density, NDRE tracks leaf chlorophyll. They are complementary — use NDVI for growth stage and NDRE for nutrition. NDRE stays sensitive to nitrogen after NDVI saturates over dense canopies, making it the mid-to-late season specialist. EVI also resists saturation but measures biomass, not chlorophyll specifically. For a complete nutrition picture, pair NDRE with NDMI (water status) since nitrogen uptake depends on available moisture.
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Frequently asked questions
What is a good NDRE value?
For a healthy, well-fertilized crop at peak growth, NDRE between 0.25 and 0.45 is typical — though absolute values vary by crop and growth stage. Do not compare NDRE directly to NDVI: it runs lower by design. Track changes over the season and map spatial variation within your field.
Is NDRE better than NDVI?
For nitrogen monitoring and mid-to-late season management, yes. NDRE stays sensitive to chlorophyll after NDVI has saturated. For early-season growth monitoring and general biomass mapping, NDVI remains the better default. The two are complementary — use NDVI to track growth and NDRE to track nutrition.
Can NDRE detect nitrogen deficiency?
Yes — this is its core use case. A falling NDRE in a still-green canopy indicates declining chlorophyll, most often caused by nitrogen depletion. Spatial NDRE maps reveal deficient zones before they become visible, enabling targeted variable-rate nitrogen rather than blanket application.
Why does NDRE use the red-edge band?
The red-edge is the narrow spectral region where reflectance transitions from red absorption (by chlorophyll) to near-infrared reflectance (by leaf structure). Its position and slope shift with chlorophyll concentration, making it a more direct probe of leaf chemistry than the broad red band NDVI uses.