Red edge vs NIR: what each band actually tells you about crop stress
Pull up a red edge composite and a NIR composite side by side on the same field. They don't agree. A patch that looks rough on red edge can look fine on NIR, and the reverse happens just as often. That's not a glitch in the processing. The two bands measure different things in the leaf, and the disagreement between them is the signal that separates a nitrogen problem from a water problem from something else entirely.
What red edge is actually measuring
Red edge sits in the narrow transition zone around 705 to 750 nanometers, right where reflectance climbs steeply from the red absorption dip into the NIR plateau. That slope is driven almost entirely by chlorophyll concentration in the leaf. When a plant is short on nitrogen, fighting early disease pressure, or hauling salt it can't process, chlorophyll production drops before any yellowing shows up from the cab. Red edge catches that drop. It flags a plant's chemistry going wrong well ahead of a visible symptom, which is why it gets paired with nitrogen scouting so often.
What red edge doesn't tell you much about is canopy structure. A thin stand with healthy chlorophyll per leaf and a thick stand with mild chlorophyll stress can read surprisingly close to each other on this band alone.
What NIR adds to the picture
Near infrared, roughly 750 to 1300 nanometers, bounces off the internal spongy mesophyll layer of the leaf and reflects strongly off healthy, turgid, structurally intact canopy. It's a biomass and leaf-area signal more than a chemistry signal. Thick canopy reflects a lot of NIR. Thin canopy, wilted canopy, or bare ground between rows reflects a lot less. This band separates a full stand from a struggling one, and it holds up well for spotting drought stress, since a plant losing turgor pressure changes its internal leaf structure in a way NIR picks up directly.
The tradeoff: once a canopy closes up and gets dense, NIR reflectance saturates. A field at full canopy with mild nitrogen stress can look nearly identical to a field at full canopy with no stress at all, because NIR stopped being sensitive to that difference once leaf area passed a certain point. Red edge keeps reading the chemistry even after NIR has flattened out.
Why you need both to tell causes apart
A scouting list built off one band alone marks a field as stressed and leaves you to guess why. Build it off both bands and the guess narrows fast. A patch that shows stress on red edge but holds normal NIR reflectance is usually a chemistry problem, nitrogen shortfall being the common one, where the plant is structurally fine but running low on chlorophyll. A patch that shows stress on both bands, chlorophyll down and canopy thinned or wilted, points more toward water stress or a salinity pocket, where the plant is losing both its chemistry and its structure at the same time. Salinity in particular tends to show up as stunted, uneven stand density alongside the chlorophyll drop, since salt stress hits root function and leaf chemistry together.
That split is the whole reason to fly both bands instead of picking one. A single NDVI-style composite from NIR alone will tell you a zone is stressed. It won't tell you whether that zone needs a soil test for salinity, a tissue sample for nitrogen, or a look at the pivot. Red edge and NIR flown together, and compared pixel by pixel, start doing that separation before anyone drives out.
That's the read we built Crop Stress Map around: a weekly red edge and NIR pass over every client field, with the stress zones tagged by likely cause so you know which fields are worth the drive this week and which ones can wait for the next pass.
A single greenness layer that lumps nitrogen, water, and salinity into one undifferentiated red zone only gets you so far. It might be worth seeing what a cause-tagged map looks like on your own fields.