What your satellite sees in April 2026 — and why 700 km altitude decides whether your N application pays off

At a glance

If you only use NDVI, you miss water stress, structural issues, and growth delays. Here is which index answers which question — without jargon.

Related:

Application maps · Monitoring

Why April matters more than many think

Spring N is the yield-defining pass of the year. Too early, too late, too much, or too little cannot be fixed later. Decisions must be field-specific — ideally zone-specific.

The problem with classic crop walks:

• You drive the field and mostly see the edges.
• You sample three to five spots.
• The rest is extrapolation.

On 30–50 ha that is systematically too thin. On 80–300 ha farm area it is rarely feasible to scout every field thoroughly.

What a satellite actually measures

ESA’s Sentinel-2 instruments orbit at ~786 km and measure reflected light in 13 spectral bands from visible to infrared.

Plants have a characteristic reflectance pattern: partial green reflection, strong red absorption, very strong near-infrared reflection.

Healthy, growing canopies differ from stressed, dry, or nutrient-limited canopies. Indices summarise those differences.

Five indices, five questions

NDVI — the classic

Normalised Difference Vegetation Index: green, photosynthetically active biomass. High NDVI = dense, healthy canopy; low = little biomass or stress. In April: where is growth, where is it missing? Limits: saturation in dense stands; also sensitive to soil moisture background.

EVI and SAVI — improved biomass estimators

Enhanced Vegetation Index and Soil-Adjusted Vegetation Index correct for soil background and atmosphere — useful in thin stands or early season when soil shows through.

LAI — leaf area

Leaf Area Index models leaf area per square metre of soil. Growth delays often appear earlier and clearer in LAI than in NDVI alone.

NDWI — water stress

Normalised Difference Water Index tracks canopy water content. Falling NDWI while NDVI still looks “okay” is often the first drought warning — days before the eye sees it.

Radar vegetation index — under cloud

Optical satellites need clear skies. Sentinel-1 radar sees through cloud and adds structure information — critical in cloudy German springs.

Why one index is not enough

Field 7 shows slightly below-average NDVI in April. What does it mean?

• Option A: water stress — NDWI would show it; NDVI alone might not.
• Option B: later drilling — LAI trajectories tell the story better.
• Option C: wet, dark soil suppresses NDVI — SAVI/EVI correct for that.
• Option D: days without optical data — radar still gives a signal.

The skill is diagnosis from combined indices — what the Xsupra platform delivers, and what Alora turns into clear guidance instead of a data dump.

What happens in April 2026 in practice

For many cereal farms the window from late March to late April is when the second N dose (stem elongation) is decided. The question is how much on which zone.

• Heterogeneity: which zones lag, which lead? Lagging zones may need more N to catch up — or less if the cause is structural (wet feet, compaction).
• Water stress: if NDWI falls, reduce N because the crop cannot use extra nitrogen.
• History: compare to 2024 and 2023 on the same field — anomaly or normal pattern?

That yields the application map for the spreader: not one rate for the field, but three to six zones with different N rates.

What that means on the bottom line

From the Xsupra customer base: typically 15–25% less N at the same or slightly higher yield; up to 50% less fertiliser on heterogeneous fields; about six hours less scouting per week because satellites show which fields need attention.

Bottom line

A satellite does not replace the farmer — it widens and regularises the view. Planning April N with five indices at the table beats guessing from one number alone.

Sources: ESA Copernicus Sentinel-1 and Sentinel-2 | Xsupra customer references | University of Bonn, Dorothee Scharpenberg on AI assistance in agriculture