What “field health” means from space
This is the question every farmer asks, every season. Until recently, the answer required either walking every hectare or buying expensive software. Now, a satellite does it for you — for free, in 30 seconds, from public Sentinel-2 data.
The answer comes in three parts: how green the canopy is (NDVI), how well-watered it is (NDMI), and how that compares to normal for this time of year and this crop.
The three signs of a healthy field
A healthy field shows three things on satellite:
- NDVI in the expected range for the crop and season. A wheat field at heading should be 0.7+; a vineyard at peak canopy should be 0.4–0.5. There is no universal “good” — it depends on what you’re growing and when.
- NDMI positive and stable. A well-watered crop has NDMI above 0.1; a falling NDMI signals developing water stress before the canopy visibly wilts.
- A trend that matches the season. NDVI should climb through spring, hold through summer, and decline at senescence — for an annual crop. Evergreens (orchards, forests) should stay relatively stable.
If all three check out, your field is probably fine. If any one is off, that is where to focus your attention.
Early warning signs
The earliest warning signs from satellite are:
- NDMI dropping while NDVI holds. Water stress is starting but hasn’t visibly affected the canopy yet. This is the earliest signal — days to weeks before visible wilting. Action: check irrigation.
- NDVI dropping suddenly. The canopy is losing green biomass — stress is now severe enough to affect growth. Causes include drought, disease, pest damage, nutrient deficiency, or herbicide drift. Action: walk the field.
- NDVI lower than the same period last year. The crop is behind its normal curve. Could be late planting, poor establishment, nutrient deficiency, or weather. Action: compare against last year’s management.
- Spatial variation across the field. One patch significantly lower than the rest signals a localised problem — poor drainage, compaction, local pest, or soil variation. Action: target that zone for inspection.
Satellite monitoring insights
Satellite vegetation indices correlate well with ground-level crop status — published R² values for NDVI vs field-measured biomass typically exceed 0.8 for most crops. The critical advantage of satellite monitoring is that leaf water content drops before chlorophyll degrades, so NDMI falls before NDVI. A crop can be measurably water-stressed for days to weeks before visible wilting or yellowing appears. This early-warning window is when intervention is cheapest and most effective.
Key metrics
| Index | What it measures | Healthy signal |
|---|---|---|
| NDVI | Canopy greenness and density | In range for crop + season, steady or climbing |
| NDMI | Leaf/canopy water content | Positive (> 0.1) and stable |
| NDRE | Chlorophyll (nitrogen) status | Stable — declining = nutrient stress |
Free report: Check if your field is healthy — enter the location, and AgroReports computes NDVI, NDMI, NDRE, compares against 3–5 years of your field’s own history, and writes a plain-language report. Free, no signup. Check my field →
How can I tell if my field is healthy without visiting it?
Satellite vegetation indices — especially NDVI (canopy health), NDMI (water stress), and NDRE (chlorophyll) — correlate well with ground-level crop status. A field with stable or rising NDVI, positive NDMI, and values in the expected range for the crop and season is almost certainly fine. A sudden drop in any index is an early warning. AgroReports computes all of these and interprets them for you.
What NDVI value means my crop is healthy?
It depends entirely on the crop and the time of year. Wheat at heading: 0.7+. A vineyard at peak canopy: 0.4–0.5. A young crop in early growth: 0.2–0.4. There is no universal threshold. The most useful reading is the trend — is your NDVI tracking the expected seasonal curve, and how does it compare to the same period last year?
Can satellite detect crop stress before I can see it?
Yes — this is satellite monitoring’s biggest advantage. Leaf water content drops before chlorophyll degrades, so NDMI falls before NDVI. A crop can be measurably water-stressed for days to weeks before visible wilting or yellowing appears. This early-warning window is when intervention is cheapest and most effective.