Spot your worst-performing fields from your phone — NDVI maps explained

There's a field on most farms that underperforms every year. You know the one — the grass never looks quite right, the yields are always a bit soft, and you can never fully put your finger on why.

NDVI maps won't fix the field. But they'll show you exactly which parts of it are the problem, and they'll show you before you can see it from the ground.

What NDVI actually is

NDVI stands for Normalised Difference Vegetation Index. It's a measure of how healthy and dense your vegetation looks from satellite imagery. Satellites measure how much light reflects off your land. Healthy, dense grass and crops reflect light differently to stressed, sparse, or damaged growth. NDVI converts that light measurement into a number between -1 and +1, and then maps it — so you can see your fields colour-coded by vegetation health.

The EU Copernicus satellite programme passes over Ireland roughly every five days with its Sentinel-2 satellites. The imagery is free. Several platforms have built tools that give you access to it without needing a technical background.

What you can see that you can't see from the ground

On a standard field walk you see what's in front of you. NDVI shows you the whole field from above — and patterns that aren't obvious at ground level become clear from satellite imagery.

Poorly drained patches that stress grass growth. Areas of poor soil fertility that consistently underperform. Compaction zones from machinery running the same tracks. Rush and weed encroachment starting before it's visible. NDVI doesn't tell you why — it tells you where. The diagnosis still needs a soil test and your own knowledge of the field.

For tillage farmers, NDVI during the growing season shows crop health variability across individual fields — useful for deciding where to take soil samples and where to investigate further.

The free tools worth knowing about

EOS Crop Monitoring has a free tier that gives you access to satellite NDVI imagery for your fields. You set up your farm by drawing your field boundaries on a map — takes about twenty minutes for a typical Irish farm. You then get regular NDVI updates as new satellite passes are processed. The free tier covers the basics.

Sentinel Hub gives you direct access to the EU Copernicus imagery but requires more technical setup. Worth knowing it exists, but EOS is the easier starting point.

Some precision ag platforms include NDVI as part of a broader package — John Deere Operations Centre, for example, integrates satellite imagery if you have compatible machinery. If you're already in one of these platforms, check whether NDVI is already available to you.

What to do with what you see

The map is the starting point, not the answer. If NDVI shows a consistently underperforming area in a field, the next step is a soil sample from that specific area — not the composite sample you might take for the whole field. Understand the underlying cause before spending on a solution.

For grass farmers, NDVI is most useful as a pattern-detection tool over time — watching which areas underperform consistently across seasons tells you something more reliable than a single snapshot.

For tillage, in-season NDVI during crop establishment can flag problems early enough to do something about them.

The honest limitation

Ireland's cloud cover is the practical problem. A satellite pass every five days means nothing if eight of ten passes are obscured. In wet years, you can go weeks without a usable image. NDVI is a useful addition to your information stack — it's not a replacement for the field walk.

It also can't tell you whether a low-performing area is a drainage problem, a fertility problem, a compaction problem, or something else entirely. That still needs ground-truth investigation.

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