NDVI crop sensors are agricultural tools that estimate plant health by measuring how crop leaves interact with visible and near infrared light. Healthy plants absorb much of the red light for photosynthesis and reflect a large amount of near infrared light because of their leaf structure. A tractor, sprayer, drone, or fixed sensor can use these measurements to create a map of crop vigor across a field.
This matters because farmers can spot stress early and apply fertilizer, water, or treatments more precisely.
Key Facts
- NDVI = (NIR - Red) / (NIR + Red), where NIR is near infrared reflectance and Red is red light reflectance.
- NDVI values usually range from -1 to +1, with healthy green vegetation often between about 0.5 and 0.9.
- Low NDVI can indicate bare soil, water stress, nutrient deficiency, disease, pest damage, or sparse crop cover.
- Active NDVI sensors emit their own light, while passive sensors rely on sunlight and need good illumination correction.
- Variable rate application uses sensor data to change fertilizer or spray rate as the machine moves through the field.
- A sensor map is only useful when readings are linked to position data, usually from GPS or GNSS.
Vocabulary
- NDVI
- Normalized Difference Vegetation Index is a value calculated from red and near infrared light to estimate crop greenness and vigor.
- Near infrared
- Near infrared is light just beyond visible red that healthy plant leaves strongly reflect.
- Reflectance
- Reflectance is the fraction of incoming light that a surface bounces back to a sensor.
- Variable rate application
- Variable rate application is the practice of changing input rates such as fertilizer or pesticide according to field conditions.
- Crop canopy
- The crop canopy is the layer of leaves and stems that covers the field surface and is scanned by the sensor.
Common Mistakes to Avoid
- Treating NDVI as a direct fertilizer requirement is wrong because NDVI measures vegetation reflectance, not nitrogen concentration by itself.
- Comparing NDVI maps from different dates without considering growth stage is wrong because normal plant development can change NDVI even when crop health is good.
- Ignoring soil background in thin crops is wrong because exposed soil can lower NDVI and make plants look less vigorous than they really are.
- Assuming all low NDVI areas have the same cause is wrong because drought, disease, nutrient stress, compaction, and poor emergence can produce similar readings.
Practice Questions
- 1 A crop sensor measures NIR reflectance = 0.72 and red reflectance = 0.18. Calculate NDVI.
- 2 Two field zones have readings: Zone A has NIR = 0.60 and Red = 0.20, while Zone B has NIR = 0.45 and Red = 0.30. Calculate the NDVI for each zone and identify which zone likely has healthier vegetation.
- 3 A field map shows a long strip of low NDVI that follows the path of a sprayer track, while nearby rows have high NDVI. Explain two possible causes and one follow up observation or measurement a farmer should make before applying more fertilizer.