Being a living organism, any plant needs an abundant supply of both major and minor nutrients in its growing environment for proper development at every growth stage. Among the essential “treats” for plants are nitrogen, phosphorus, potassium, calcium, sulfur, magnesium, and iron.
When any of these are lacking, one common symptom follows — a decrease in leaf chlorophyll concentration. This interferes with the plant’s ability to absorb sunlight, which jeopardizes the effectiveness of photosynthesis.
Assessing chlorophyll levels is thus a critical indicator of various stress factors impacting plant health, including both living (biotic) and non-living (abiotic) factors such as inadequate light, water scarcity, and damage from herbicides that inhibit pigment formation, all of which significantly affect chlorophyll levels in leaves.
Utilizing chlorophyll measurements in farming practices allows growers to quickly and effectively understand plant health and potential yields, enabling the creation of precise and optimized nutrient management plans. But how does one measure chlorophyll? Short answer — sensors, CI index, and advanced analytics. However, there is much more to it.
Measuring Chlorophyll Content
To understand chlorophyll measurement better, we must look back at the past. Historically, assessing chlorophyll levels was a cumbersome and imprecise process. It involved extracting the pigment using acetone and analyzing it spectrophotometrically at wavelengths of 663 nm and 645 nm. Naturally, this method proved to be time-consuming and inconsistent.
In contrast, modern techniques like the SPAD-502 meter have revolutionized the way we “see” chlorophyll. This device estimates chlorophyll content by detecting the greenness of leaves, providing more reliable data. It requires placing the leaves between sensors for accurate measurement.
Moreover, advancements in digital technology have introduced methods involving Computer Vision algorithms and smartphone cameras to measure and analyze the greenness of leaves based on specific index values. Additionally, satellite monitoring has become an invaluable tool for chlorophyll analytics, offering large-scale, efficient, and continuous observation of vegetation health and productivity relying on different indices values.
Among the most common indices used to measure chlorophyll in plants are:
Green Chlorophyll Index or the GCI index is a vital tool for assessing the total chlorophyll content in leaves. It utilizes the ratio of near-infrared reflectance to green reflectance, making it very sensitive even to the slightest deviations in chlorophyll levels and reliable across various plants.
The green chlorophyll vegetation index is calculated using the following formula:
GCI = ρNIR / ρgreen – 1 = ρ730/ρ540 – 1
Similarly, the MCARI index is designed to correct the soil background influence and improve sensitivity to chlorophyll concentration, primarily focusing on the red-edge and green bands. This index is particularly effective in assessing the chlorophyll content, indicating plant health and vigor.
Formula: MCARI = (ρ700 – ρ670) – 0.2 * (ρ700 – ρ550) * (ρ700/ρ670)
Red-edge chlorophyll Index (RECI), calculated using the near-infrared and red-edge spectral bands, indicates chlorophyll content, especially in mid to late-stage crops with higher chlorophyll levels. The red-edge spectral region, a narrow band in the vegetation reflectance spectrum, transitions from red to near-infrared and is susceptible to variations in chlorophyll.
Formula: RECI = ρNIR/ρred_edge – 1 = ρ850/ρ730 – 1
The total chlorophyll content shows a linear relationship with the difference between the reciprocal reflectance of the green/red-edge bands and the NIR band. Therefore, indices like GCI and RECI are broadly utilized for their sensitivity and accuracy.
Normalized Difference Red-Edge Index (NDRE) is another index that enters the stage when the red-edge band is accessible. It’s particularly sensitive to moderate and high chlorophyll levels, making it a reliable indicator of crop health in later growth stages and an effective tool for mapping within-field variability of foliar nitrogen. This aids in understanding and managing crop fertilizer needs.
Formula: NDRE = (ρNIR-ρred_edge)/(ρNIR+ρred_edge)
The red-edge band is adept at penetrating foliage deeper than the red band, which allows for a more accurate assessment of the total chlorophyll content and, by extension, the health and vitality of the crop.
As mentioned, those indices can be used by different software analyzing data from a particular type of sensor, be it ground devices, drones, or satellites. We’re going to concentrate on the letter to explain how it works.
Chlorophyll Measurement on EOSDA Crop Monitoring
EOSDA Crop Monitoring, a product by EOS Data Analytics, is an online platform designed for detailed field observation using satellite technology. This comprehensive tool gathers a wide spectrum of data — from crop health and weather patterns to soil moisture and elevation — all integrated into a streamlined, user-friendly system.
Among the standout features of the platform is its advanced chlorophyll measurement capabilities, particularly through the red-edge-based chlorophyll index (ReCl). Understanding the chlorophyll levels of crops helps in determining the optimal time for seed harvesting, ensuring the seeds are of the highest quality. Moreover, by monitoring and analyzing plant performance based on digital data, seed suppliers can select the most suitable seeds and create more resilient crop varieties.
In the platform’s toolset, the ReCl index, alongside other metrics, is particularly useful during the peak growth periods of plants, before early senescence sets in, to maximize efficiency. As the harvesting period nears, transitioning to indices like NDRE continues to provide valuable, targeted information for ongoing crop health monitoring. Such detailed, accessible, and actionable data is transformative, offering a competitive edge to those in a farming business by enabling more strategic and responsive operations.
