For many modern agronomists, the crux of precision crop management is to apply only necessary amounts of nitrogen when and where it is needed. To achieve optimum crop yield and quality, an adequate supply of nitrogen—which enhances a plant’s ability to form green tissue for photosynthesis—must be present during the period of most rapid nitrogen uptake by the plants in the crop. In addition, the right amount of nitrogen supplied at the right time (and in the right places) to a crop can also reduce fertilizer costs and protect the environment.
Field-based measurements of nitrogen levels in a given crop can be a challenge, however, especially in real time. Different leaves reflect, transmit, and absorb varying amounts of light; in addition, polarized light entering a leaf’s interior can exit as depolarized light. Nitrogen used by vegetation can be difficult to detect and measure with conventional nitrogen-stress-sensing methods such as soil sampling or multispectral imaging. An Illinois research team, however, has developed a field-based optical sensing system that appears to overcome this problem by collecting and analyzing the depolarized light in real time.
Steve Finkelman, Paul Nordine, and their colleagues at Containerless Research (Evanston, IL), Louise Egerton-Warburton and partners at the Chicago Botanic Garden, and graduate student Tim Smith of the University of Illinois (Urbana-Champaign) say that by firing rapid pulses of polarized light at corn, spinach, and other crops, their nitrogen analyzer—dubbed “N-Checker”—can differentiate minute differences in leaf colors, which act as indicators of over- or underfertilization, crop-nutrient levels, and even disease (see figure). Because the system is tractor-mountable, it can be used in the field to help farmers determine in real time how much fertilizer to apply and where. In addition, by preventing waste, the system could decrease the cost of crop production and dramatically cut the nitrogen-laden runoff responsible for algal blooms and other damage to wetlands, waterways, and fisheries.
“Other field devices use both red and infrared wavelengths,” Finkelman says. “Those devices tend to be imprecise because they measure bulk chlorophyll content, not the relationship of the different chlorophyll types. Also, they do not have the sensitivity to the necessary red wavelengths due to interference from reflections. With our technology, we are able to easily see what is hidden from conventional instruments—the strongly absorbed spectral bands that are obscured by noise. The system eliminates interference from light reflected at a leaf’s surface and allows us to see light re-emitting from within. By using polarization techniques we can isolate out the surface reflectance.”
Two versions
The researchers have experimented with two versions of the N-Checker. The original lab-based system channels broad-spectrum light from a xenon flashlamp through a series of calcite crystals to illuminate each corn, sugar beet, cotton or other broad-leaf crop with a transient spot of polarized light that is detected by a broadband spectrometer. Moving from leaf to leaf, this system can take 60 measurements per minute.
Instead of a broad-spectrum lamp as its source, the field-based N-Checker uses multiple red diode-laser sources in the 640- to 780-nm range, which cuts down on sensor and optical-component costs and increases system speed. According to Finkelman, this region of the electromagnetic spectrum reveals not only total chlorophyll content but also relative amounts of the various types of chlorophyll-related complexes, differentiating among them and thus revealing nitrogen-dependent plant health information. The field-based N-Checker, which uses two silicon detectors to collect the reflected light, can take 1000 measurements per second. At roughly 5 mph, a farmer could survey and fertilize tens of acres in a day, or hundreds of acres per day with a multisensor system, Finkelman says.
“Infrared light is not absorbed by chlorophyll, but red light is,” Finkelman says. “So if you do a dual measurement (both red), you can measure how much chlorophyll the plant has. Red and IR light are both absorbed by chlorophyll; however, a component of IR absorption is not related to chlorophyll so this is not a pure signal. With red and red, each red wavelength is related to a specific chlorophyll type, and it appears that the ratio of the parts is more predictive than the total. By analyzing the spectrum of the leaves, we can determine the nitrogen level in the leaves.”
While the current focus is on nitrogen analysis, Containerless also sees opportunities for this technology in the detection of potassium, phosphorous, and other nutrients and stresses, according to Nordine. The company has been granted a patent relating to polarized applications in plants.