Using Light to Sense Plants’ Health and Diversity

When it’s time to fertilize fields, farmers typically grab a soil sample every few acres and measure how much nitrogen and potassium each sample contains. This approach eventually creates a map reflecting the fields’ need for these plant nutrients. A new experimental laser device promises speedier and more-detailed maps of crop-nutrition needs by taking readings from plants themselves as a tractor or other vehicle moves through a field.

Farmers might reap big benefits because such maps would allow them to better adjust fertilizer application and thus limit costly fertilizer waste or diminished yields due to the underfeeding of some plants.

The technology works by shining polarized laser light on plants and then analyzing the wavelengths that are reflected back by the leaves. By knowing the spectrum of light that typically comes from healthy, well-fed members of a plant species, the computerized system could tell when a crop is getting less-than-optimal nutrition.

Currently, the system can discern only a plant’s nitrogen status. However, the device’s developers expect to someday give the system the ability to also measure a plant’s phosphorus status and perhaps to find early evidence of disease or insect infestation. Moreover, because plant species can differ substantially in the spectra they reflect from a given beam of light, this system might also be used to identify undesirable biodiversity in crop fields—weeds!

Weighing less than 10 pounds, the current device could easily be mounted on a tractor, notes Steven Finkelman of Containerless Research, the project’s lead developer. He expects that in one pass through a field, it could diagnose how much fertilizer an individual plant needs and tell sprayers on the same vehicle to dispense the appropriate amount of fertilizer to that plant.

In addition to boosting farm productivity, this technology might also pay environmental dividends, Finkelman says. By limiting overfertilization, it could dramatically reduce the excess plant nutrients that ultimately wash from fields into streams. That’s important because those same substances can fertilize algal blooms that create oxygen-starved zones in the Gulf of Mexico and other coastal waters. Recent research has shown that these dead zones not only kill fish (see Dead Waters), but also trigger more-subtle health problems in aquatic life (see Choked Up: How dead zones affect fish reproduction).

Polarized views

Although much of the light that hits a leaf bounces off, some is absorbed and then, after being affected by the plant’s internal chemistry, is reemitted from the leaf. When polarized laser light is beamed onto the leaves, the absorbed-then-reemitted energy is depolarized light, Finkelman says. It’s these depolarized emissions that reveal information about a plant’s nitrogen status and could indicate other features.

The prototype N-Checker—for nitrogen checker—system shines red light at two different wavelengths onto crop leaves. A spectrometer then analyzes the light coming back from the plants in two wavelength bands. The resulting spectral fingerprint provides clues to chemical action centers inside that leaf, such as the presence and quantities of nitrogen-rich chlorophylls and some other plant pigments.

Taking readings from plants up to 18 inches away, N-Checker can move through a field and evaluate the status of 60 plants per minute. In tests at the University of Illinois, the system has detected distinct and characteristic differences in the reflected spectra from plants that had been fed different amounts of nitrogen, Finkelman and his colleagues reported in July at an agricultural-science conference.

The National Science Foundation had been supporting the system’s development with Small Business Innovation Research grants. However, “we’ve recently had interest from tractor companies,” Finkelman says, “so we’re in discussions now about commercializing this particular [nitrogen-analyzing] instrument.”

Mapping biodiversity

Such systems might also become research tools. Finkelman thinks it’s possible that an N-Checker–like device that can distinguish plant species might one day be the heart of a system for mapping areas’ biodiversity. A truck-based unit could scan roadside foliage or one mounted on an all-terrain vehicle could identify plants while moving through wild lands. If coordinated with a global positioning system, the readouts made during such treks might instantly map the locations and abundance of plant species in any prairie, field, or forest, he says.

Finkelman’s collaborator Louise Egerton-Warburton, an environmental scientist with the Chicago Botanic Garden, has shown that the N-Checker detects characteristic spectra coming from greenhouse specimens of spinach, corn, sunflowers, beans, and sweet potatoes. Tested on tree leaves, the system detected differences among oak, maple, pine, and cottonwood trees. N-Checker even distinguished two closely related cultivated varieties of a single species—in this case, poinsettias—notes Edgerton-Warburton.

She proposes that such performance could interest researchers studying the ecological impacts of climate change. She explains that plants’ spectral fingerprints might someday reveal both the nitrogen and carbon content of a plant. The higher the nitrogen content of a plant, relative to its carbon stores, the more quickly its leaves, bark, and twigs will decompose, a process that sends the greenhouse gas carbon dioxide into the atmosphere. The device might therefore be used to gauge how prone a particular environment is to spewing the climate-altering gas.

But Edgerton-Warburton also expects N-Checker–like devices to see more-prosaic use. For instance, equipment-rental shops might keep some on hand to loan to homeowners who want to evaluate whether and how much their lawns need fertilizing.