By Mary-Russell Roberson
Synthetic fertilizers have been widely used for more than a century, and they allow farmers to feed the world more efficiently. But there’s a downside: the manufacture of nitrogen-based fertilizer contributes significantly to climate change and air pollution, as does the overuse of fertilizers on farms. Furthermore, nitrogen-rich runoff from farms harms the environment by creating algal blooms and areas of oxygen depletion in streams, lakes and oceans. A well-known example is the “dead zone” at the mouth of the Mississippi River in the Gulf of Mexico.
In a perfect world, fertilizer would be applied in the right place, at the right time, and in the right amount, and the intended plants would soak it all up. In the real world,
experts estimate that well over half of nitrogen-based fertilizer is wasted because farmers don’t always know which plants need an influx of nutrients at any given time, and they may prefer to err on the side of applying too much rather than too little. The result? Farmers lose money and the excess nitrogen adds to environmental problems.
What if there were a simple, inexpensive way to help farmers use only the fertilizer they need and no more?
A Camera for Improving Crop Health
Duke professor Maiken Mikklesen has developed a pocket-sized camera that is making it easier and quicker for farmers to asses the heath of their crops. The benefits are numerous—less water use, a reduction in pesticides, and overall better, more sustainable food. Learn how it works.
Maiken Mikkelsen, Ph.D., is working on an invention she hopes will make this possible. Mikkelsen is the James N. and Elizabeth H. Barton Associate Professor of Electrical and Computer Engineering and she holds appointments in the Thomas Lord Department of Mechanical Engineering & Materials Science as well as physics.
Her invention incorporates a new way of capturing wavelengths of light — not only the wavelengths we can see, but also infrared light, which is invisible to human eyes. The combination of visible and invisible wavelengths creates a “spectral fingerprint” that can indicate whether crops are flourishing or malnourished. And that’s not all. The image can also show whether crops are dehydrated or covered in insects, information that could save water and reduce pesticide use by helping farmers use those resources only when and where necessary.
Multispectral imaging is already used in agriculture, but the equipment is bulky and prohibitively expensive. Even those farmers with access to multispectral imaging typically use it only once or twice a growing season.
“How to translate the spectra into information about plants is well known,” Mikkelsen says. “But the bottleneck was capturing those images. I stumbled onto a way to take images in a simple and elegant manner that’s also much less expensive.”
The multispectral imaging camera was not even a gleam in Mikkelsen’s eye when she first started working with the technology that makes it possible. She was doing basic research in order to learn more about the interaction between light and matter, and how materials behave in different and unexpected ways when they are only a few nanometers in size. (For comparison, a strand of human hair is between 60,000 and 100,000 nanometers thick.)
“I do fundamental research,” she says. “I didn’t start out with, ‘I have this problem; how can I solve it?’ I started with, ‘How does the optical properties of materials change on the deep nanoscale?’ and the application came later on. That’s probably the reason it’s such a fundamentally new approach to solving the problem.”
Her creation consists of a gold film topped with silver nanoparticles shaped like cubes. She discovered that by changing the size of the nanocubes and the sizes of the gaps between them with atomic-scale precision, she could create arrangements that absorb one particular wavelength while reflecting all others.
That was exciting — and it suggested a next step. “It felt like a pity not to detect the wavelengths that were absorbed,” she says. The absorption creates heat, so she made a detector that converts thermal energy into electric current. The size of the current indicates the temperature change, which in turn indicates the size of the absorbed wavelength. As a bonus, the entire process occurs nearly instantaneously, much faster than conventional multispectral cameras that rely on mechanical filters.
Placing multiple squares of gold foil with different nanocube arrangements on one small chip means a camera could capture multiple wavelengths at one time. “It’s efficient and fast,” she says. “It’s low-cost and all on one chip.”
Mikkelsen plans to use her new technology to produce an inexpensive, lightweight camera that could be used by farmers as frequently as they wished, essentially making precision agriculture more accessible to all, even those in low-resource settings.
“Making agricultural fertilizer uses 1% to 2% of the global energy supply and produces about 3% of the global carbon dioxide emissions,” Mikkelsen says. “Since 50% to 60% of fertilizer is wasted, you could imagine cutting these numbers in half.”
Although Mikkelsen is perfecting multispectral imaging for use in agriculture, she says it has the potential to have a profound impact on other areas as well. For example, it could be used to diagnose skin cancer, check food and water for pathogens, or perform vision tasks for automated systems like self-driving cars.
Mikkelsen says her work benefits from the collaborative environment at Duke. In her early explorations, which laid the foundations for her current work, she collaborated closely with David Smith, Ph.D., the James B. Duke Distinguished Professor of Electrical and Computer Engineering.
And she’s looking forward to partnering with others at Duke later in the camera’s development. She anticipates working with the robotics group, because she’d like to put her camera on drones, and with experts in signal and image processing, because her camera will be generating large amounts of data.
“Duke is an absolutely wonderful environment for collaboration,” she says. “There are no barriers to contacting people across departments and across schools, and there is strong support from the university to form new interdisciplinary collaborations.”