One of the most intriguing demonstrations at this year’s AmericanHort Cultivate Expo was from Dr. Krishna Nemali, Ph.D., assistant professor of controlled environment agriculture at Purdue University. He showed how floriculture and nursery growers, using nothing more than the cameras on their smartphones, can measure plant nitrogen status.
First, though, Nemali explained why plant nitrogen status should be monitored. Many factors, like the form of nitrogen, pH, sensor calibration and crop environments, can affect the nitrogen available to plants. Ideally, he said, nitrogen should be 3% to 4% of dry matter.
The current available monitoring technologies, in order from least sophisticated to most, include visual inspections, lab analysis, handheld meters and remote sensors. Nemali said visual inspections are not reliable because of the plants themselves. Some varieties are dull green rather than vibrant green; some varieties are more tolerant than others to nitrogen deficits – “but visual is still the most common practice in greenhouses because it’s free and people think they can rely on experience,” he said.
Lab tests can cost $25 per sample and there is a waiting period for results – two marks against that method. As for the handheld meters, Nemali said a SPAD sensor is “a nice tool, and it correlates well with lab results, but it can cost up to $3,000.” Remote sensors are even more expensive and complicated, with software, data analysis and calibration to consider.
“Growers need something effective and inexpensive. Three years ago, we thought why not use a smartphone?” Nemali said. “It can take hundreds of measurements in a few minutes. We call it smart sensing.” It’s based on images, has low or no cost, is rapid, is precise and is easy to use.
The smart sensing is based on plant reflectance. The digital camera sensor collects color and reflectance intensity of light from objects to form images. Information can then be extracted by image processing software. These cameras work on the RGB color model, in which red, green and blue colors of light are added together in various ways to reproduce a broad array of colors. When plants are green, they’ll absorb the color green, and red and blue will get reflected back. If too much is reflected, that indicates something is wrong. The camera sensor will tell you how much light is being reflected; the image is the digital signature of that reflectance.
The image analysis in the application estimates the “smart ratio” of RGB. The brighter the image, the more that color is being reflected. As nitrogen levels increase, the ratio will increase, Nemali explained, and thus far the app data are comparable to those from SPAD and lab results. “We saw a great relationship for impatiens, marigold, salvia, zinnia and hydrangea. The phone measurements were validated,” he stated.
Right now, the smart sensing app can simply tell if you if a plant is nitrogen deficient. In the future, though, the goal is to get that smart ratio and from that you’ll learn your percentage of nitrogen content.
“You can do individual plants or an average of several plants – the potential of this technology is huge,” Nemali said. The Purdue team that is putting it all together is aiming for the app to launch in spring 2022. It’s called APTUS (Analyzing Plant Traits Using Smartphones). The small fee to download the app will directly fund more graduate research.
Nemali noted his team is still gathering input from growers about what’s important to them in this application. You can learn more about his research and contact him via purdue.edu/hla/sites/cea.
To see a demonstration of APTUS at Cultivate’21, check out tiktok.com/@cfgrower.