Key takeaways 

• Using drone imagery, we can determine not only whether a farmer is using enough fertilizer but also whether they are using too much.
• This measurement is typically calculated from physical plant samples and lab tests, which require significant time and labor.
• New research shows that we can accurately determine fertilizer use efficiency using data collected from remote sensing tools, such as drones or satellite imagery.

Nitrogen is a major nutrient in agricultural production

Farmers often apply fertilizer in excess to reduce the risk associated with low plant nutrition, resulting in nitrogen pollution in our water and air. Although fertilizer can be expensive, the cost is generally lower than the loss of income from under-fertilization and a low-yielding crop. 

In California, growers are required to report the amount of fertilizer applied, among other management decisions, through the Irrigated Lands Regulatory Program¹. The goal is that, by doing so, the state can monitor farmers who apply fertilizer at the wrong time or rate, leading to water and air pollution. 

Synthetic fertilizers—industrially made products with specific combinations of nutrients—are high in nitrogen, one of the most limiting nutrients for plant growth. Nitrogen pollution enters waterways either by running off the soil surface into rivers and lakes, or by percolating through the soil into groundwater. Both can be dangerous to human and wildlife health. The Irrigated Lands Regulatory Program was established to address issues in the Central Valley and other agricultural regions of the state, where nitrogen, pesticides, and other pollutants were found in drinking water and natural habitats. 

To make things more complicated, nitrogen is not an easy chemical to track. Nitrate, a form of nitrogen that is easily taken up by plants, is what agronomists call “highly mobile,” meaning it can move through the soil environment easily based on the conditions. So, growers also face the challenge of high nitrogen loss potential, contributing to the risk of income loss from under-fertilization. 

Quantifying fertilizer use efficiency—that is, the proportion of fertilizer nitrogen actually used by the plant, relative to the amount applied—is straightforward, but it takes time and access to a laboratory. Knowing a crop's nitrogen use efficiency can help growers maximize yield while limiting nitrogen pollution and save money. Therefore, exploring new ways to estimate it remains a challenge in agriculture.

Our drone-based index explains how nitrogen is used in a system

The concept of nitrogen use efficiency is founded on basic agronomic principles. That is, when plants receive more nitrogen, both their growth and nitrogen concentration increase. At some point, plants reach their growth limit, and nitrogen application only increases plant nitrogen concentration. The point at which plants’ growth stops is a threshold that agronomists use to determine the most efficient nitrogen application and use for a specific crop. Any additional fertilization is considered excess and may not be used by the plant. Previous research in various cropping systems has determined this threshold for different plants and in different areas of the world². Researchers can then place a specific sample above, on, or below this threshold based on its growth and nitrogen content. The ratio between a specific plant’s values and the model threshold is measured by the nitrogen nutrition index (NNI). 

NNI may seem like a complicated term, but it can easily be broken down. Nitrogen, we know. Nutrition is just that, i.e., the health of the plant. An index is just a metric for measuring. So, altogether, the nitrogen nutrition index is a measure of how healthy a plant is in terms of nitrogen. It is a way to quantify plant health, similar to human health metrics like blood pressure.

Our research team in the Department of Plant Sciences at UC Davis found that an NNI calculated from remote sensing data is not only an accurate proxy for actual plant growth and nitrogen concentration but may also support a better understanding of the plant's nitrogen use efficiency. We tested our drone-based NNI using commonly used nitrogen use efficiency metrics that explain both how fertilizer applied is taken up by the plant and how it is used within the plant. And the most exciting part is that we can now use this as a tool to get all this information almost instantly.

We can use remote sensing to measure important plant traits quickly and effectively

Researchers and growers have been using remote sensing—non-destructive data collection using various tools—to diagnose plant and environmental health for decades³. The new research we are conducting at UC Davis shows that we can use these tools to assign an actual value to plant nitrogen status rather than a relative one. Think of it like using a measuring tape to determine the length of a table to see if it will fit in your room, rather than just knowing it is bigger than the last one you had. 

We tested the ability to estimate plant growth and nitrogen by growing wheat and related crops over multiple years across diverse regions of California. We flew a drone over each field throughout the season. Every time our drone flew over a field, it took thousands of high-resolution images that capture not only what our eyes can see—the visible light spectrum—but also light beyond that, in the near-infrared and red-edge regions. 

We then fed these values, along with weather and other data, into a machine learning model that produced highly precise estimates of plant growth and nitrogen concentration, enabling us to calculate NNI. 

The NNI can be used to monitor nitrogen cycling within a farming system

The new research outlined here can help make a timely quantification of nitrogen use efficiency. This measurement, in combination with other environmental data, may help us estimate nitrogen losses. Consequently, it can help growers amend fertilizer plans and more accurately enforce regulations under initiatives such as the Irrigated Lands Regulatory Program.

To protect our drinking water and environment from nitrogen pollution without jeopardizing growers' livelihoods, we need accurate, easy-to-use tools to measure nitrogen. By using a remote sensing metric, such as the NNI presented here, to target fertilizer use efficiency rather than fertilizer use alone, we can identify fields that need improvement while protecting growers who are applying nitrogen fertilizer effectively to minimize losses.

In the long run, this tool may help refine policy measures and expedite research on nitrogen use efficiency, thereby improving the health of natural habitats and agricultural communities alike.

Maya Shydlowski is a PhD student in the Department of Plant Sciences at UC Davis. This research was conducted in the Grain Cropping Systems Lab with Dr. Mark Lundy.

¹ Central Valley Regional Water Quality Control Board, “Irrigated Lands Regulatory Program (ILRP),” accessed March 31, 2026; https://www.waterboards.ca.gov/centralvalley/water_issues/irrigated_lands/.

² Ignacio M. Rodriguez, Josefina Lacasa, Emmanuela van Versendaal, Gilles Lemaire, Gilles Belanger, Guillaume Jégo, Patricio G. Sandaña, et al., “Revisiting the Relationship between Nitrogen Nutrition Index and Yield across Major Species,” European Journal of Agronomy 154 (March 2024): 127079. 

³ Ben Belton, Leo Baldiga, Scott Justice, Bart Minten, Sudha Narayanan, and Thomas Reardon, “Can the Global Drone Revolution Make Agriculture More Sustainable?” Science 389, no. 6764 (September 4, 2025): 972–76.