Two peer-reviewed studies examine biological nitrogen fixation, drought conditions, and context-specific fertilizer management

FRANKFORT, Ky. — Soybean is different from many other major crops because it can partner with soil bacteria to produce much of the nitrogen it needs.

That biological advantage can reduce the need for added fertilizer, but it also raises a practical management question: When does supplemental nitrogen help, and when does it simply add cost without improving yield?

Two recent peer-reviewed publications from Kentucky State University examine that question from two perspectives. One study tests supplemental nitrogen applications under Kentucky rainfed field conditions. A companion review examines biological nitrogen fixation in soybean and how the process supports productivity, sustainability, and future crop improvement.

The studies were led by Manish Pandit under the mentorship of Dr. Anuj Chiluwal, assistant professor of agronomy at Kentucky State. Dr. Chiluwal served as corresponding author on both papers.

Together, the publications reinforce the importance of context-specific nitrogen management, especially in rainfed, drought-prone, and low- to moderate-yield environments.

The field study reported results from a two-year experiment at Kentucky State’s Harold R. Benson Research and Demonstration Farm. Pandit evaluated supplemental nitrogen applications at different reproductive stages in soybean under rainfed conditions.

Across two growing seasons marked by extended dry periods during pod formation and seed filling, the team found no significant benefit from a single nitrogen application on plant growth, yield, or seed composition.

“From a farmer’s perspective, the message is clear,” Pandit said. “Under drought-prone and low- to moderate-yield conditions like we saw in our trials, nitrogen top dressing during reproductive stages is unlikely to provide a positive return or pay for itself. The crop already relies heavily on nitrogen fixation and internal nitrogen cycling, and without adequate moisture, extra fertilizer simply does not translate into increased yield or quality.”

Dr. Chiluwal said the findings reinforce the need to avoid one-size-fits-all fertilizer recommendations.

“Many earlier studies that reported strong responses used higher nitrogen rates, split nitrogen applications, or fully irrigated environments,” Dr. Chiluwal said. “Our work shows that in a typical rainfed field where water is a major limiting factor, late-season nitrogen is not the ultimate solution. It reinforces why blanket nitrogen fertilizer recommendations for soybean do not work across all environments.”

Pandit’s second paper, “Biological Nitrogen Fixation in Soybean: Mechanisms, Benefits, Sustainability, and Future Prospects,” takes a broader view of how soybean plants rely on their symbiotic relationship with Bradyrhizobium to support productivity and sustainability.

The review notes that well-nodulated soybean typically derives about 50 to 70 percent of its nitrogen from biological nitrogen fixation during the growing season. That process can reduce reliance on energy-intensive synthetic fertilizers, lower greenhouse gas emissions, and leave nitrogen credits for crops that follow soybean in rotation.

At the same time, the review explains that modern high-yielding soybean cultivars can experience a nitrogen gap when crop demand during peak reproductive growth exceeds what biological nitrogen fixation and soil nitrate can supply. In those cases, targeted nitrogen supplementation may be justified, particularly in high-yield systems where fertilizer complements the plant’s natural nitrogen-fixing capacity rather than suppressing it.

“In most low- to moderate-yield situations, well-managed nodulation and soil nitrate are enough, and extra fertilizer often just replaces what the plant would have fixed on its own,” Pandit said. “But as breeders push higher yields, crop demand can exceed the biological supply. Our review emphasizes that we have to close that gap in ways that do not hurt nodulation or create more environmental problems.”

Dr. Chiluwal said pairing the field study with a global synthesis was intentional.

“The field study gives us a very concrete, producer-relevant context showing why nitrogen applications may not be useful under low-yielding and drought-stressed conditions,” he said. “The review reminds us that the real long-term leverage comes from strengthening biological nitrogen fixation and then using nitrogen fertilizer only when it truly complements, rather than competes with, the soybean plant’s natural symbiosis.”

Pandit recently completed his master’s degree at Kentucky State. The publications follow a strong finish to his graduate work at Kentucky State, where he successfully defended his master’s thesis and was recognized as an Outstanding Graduate Student by the School of Agriculture and Natural Resources.

Pandit published “Impact of supplemental nitrogen application at different reproductive stages on soybean yield and seed composition” in the Journal of Agriculture and Food Research. He also published “Biological Nitrogen Fixation in Soybean: Mechanisms, Benefits, Sustainability, and Future Prospects” in Agronomy, and the article was selected as the cover story for the journal issue.

Co-authors on the field study include Anjan Timilsina, graduate research assistant; Dr. Theoneste Nzaramyimana, assistant professor for urban agriculture; and Dr. Suraj Upadhaya, assistant professor of sustainable systems. Surekha Panthi, graduate research assistant, co-authored the review article.

Both studies were supported by a USDA-NIFA 1890 Capacity Building Grant (Award Number 2023-38821-39960) and a USDA-NIFA Evans-Allen Grant (Accession Number: 7004460).