Cotton growers in South Carolina are being advised to reconsider their nitrogen fertilizer practices following a comprehensive study by Clemson University. The research indicates that many farmers may be applying excessive nitrogen, which not only incurs unnecessary costs but also poses environmental risks. The findings suggest a shift towards “site-specific” nitrogen management rather than implementing uniform recommendations across diverse agricultural fields.
Led by soil fertility specialist Bhupinder Farmaha, the three-year study encompassed 50 trials throughout various regions, including the Coastal Plain, Flatwoods, Sandhills, and Piedmont. The team evaluated nitrogen requirements at each location, taking into account factors such as soil type, crop history, tillage methods, and long-term conservation practices like cover crops and poultry litter application.
The current statewide guidelines recommend between 70–120 pounds of nitrogen per acre without specific instructions on application timing or location. However, the study revealed that a significant 68% of the fields examined required less nitrogen than these guidelines suggest. Notably, some fields in the Piedmont region demonstrated a complete lack of need for nitrogen fertilizer to achieve optimal yields.
Farmaha highlighted the variation among different farms and regions, stating, “Using the same fertilizer plan everywhere just isn’t serving growers anymore.”
The research underscores the critical role that soil type plays in determining nitrogen needs. For instance, sandy soils prevalent in the Coastal Plain often experience nitrate leaching. Approximately two-thirds of the fields in this area required nitrogen, but typically less than what growers currently apply. In contrast, soils in the Flatwoods region, which tend to have more organic matter, frequently required little to no fertilizer.
In the Piedmont region, characterized by clay soils, all five study sites showed no positive response to additional nitrogen. Many of these farms had utilized poultry litter for years, significantly boosting soil nitrogen levels.
One of the most significant findings of this study is the impact of long-term conservation practices on cotton yields. Farms that incorporated cover crops or poultry litter for over ten years achieved higher yields with minimal to no added nitrogen. Fields with long-term cover crops averaged 1,160 pounds of cotton lint per acre without any nitrogen, compared to just 690 pounds on fields lacking such practices. Farmaha noted, “That’s about a $350-per-acre difference in lint value, before factoring in fertilizer savings.”
The financial implications for growers are substantial. With nitrogen priced at approximately $1 per pound, reducing application rates from 90 to 45 pounds per acre could save farmers $45 per acre. For larger operations, such adjustments could lead to savings of tens of thousands of dollars. For example, cutting rates on 500 acres from 90 to 45 pounds could yield savings of $22,500.
To help growers determine if they are using excessive nitrogen, Farmaha recommends implementing a zero-nitrogen check strip in fields this season. If the strip produces yields comparable to the fertilized sections, it indicates that too much nitrogen is being applied. The study found that 48% of the zero-nitrogen strips yielded similarly to fertilized plots.
The research team has outlined several practical steps for growers to personalize their nitrogen management plans. These include:
1. Understanding regional soil types to refine nitrogen application ranges.
2. Tracking field conservation histories, as long-term practices significantly reduce nitrogen needs.
3. Conducting soil nitrogen tests before planting by sending samples to the Clemson Agricultural Service Laboratory.
4. Considering the previous year’s crops, as crops like soybeans and peanuts leave more nitrogen in the soil than cotton.
5. Adjusting based on tillage methods; no-till fields typically require less nitrogen than strip-till fields due to higher soil nitrate availability.
6. Splitting nitrogen applications on sandy soils to minimize losses and optimize crop access.
The Clemson Agricultural Service Laboratory offers nitrate-nitrogen testing, with additional ammonia testing available for a fee. For details, farmers can refer to the laboratory’s soil testing webpage.
Given that soils across neighboring states such as Georgia, North Carolina, and Alabama share characteristics similar to those in South Carolina, these findings may be relevant to cotton growers throughout the Southeastern United States. Additionally, modern cotton varieties are more efficient in nitrogen usage than older strains, suggesting that traditional recommendations may no longer apply.
Looking ahead, Clemson Extension plans to provide practical tools to assist growers in determining field-specific nitrogen needs. Initiatives include developing online nitrogen rate calculators, establishing on-farm demonstration sites, and expanding check-strip trial networks.
Farmaha emphasized, “We want growers to feel confident reducing nitrogen when the data indicates it’s safe. This study shows we have a tremendous opportunity to improve profits while protecting our natural resources at the same time.”
Contributing to this research, Shikha Dubey, a post-doctoral researcher at Clemson University, brings an international perspective to the project. Dubey’s work is focused on refining nitrogen recommendations for cotton, addressing existing guideline shortcomings, and developing strategies that reflect the variability in soil types and management practices.
Having joined Clemson in January 2021, Dubey expressed enthusiasm about the hands-on experience gained through this project, which aligns with her interests in soil fertility and nutrient management. She aims to continue her research in soil science and precision agriculture.
The project, focusing on site-specific nitrogen recommendations for cotton, is funded by Cotton Incorporated, the South Carolina Cotton Board, and the USDA-Natural Resources Conservation Service.
