Brenda S. Tubaña, Dennis Burns, Marilyn Dalen, Daniel Forestieri and Ralph L. Frazier Jr.

Nitrogen is one of the 14 mineral nutrients essential to plants. It is considered the most limiting among these nutrients mainly because plants’ requirement for nitrogen is high. On average, nitrogen makes up about 5 percent of total plant dry matter. It is required for the production of chlorophyll molecules, which are pigments that help harvest sunlight energy that is converted to chemical energy through photosynthesis. Plants also use nitrogen to produce amino acids, the building blocks of proteins. The pivotal role of nitrogen in plants is for growth and development of vital plant tissues and cells. This is why the symptoms of nitrogen deficiency in plants are evident, especially during the vegetative growth stage. Nitrogen deficiency in sugarcane leads to stunted growth with leaves that are pale green or yellow. This leaf chlorosis in a nitrogen-deficient plant begins at the lower canopy or older group of leaves.

Nitrogen availability in the soil can be compromised by several transformation processes. Many of these can lead to nitrogen losses to the atmosphere and to surface and ground water. For this reason, and knowing that the value of nitrogen in crop production is absolute, growers have used liberal applications of nitrogen fertilizer on many crops to avoid deficiency and yield losses. Apart from the fact that excessive nitrogen supplied to sugarcane can also result in yield reduction, such an approach is not economically and environmentally sustainable.

About 2 pounds of nitrogen are taken up per ton of sugarcane stalks produced. Based on this removal rate, 60 to 120 pounds of nitrogen per acre are typically applied to meet the crop’s nitrogen requirement. Moreover, farmers’ standard practice is to uniformly apply the recommended nitrogen rate to the entire field. Research was initiated a few years ago to refine this approach by using the well-established concept of a plant’s quick and evident response to nitrogen. Using a nitrogen-rich strip as a diagnostic tool involves establishing a small portion of a field that is well-fertilized with nitrogen a few weeks before the scheduled application of fertilizer to the entire field.

Ideally for sugarcane, the nitrogen-rich strip should be established in early March or at least three weeks before the scheduled nitrogen application, which is commonly done during April. Photo 3 shows a five-row-by-600-foot nitrogen-rich strip in a producer’s field in Napoleonville, Louisiana, four weeks after it was fertilized with 120 pounds nitrogen per acre. The plants growing on the nitrogen-rich strip are greener than the rest of the plants, suggesting a low supply of nitrogen in the soil; therefore, sugarcane will likely benefit from applied nitrogen. On the other hand, if the nitrogen-rich strip is not visible, this suggests enough nitrogen may be present, and producers should consider reducing the rate of nitrogen fertilizer to be applied.

The latter part of the research involved pairing a nitrogen-rich strip with remote sensing to develop the technology capable of applying nitrogen fertilizer on a plant-need basis. The nitrogen-rich strip provides the platform for estimating nitrogen available from the soil while the remote sensor gives corresponding values for what the soil can supply. These values are used together to estimate appropriate nitrogen application rates. Unlike the traditional soil and plant testing procedures, nitrogen recommendations from this technology are obtained from real-time, site-specific information.

Trials were conducted on producers’ fields in Napoleonville and at the LSU AgCenter Sugar Research Station in St. Gabriel from 2013 to 2015 to compare the performance of this technology against the farmers’ standard nitrogen practices. The remote sensor component of the technology takes the canopy readings, called normalized difference vegetation index or NDVI (Photo 4a). The technology instantly computes the difference between the nitrogen-rich strip NDVI and the NDVI of the sugarcane from the rest of the field on-the-go. This approach accounts for variability in the field as opposed to the uniform nitrogen rate application under the farmers’ standard practice and distributes different rates of nitrogen fertilizer based on NDVI values in the field (Photo 4b).

The trials showed the potential of the nitrogen-rich strip and remote sensing technology in improving both nitrogen fertilizer use efficiency and profitability in sugarcane production (Figure 1). Across the three cropping years, the reduction in applied nitrogen was 18 percent for the Dugas site, 43 percent for the Gravois site and 49 percent for the research station site. Overall, this is about a 36 percent reduction, which is equivalent to 40 pounds of nitrogen fertilizer saved per acre. The 49 percent reduction in applied nitrogen at the research station rendered a 1.4 percent reduction in sugar yield. This loss was offset by the savings from applying less nitrogen fertilizer, making a positive net return of $27 per acre. The Dugas site had a 2.6 percent increase in sugar yield, and the Gravois site had a 4.1 percent increase in sugar yield, despite reductions in applied nitrogen. This resulted in significant improvements in returns at $156 per acre for the Dugas site and $315 per acre for the Gravois site.

Applying excessive nitrogen fertilizer to compensate for any nitrogen losses that may or may not occur later in the season does not guarantee high crop yields. In sugarcane production, excess amounts of applied nitrogen often result in lower sugar yield because of reduced sucrose content, lodging, and increased pest and disease presence. For this reason, it is imperative that the appropriate amount of nitrogen is distributed where it is needed in the field to maximize yield and profitability, while minimizing nitrogen losses from the soil.

A nitrogen-rich strip is a stand-alone diagnostic tool to determine the soil’s ability to supply nitrogen. When combined with remote sensing, the resulting technology can adjust the nitrogen fertilizer rate recommendation to meet plant needs. This technology is efficient and precise because it considers all the site-specific factors that are highly influenced by nitrogen dynamics in the soil.

Brenda S. Tubaña holds the Jack E. and Henrietta Jones Professorship in the School of Plant, Environmental, and Soil Sciences. Dennis Burns is an extension agent in Tensas Parish. Marilyn Dalen is a postdoctoral researcher in the School of Plant, Environmental, and Soil Sciences. Daniel Forestieri is a graduate student in the School of Plant, Environmental, and Soil Sciences. Ralph L. Frazier Jr. is an extension agent in Madison Parish.

(This article appears in the winter 2019 issue of Louisiana Agriculture.)

Brenda Tubaña, professor in the School of Plant, Environmental and Soil Sciences, was the winner of the 2018 G&H Seed Company Research Award, which recognizes an LSU AgCenter researcher who has conducted exemplary work during the past five years. Photo by Olivia McClure

Keith Dugas was one of the sugarcane growers who allowed his field to be used in the demonstration trials. To his right is a five-row-by-600-foot nitrogen-rich strip that is noticeably greener in color than the other rows in his field. Photo by Marilyn Dean

Figure 1. Change in sugar yield, amount of nitrogen fertilizer saved, and profit of sugarcane that received nitrogen recommendation from the nitrogen-rich strip and remote sensing technology. Data was averaged over three years and compared to farmers' standard nitrogen practice.

Maps of normalized difference vegetative readings (a) and nitrogen fertilizer applied (b) to a sugarcane field demonstration trial in Napoleonville, Louisiana. White arrows point to the nitrogen-rich strip.