Arthur Villordon, Don LaBonte and Tara P. Smith

Sustainable and profitable commercial sweet potato production requires optimum nitrogen fertilizer. Although sweet potatoes can be grown in marginal production environments, a certain amount of nitrogen is necessary for normal shoot development and photosynthetic activity required for storage-root growth. If the nitrogen supply is suboptimal, the nutrient becomes a limiting factor, and this condition generally reduces marketable yield. On the other hand, excessive or improperly timed nitrogen fertilizer applications can potentially decrease marketable yield and represent an unnecessary waste of resources.

The objective of nitrogen management is to maximize the use of the nutrient and minimize potential leaching and possible contamination of surface and ground water. The need for optimum nitrogen management information for Louisiana-grown sweet potatoes is especially critical in view of increasing farm input costs.

Several past studies of the nitrogen fertilizer requirement for sweet potatoes have concluded that yield response to nitrogen fertilizer varies from location to location and from year to year. This response has been attributed to variability in nitrogen soil availability, climate and sweet potato variety. This suggests that nitrogen management needs to be optimized for a specific variety grown under a specific range of production environments.

These approaches include delaying or "splitting" nitrogen fertilizer application. Delayed application means the entire amount of nitrogen fertilizer is side-dressed three to four weeks after transplanting. In a split application, a portion of the nitrogen fertilizer (typically half) is applied preplant, and the remainder is applied three to four weeks later. The positive yield response generally associated with these approaches is partly attributed to the fact that a high nitrogen level is not required for storage root initiation. Storage root initiation, defined as the appearance of secondary meristematic activity in adventitious roots, signals the onset of storage root development. In Beauregard, this critical developmental phase has been observed to occur as early as 13 days after transplanting (DAT).

Synchronizing storage root initiation with delayed nitrogen fertilizer application helps to explain the yield response associated with this management approach. In Louisiana, research has indicated that the favorable period for in-season nitrogen application is about three to four weeks after transplanting. To help explain this time frame, 15N-labeled nitrogen fertilizer was applied at various days after transplanting to determine how much applied nitrogen fertilizer was recovered in a specific period (Figure 1). 15N is a label or tracer that enables specialized equipment to detect and precisely quantify the presence of the labeled material in plant samples.

The results of this study indicated that maximum recovery of nitrogen by leaves occurred at 40 DAT. Recovery of nitrogen by storage roots gradually increased beginning at 23 DAT and was maximum at 40 DAT. Compared to preplant-applied nitrogen fertilizer, delayed or split application potentially increases the recovery of the nutrient from 31 percent (nitrogen fertilizer applied at 23 DAT) to 81 percent (nitrogen fertilizer applied at 32 DAT). Some earlier studies have suggested that applying nitrogen fertilizer past 40 DAT favored shoot growth at the expense of storage-root growth.

Bedding seed roots represents the initial phase of each year’s sweet potato production activities. Despite its importance, relatively little information is available about the role of bed-applied nitrogen fertilizer in crop performance. To document how plant bed-applied nitrogen fertilizer interacted with field-applied fertilizer, researchers incorporated 15N-labeled ammonium nitrate in plant beds and set transplants in plots applied with various rates of non-labeled nitrogen fertilizer. At the end of the season, storage-root and leaf samples were collected and analyzed for the presence of the 15N label (Figure 2).

Compared to the natural amount of 15N in control samples, only storage roots from plots applied with the lowest rate of nonlabeled nitrogen fertilizer showed a slight increase in the labeled compound. The results suggest that transplants (specifically the developing adventitious roots) used or metabolized relatively more of the plant bed-applied nutrient under a reduced rate versus higher rates of field-applied nitrogen. This indicates that the potential benefits of split or delayed in-season application of nitrogen fertilizer appear to be somewhat influenced by the fertility regime in plant beds, assuming other variables such as soil moisture are not limiting during the critical storage root initiation phase. The results also indicate the need to optimize the fertilizer regime in plant beds.

Results from this research can be used in future studies that seek to further reduce the total nitrogen fertilizer required to produce a profitable sweet potato crop. The apparent link between plant bed-applied nitrogen and its uptake and recovery by storage roots also provides clues about the role of the nutrient in adventitious root initiation, early storage-root development and yield.

Acknowledgment
Louisiana Sweet Potato Commission for providing funds for the 15N study.

Arthur Villordon, Associate Professor, Sweet Potato Research Station, Chase, La.; Don LaBonte, Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; and Tara P. Smith, Assistant Professor, Sweet Potato Research Station, Chase, La.