(04/23/19) WINNSBORO, La. — Candied, baked, boiled, roasted, mashed or fried, sweet potatoes are a culinary staple in Louisiana.

The versatile vegetable is available in a wide array of colors, shapes and sizes as a result of extensive research and breeding protocols designed to match develop what consumers want.

LSU AgCenter professor Arthur Villordon is researching how growers can manipulate agricultural inputs, such as fertilizer, to control the shape of the sweet potato during growth.

“Knowledge about factors that determine sweet potato storage root shape is of scientific and practical importance, and growers want a predictable storage root shape,” Villordon said.

In the processing industry, especially in french fry production, more uniform, round-shaped roots are desirable, producing more uniform slices.

Consistency in root shape is also needed for increased mechanization, leading to overall reduced cost of production.

Because root shape is essentially determined by length and width, Villordon’s research goal is to control the downward root growth, or length, stopping it at some point and then focusing on a subsequent increase in root diameter.

“Learning more about the variables that determine root length and width will lead to production practices that determine the shape of the sweet potato,” he said.

Past data indicated that potassium is responsible for increases in diameter, Villordon said.

Based on findings in other plant species, growing evidence suggests that phosphorus is a likely candidate for determining root length.

Focusing on fertilizer inputs, Villordon is looking for the key nutrient associated with root length.

“We were quite lucky to find prior research in a model system where phosphorus availability was shown to control the plant root’s downward growth,” Villordon said.

Phosphorus is a relatively expensive fertilizer input and can be a source of nonpoint source pollution, so optimizing its use can potentially increase production efficiency and reduce environmental impact, Villordon said.

A review of existing recommendations for phosphorus fertilization indicates significant differences in phosphorus requirements among sweet potato varieties.

In a greenhouse study, Villordon is testing the simultaneous effects of phosphorus and slip length, two variables that farmers can manipulate to help improve shape consistency.

Villordon has shown that some sweet potato varieties are more efficient at acquiring phosphorous from the soil.

“We are almost to a point where we can recommend to the grower the appropriate amount of phosphorus to apply in the field based on the variety,” he said.

Existing recommendations for phosphorus fertilization can be reduced by as much as 30% in some of the newer varieties such as Bayou Belle while maintaining yield levels, he said.

Because phosphorus levels vary across fields depending on what crop was grown the year before, information from soil testing is used to adjust fertilizer recommendations so growers can more precisely apply the amount needed based on the variety.

Only a few tools and methods have been available until recently to capture three-dimensional measurements for sweet potato shape.

New experimental procedures to capture 3D models of root shape using low-cost 3D scanners and open-source software have been successful.

“This focus on phosphorus inputs has increased knowledge of optimum phosphorus fertilization rates in relation to soil phosphorus tests, and some growers are already adopting variable-rate application methods,” Villordon said.

Results from his two-year greenhouse study have been published in the American Society for Horticulture Science. This information is guiding current field trials at the LSU AgCenter Sweet Potato Research Station near Chase.

LSU AgCenter professor Arthur Villordon conducts research to understand how sweet potato growth can be controlled by adjusting phosphorus rates at planting. Greenhouse research trials conducted at the LSU AgCenter Sweet Potato Research Station near Chase will help determine guidelines for use in variety field trials this year. Photo by Karol Osborne/LSU AgCenter

By controlling the sweet potato length and diameter during growth stages, farmers can produce a more uniform potato desired by both consumers and the sweet potato processing industry. Photo by Karol Osborne/LSU AgCenter