Onion Sweetness: Soil Conditions, Fertilizers Affect Vidalia Flavor

Clint Thompson Georgia, Onion, Top Posts

UGA CAES photo: With high-margin vegetable crops such as Vidalia onions — where an increased yield on a smaller plot can mean much higher profits — determining whether fertilization rates affect quality and yield could make the case for investing in precision agriculture technology to produce a better-quality product.

Source: UGA CAES Newswire

University of Georgia Agricultural and Environmental Services Laboratories (AESL) researchers are ensuring sweetness is a quality of all Vidalia onions.

Vidalia onions are known for their sweetness because of the low sulfur environment of the sandy Coastal Plain soils in South Georgia where they are grown, said Daniel Jackson, manager of the Crop Quality Lab, a unit within AESL in the College of Agricultural and Environmental Sciences.

Field Features

With around 30 approved varieties of Vidalia onions produced, Jackson and fellow researchers are trying to determine how to ensure consistently sweet onions — in every field, under various growing conditions, in every harvest season.

“We’re talking about 20 counties where Vidalia onions are grown, all of which have similar soil types. But there’s a lot of variability, even within a single field. The main point of this research is to determine if the in-field variability of soil characteristics is enough to change onion pungency or onion flavor profile,” said Jason Lessl, program coordinator of the AESL’s Soil, Plant and Water Analysis Laboratory.

“What we’ve seen is that these soils are sandy on the surface — the top 8 to 20 inches — almost like beach sand. Below this sandy soil is a thick clay layer that can loosely absorb certain nutrients, like sulfur.  As the sulfur moves down through the soil profile, it can build up in the clay layer, which then provides a large reservoir of sulfur for the onion. We hypothesize that this pool of sulfur in the clay can be problematic for onion flavor quality, the shallower it is, since the onion roots will have easier access to it,” he said.

Many fertilizer inputs contain sulfur, and while sulfur is a critical nutrient for all plants, including onions, surplus sulfur in the soil is readily absorbed by onions. With sandy soils, sulfur leaches deeper into the soil, distributing the nutrients and preventing excess accumulation. A shallow claypan — a dense layer of the subsoil with a much higher clay content than the overlying material — limits or slows the downward movement of water and some nutrients, like sulfur, through the soil making them more readily available for uptake into the plants.

“That’s when you get pungent onions,” Jackson said. “Sulfur is a mobile nutrient, and readily moves through sandy soils. However, sulfur binds to clay, so if you apply a lot of sulfur to soils with high clay content, the sulfur can stick around and the excess is accumulated by the onions, which affects the onion flavor. The big picture purpose of this research is to figure out how important this process is to producing sweet onions and how much it varies within a single field or among fields within the region due to differences in soil type.”

Precision Technologies

Working with three different fields, the AESL crop quality team tested the soil in each 20- to 25-acre field in half-acre grids. It collected a soil sample from the center of each grid before the onions were planted to know what the background growing environment is for each section of each field.

The team used a probe to measure the electrical conductivity signature of the soil in each field and worked with colleagues in UGA’s Enterprise Information Technology Services and the Warnell School of Forestry and Natural Resources to survey the fields using drones with multiple types of sensors to create a visual, 2-D map of soil conditions in each field just before planting.

“We’ve got one field we’re working on that has six different soil types within 25 acres, so it’s really variable in terms of soil characteristics. We’re trying to figure how much that matters to onion pungency with the ultimate goal of developing management practices to suit all of those specific soil conditions,” Jackson said.  

Once the onion plants were well-established and growing in each field, the EITS and Warnell team members used drones to measure the vegetative index to determine the health of the plants.

“Specific wavelengths of light can show you where the plants are healthy and where they are not as healthy within each field. This helps us to see if there’s an overlap between onion performance and soil characteristics based on those measurements,” Jackson said.

UGA CAES photo/UGA Crop Quality Lab manager Daniel Jackson with a load of research samples from the Vidalia Onion Research Center.

Balancing Act

Past research has shown that various environmental conditions can affect the onion pungency. These include irrigation, soil temperature, and both nitrogen and sulfur application rate. However, sulfur application rate is the leading factor.

“Sulfur is the No. 1 thing that influences onion flavor. Nitrogen and other environmental factors do play a role, but they don’t seem to be a consistent determining factor like sulfur,” Jackson said. “The crux of this is that we’re trying to basically create a tool growers could use to characterize their soil if they want sweet onions. The whole point is to help them with a management program that will allow them to grow consistently sweet onions across the region regardless of their specific soil conditions.”

No More Tears

In May, the team collected onion bulbs from each grid in each field. They will test the nutrient profile of each sample to check levels of accumulated nutrients in the plant tissues and seed bolts, as well as measuring the lachrymatory-factor — the compound that causes your eyes to tear when you cut an onion — as well as the methyl thiosulfinates — the sulfur-containing compounds responsible for onion breath.

“When you eat an onion, those are the compounds that create the burning sensation in your mouth. They kind of have a metallic, grassy green flavor. All the kinds of things that stick with you,” Jackson said. “Those parameters give us a snapshot of how pungent or how intense the flavor is, with the ultimate idea of trying to find out if there’s a correlation between soil conditions and onion flavor.”

Last year’s study showed a correlation between the quality, the yield and the production of these onions based on soil type. If this year’s tests show similar correlations, future research will be focused on how to manage production in each soil type.

“So, if you have shallow clay, how should we manage those areas versus the areas where you have deep clay? At that point we could potentially move into using precision agriculture techniques to control sulfur applications and get a more consistent crop,” Jackson said.

For more information on research at CAES, visit caes.uga.edu/research.