Previous research has shown that variability in soil characteristics within fields directly relates to the flavor profile of Vidalia onions produced. Several factors have been identified that play a role in this process, including sulfur fertility, soil clay content, depth to the claypan, soil pH, and organic matter content. Each of these characteristics affects sulfur availability within the rooting zone.
While the link between these individual attributes and onion flavor appears to be straightforward, a substantial degree of variability exists among these characteristics across the Vidalia production region, which complicates our ability to predict onion flavor based on specific soil conditions. This study seeks to understand the extent of variability in soil properties across the Vidalia production region, and how those differences are related to onion yield and pungency.
Material and Methods
In 2023, 97 soil and onion samples were collected from 13 fields distributed across the Vidalia region (representing six counties). Each sample represented approximately 10 acres, translating to roughly 10% of the industry. Sampling locations were selected to represent the relative distribution in soil types in the region.
At each location, soil profiles were described and photographed, foliar samples were collected for nutrient analysis, onion bulbs were sampled for yield and flavor profile, and soil samples were collected at multiple depths for analysis of organic matter, texture, pH, and plant available and total nutrient content.
The study was repeated in 2024, with 117 samples collected from 17 fields representing approximately an additional 1,000 acres, along with resampling about 200 acres from the 2023 season to quantify any seasonal variability in the data.
Results
While typically very sandy, soils within the Vidalia onion region can be highly diverse, as illustrated in the photos in Figure 1. The most common soil in the region is the Tifton series, which is generally characterized by a sandy loam surface horizon above a claypan that normally begins at a depth of 8–20 in. and has abundant ironstones within the upper soil horizons. However, even within the same soil series classification, differences in surface texture, claypan depth, and the size and abundance of ironstones can greatly influence water and nutrient availability.
Other soils identified were extremely sandy and contained no claypan within the upper 50–60 in. of soil, while soils from low-lying areas often drained poorly and had a very shallow water table (see grey-colored subsurface horizon soils in Figure 1). These factors influence how onions access water and nutrients.
As shown in Table 1, fertilization rates, claypan depths, and onion yield and pungency were highly variable across the growing region in both the 2023 and 2024 seasons.
Particularly dramatic were the differences in fertilization rates, where reported sulfur fertility rates differed by 72 lb/acre in 2023 and 95 lb/acre in 2024, and nitrogen rates varied by 91 lb/acre in 2023 and 86 lb/acre in 2024. Claypan depth was also highly variable, ranging from extremely shallow (1 in. for two samples in 2024) to over 50 in.
Onion pungency was highly irregular in both seasons; some onions were exceptionally mild while others were relatively pungent. Onion lachrymatory factor was substantially lower in 2024 compared to 2023, which is likely climate-related.
| Year | Value level | S applied (lb/acre) | N applied (lb/acre) | Yield (g/bulb) | Claypan depth (in.) | Pyruvic acid (µmol/ml) | Lachrymatory factor (µmol/ml) |
|---|---|---|---|---|---|---|---|
| 2023 | Min | 26 | 75 | 27 | 7 | 2.8 | 0.5 |
| 2023 | Average | 63 | 113 | 238 | 19 | 4.6 | 2.9 |
| 2023 | Max | 98 | 166 | 437 | > 50 | 7.2 | 6.8 |
| 2024 | Min | 44 | 62 | 112 | 1 | 2.8 | 0.4 |
| 2024 | Average | 65 | 113 | 221 | 15 | 4.7 | 0.8 |
| 2024 | Max | 139 | 148 | 318 | > 50 | 6.5 | 1.3 |
Variety selection is known to be an important contributor to onion pungency, and that was supported by this study. Despite some variation, pyruvic acid content was generally lower in some varieties, like Vidora, compared to other varieties like Tania (see Figure 2).
We know from many previous studies that onion pungency and sulfur fertility are tightly linked, and higher sulfur-fertilizer application rates produce more pungent onions. However, in both the 2023 and 2024 growing seasons, onion flavor profile was not related to the amount of reported sulfur in the grower’s fertility program (Figure 3). This indicates that sulfur fertilization rates alone are not determining onion pungency in the Vidalia region.
While 2023’s values were not quite statistically significant (p = 0.078), in the 2024 season, onion flavor—as measured by pyruvic acid, onion lachrymatory factor, and methyl thiosulfinates—was highly negatively related to soil claypan depth, indicating that onions grown on soils with a deep claypan were generally milder in flavor (Figure 4).
As Figure 5 demonstrates, onion pungency was somewhat variable based on soil type, but soil series classification alone is not enough to predict onion pungency. For example, the most common soil type sampled was the Tifton soil series, which produced both extremely mild and more pungent onions.
Conclusion
Onion pungency is complex and is determined by a combination of many factors, including onion variety, soil characteristics, weather conditions, and the grower’s cultural practices, including their fertility program. While soil characteristics, like claypan depth, clearly play a major role in onion productivity and flavor profile, soil series classification alone is not sufficient to describe how soil characteristics influence onion yield and pungency. Further research is needed to determine the interactions and contributions among these factors toward onion pungency.
Learn more and stay up to date at our onion mapping project website (https://aesl.ces.uga.edu/OnionMapping).
