Sulfur (S) availability is closely associated with onion pungency. Previous research identified that sulfur is accumulating in the soils of the Vidalia onion region, particularly in the subsurface claypans. The depth of the claypan appears to play an important role in the ability of onions to access these subsurface sulfur deposits.
When sulfur is contained in relatively shallow claypans (less than 12 in.), it is easily accessible to plant roots, but a claypan deeper than 18 in. appears to be relatively inaccessible for much of the growing season.
This study seeks to determine the relative availability of sulfur from various soil profile depths by growing onions on raised beds with differing depths to the claypan and varying sulfur content.
Material and Methods
‘Sapelo Sweet’ and ‘Plethora’ onion varieties were grown on 21 raised beds at the Vidalia Onion and Vegetable Research Center during the 2022, 2023, and 2024 growing seasons. Raised beds (4 ft wide x 6 ft long x 30 in. deep) were constructed using pressure-treated lumber. Beds were filled with topsoil (A horizon) and claypan (Bt horizon) soils to differing depths using soil collected from an area of uncultivated land (Tifton soil series) at the research center. Soil profiles in the raised beds were constructed such that the claypan began at depths of 6, 12, or 18 in., or no claypan (24 in. topsoil) for the control. Gypsum (calcium sulfate) was spiked into the soil of the claypan layers prior to each season to produce low- and high-sulfur treatments.
The data collected included foliar nutrients, which were analyzed every 2 weeks between bulb expansion and maturity, plant available and total nutrients within each soil layer, depth of root penetration in the soil profile, both total plant and bulb yield, and concentrations of flavor-associated compounds in onion bulbs.
Results
In 2022 and 2023, the soil claypan represented a significant barrier to onion root penetration (Figure 2 and Table 1). The 2022 season was an unusually hot and dry year, particularly in the last few weeks prior to harvest, when plant growth and bulb expansion are the greatest. As a result, onions grown on beds with relatively shallow claypans (6 or 12 in.) had decreased water availability, which negatively affected yields. Neither claypan depth nor S application rate was associated with onion pungency in 2022, with pyruvic acid measurements ranging from 3.2–4.1 µmol/ml, all of which would be considered mild-flavored.
| Onion Variety | Treatment1 | Rooting depth (in.) | Bulb weight (g) | Pyruvic acid (µmol/g) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 2022 | 2023 | 2024 | 2022 | 2023 | 2024 | 2022 | 2023 | 2024 | ||
| ‘Sapelo Sweet’ | 6-in. low S | 8.0 b2 | 12.3 c | 14.0 | 178 d | 251 | 349 | 3.3 | 2.8 | 3.8 a |
| 6-in. high S | 8.7 ab | 12.7 bc | 16.0 | 168 cd | 242 | 423 | 4.2 | 2.7 | 3.9 a | |
| 12-in. low S | 15.3 ab | 14.3 abc | 14.3 | 196 bcd | 300 | 370 | 3.6 | 2.9 | 3.1 ab | |
| 12-in. high S | 11.7 ab | 12.7 bc | 17.3 | 205 bcd | 273 | 372 | 3.6 | 3.3 | 3.9 a | |
| 18-in. low S | 16.0 a | 17.7 ab | 16.7 | 232 abc | 282 | 393 | 4.0 | 3.2 | 2.9 ab | |
| 18-in. high S | 15.7 ab | 15.7 abc | 15.3 | 250 ab | 311 | 384 | 3.5 | 3.0 | 3.5 ab | |
| Control | 14.0 ab | 19.3 a | 17.7 | 274 a | 251 | 391 | 4.0 | 2.9 | 2.6 b | |
| p-value | < 0.05 | < 0.05 | 0.48 | < 0.05 | 0.10 | 0.4 | 0.67 | 0.74 | < 0.05 | |
| ‘Plethora’ | 6-in. low S | 98.0 ab | 14.0 a | 15.3 | 226 bc | 335 | 384 b | 3.2 | 2.5 | 2.6 |
| 6-in. high S | 8.3 b | 13.7 a | 15.3 | 195 c | 320 | 480 ab | 3.7 | 2.6 | 2.9 | |
| 12-in. low S | 11.0 ab | 14.0 a | 16.0 | 250 bc | 331 | 431 ab | 3.5 | 2.2 | 2.7 | |
| 12-in. high S | 11.3 ab | 14.3 a | 18.7 | 235 bc | 346 | 508 a | 3.9 | 1.8 | 2.9 | |
| 18-in. low S | 14.3 ab | 18.7 a | 17.3 | 293 ab | 282 | 397 ab | 3.7 | 3.4 | 2.7 | |
| 18-in. high S | 16.0 a | 16.7 a | 15.7 | 268 abc | 261 | 484 ab | 4.0 | 2.5 | 2.6 | |
| Control | 14.3 ab | 18.7 a | 16.7 | 341 a | 311 | 402 ab | 4.1 | 1.9 | 2.3 | |
| p-value | < 0.05 | < 0.05 | 0.33 | < 0.05 | 0.48 | < 0.05 | 0.84 | 0.56 | 0.30 | |
| † Onion rooting depth was estimated by the maximum depth of the majority of roots. ¹ Treatment denotes the depth of the claypan (soil Bt horizon) and the sulfur application rate. For example, the 12-in., high-S treatment had a claypan at a depth of 12 in. in the soil profile that was treated with a high rate of sulfur. ² Means followed by the same letter in each column are not significantly different according to Tukey’s HSD at p < 0.05. | ||||||||||
Irrigation frequency was increased in 2023, and while rooting depth was still statistically different among treatments, root penetration was deeper overall, and no differences were seen in yield among treatments. Onion pyruvic acid content was also lower in 2023 compared to 2022, suggesting the higher irrigation may have leached more S from the rooting zone, or the cooler temperatures may have resulted in lower S uptake. Onion pungency was not statistically different among the treatments in 2023.
Soil moisture sensors installed during the 2024 season revealed the raised beds were drying much quicker than soil in neighboring field plots, and irrigation frequency was increased. As a result of increased irrigation, no differences were seen in onion rooting depth among treatments, as roots were able to penetrate the claypan under adequate soil moisture.
No differences in yield were observed for ‘Sapelo Sweet’, while for ‘Plethora’, the 6-in. claypan with low-S treatment produced significantly lower yields than the 12-in. claypan with high-S treatment.
Unlike previous seasons, the pyruvic acid content was different among the treatments, with the ‘Sapelo Sweet’ onions grown on the control treatment beds (with 24 in. of topsoil) having lower pungency than onions under 6-in. low-S, 6-in. high-S, and 12-in. high-S treatments.
Foliar S concentrations were highly variable based on timing of sample collection in all three growing seasons (Figure 4; data from 2022 and 2023 are not shown). In both the 2023 and 2024 seasons, onions grown on high-S treatments accumulated more S than the low-S and control treatments. These differences were first observed in late March to early April each season, suggesting that throughout the bulb expansion and maturation period, onions are able to accumulate nutrients contained within claypans as deep as 18 in.
Conclusion
Claypan depth plays a significant role in the availability of water and nutrients within the soil profile. Under a water deficit, the claypan acts as an impenetrable layer preventing further root penetration. This restriction of water and nutrients can affect onion yield and nutrient status.
Under adequate soil moisture, onion roots are able to penetrate deep within the soil profile and access the water and nutrients contained within the claypan layer. Onion yields are stabilized across claypan depths under these free-moisture conditions; however, onion pungency becomes a factor—increased sulfur availability, combined with genetic and other environmental factors, can result in higher onion pungency.
