Recent studies on fertilizer requirements for Vidalia onions have demonstrated the potential to reduce recommended nitrogen (N) fertilizer rates without affecting yields. Furthermore, understanding N requirements of onions at different growing stages is crucial to adequately manage N fertilizer applications and enhance fertilizer use while minimizing nutrient losses. In this study, the fertilizer N use efficiency (FNUE) of onions was investigated. The goal is to create a guideline for the timing and rate of N fertilizer applications to help onion growers in Georgia make fertilizer management decisions.
Material and Methods
Field experiments were conducted in 2021 and 2022 at the UGA Vidalia Onion and Vegetable Research Center located in Lyons, GA. Vidalia onions (cv. Vidora) were transplanted on Dec. 10, 2020, and Dec. 7, 2021. Onions were grown using standard spacing practices. Experimental plots were 20 ft long with 10-ft buffers between plots within each bed. Crop management practices associated with soil preparation, transplanting, and management of pests, weeds, and diseases followed UGA recommendations.
Treatments consisted of five ¹⁵N fertilizer application timings (see Table 1): transplanting (¹⁵NT), vegetative growth (¹⁵NV), bulb initiation (¹⁵NBI), bulb swelling (¹⁵NBS), and prematuration (¹⁵NBM). We used ¹⁵N ammonium nitrate (¹⁵NH₄¹⁵NO₃), which is a stable isotope of N that can be traced in our samples and used to determine the crop N uptake from applied fertilizer. The ¹⁵N was applied in each application-timing treatment at a rate of 21 lb N/acre for a total of 105 lb N/acre. Plots receiving the ¹⁵N fertilizer also received 21 lb/acre of unlabeled (regular ammonium nitrate) fertilizer in the other four applications. For example, when ¹⁵N fertilizer was applied at transplanting, the next four application timings received unlabeled N fertilizer.
| Treatments | N rate (lb/acre) | |||||
|---|---|---|---|---|---|---|
| NT | NV | NBI | NBS | NBM | Total N | |
| 15NT | 21* | 21 | 21 | 21 | 21 | 105 |
| 15NV | 21 | 21* | 21 | 21 | 21 | 105 |
| 15NBI | 21 | 21 | 21* | 21 | 21 | 105 |
| 15NBS | 21 | 21 | 21 | 21* | 21 | 105 |
| 15NBM | 21 | 21 | 21 | 21 | 21* | 105 |
| *The time 15N fertilizer is being applied; NT = N applied at transplanting; NV = N applied at vegetative stage; NBI = N applied at bulb initiation; NBS = N applied bulb swelling; NBM = N applied at prematuration. | ||||||
Onions were harvested on April 20, 2021 (131 DAT), and April 19, 2022 (133 DAT; Table 2), then cured and graded according to USDA standards. Statistical analyses were performed to compare the marketable yield and bulb size distribution, the amount of N in the plant derived from fertilizer (Ndff), N leaching, and fertilizer N use efficiency (FNUE) among treatments.
| Year | Days after transplanting (DAT) | |||||
|---|---|---|---|---|---|---|
| 15NT | 15NV | 15NBI | 15NBS | 15NBM | Harvest | |
| 2021 | 5 | 27 | 49 | 78 | 99 | 131 |
| 2022 | 9 | 30 | 57 | 78 | 99 | 133 |
| NT = N applied at transplanting; NV = N applied at vegetative stage; NBI = N applied at bulb initiation; NBS = N applied bulb swelling; NBM = N applied at prematuration. | ||||||
Results
Marketable yields of onions were significantly higher in 2022, with 1,303 40-lb bags/acre, compared to 1,101 40-lb bags/acre in 2021 (Table 3). In both years, FNUE was significantly different among application timing treatments (Table 4).
In 2021, the FNUE when N was applied at transplant was only 8.87%. When N was applied at vegetative growth and bulb initiation stages, FNUE increased, but less than 40% of the N fertilizer applied was taken up by the plant. At bulb swelling and bulb maturation, FNUE was significantly higher, with over 90% of the N fertilizer applied being taken by the plant.
In 2022, the N use was significantly lower at transplant (25.22%) compared to N applied at vegetative growth, bulb initiation, and bulb swelling. When applied at bulb maturation, the FNUE decreased (53.34%).
| Year | Yield, 40-lb bags/acre | ||||
|---|---|---|---|---|---|
| Marketable | Colossal | Jumbo | Medium | Culls | |
| 2021 | 1101 b | 2 b | 932 b | 167 a | 30 b |
| 2022 | 1303 a | 229 a | 1027 a | 46 b | 103 a |
| *Values followed by the same letters indicate no significant difference by the Tukey test (p < 0.05). NT = N applied at transplanting; NV = N applied at vegetative stage; NBI = N applied at bulb initiation; NBS = N applied bulb swelling; NBM = N applied at prematuration. | |||||
| Treatment | Ndff (lb/acre 15N) | N leached (lb/acre) | FNUE (%) | |||
|---|---|---|---|---|---|---|
| Bulbs | Leaves | Roots | Total plant | |||
| 2021 | ||||||
| 15NT | 1.03 c* | 0.81 d | 0.02 c | 1.86 c | 16.12 a | 8.87 c |
| 15NV | 3.01 bc | 2.50 c | 0.04 bc | 5.54 b | 12.5 b | 26.38 b |
| 15NBI | 4.17 b | 3.20 c | 0.04 bc | 7.41 b | 10.66 b | 35.28 b |
| 15NBS | 10.48 a | 11.53 a | 0.07 a | 22.08 a | 0.00 d | 105.16 a |
| 15NBM | 11.38 a | 8.92 b | 0.05 ab | 20.34 a | 0.00 c | 96.87 a |
| 2022 | ||||||
| 15NT | 4.12 b | 1.14 b | 0.03 a | 5.30 b | 11.13 a | 25.22 b |
| 15NV | 12.63 ab | 3.18 a | 0.09 a | 15.91 ab | 1.75 ab | 75.74 ab |
| 15NBI | 18.04 a | 3.52 a | 0.07 a | 21.63 a | 0.00 b | 103.02 a |
| 15NBS | 13.72 ab | 2.53 ab | 0.05 a | 16.30 ab | 1.02 ab | 77.64 ab |
| 15NBM | 9.78 ab | 1.38 b | 0.04 a | 11.20 ab | 6.29 ab | 53.34 ab |
| *Values followed by the same letters indicate no significant difference by the Tukey test (p < 0.05) among N fertilizer timing treatments within a year. NT = N applied at transplanting; NV = N applied at vegetative stage; NBI = N applied at bulb initiation; NBS = N applied bulb swelling; NBM = N applied at prematuration. | ||||||
Conclusion
The application of N fertilizer right after transplant is inefficient, since only a small portion of what is applied is being used by the plant, while most of the N is being leached. In 2021, the excessive rainfall in the first half of the onion season favored N loss; consequently, most of the N accumulated in the plant came from the last two N fertilizer applications. In 2022, targeting the periods of heavy vegetative growth and bulb initiation was most efficient in providing N fertilizer for onion plants. However, as bulbs mature and senesce, they take up less fertility, so part of the N applied at bulb maturity was not used by the plant.
