Impact of Nitrogen Fertilizer Rate and Timing on Short-day Onion Production

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Hanna Ibiapina de Jesus Department of Horticulture, University of Georgia, 1111 Miller Plant Sciences, Athens, GA 30602, USA

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Andre Luiz Biscaia Ribeiro da Silva Department of Horticulture, Auburn University, 124 Funchess Hall, Auburn, AL 36849, USA

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Bhabesh Dutta Department of Plant Pathology, University of Georgia, 2360 Rainwater Rd., Tifton, GA 31793, USA.

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Timothy Coolong Department of Horticulture, University of Georgia, 1111 Miller Plant Sciences, Athens, GA 30602, USA

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Abstract

Onions (Allium cepa) are typically planted late fall and harvested in spring in the Vidalia, GA, USA, region. Onions grown here are renowned for their for sweetness and are marketed to consumers as Vidalia onions. High rainfall during the relatively long growing season (4 to 5 months) may result in nitrogen (N) leaching during production. Therefore, fertilizer applications are usually aligned with stages of crop development to ensure nutrient availability for the entire season. Although the impacts of N application rate have been previously investigated for Vidalia onion production, the optimal timing for the final N application of the season has not been determined. The objectives of this study were to determine the optimal timing of the last fertilizer N application (at bulb initiation, during bulb growth, or during bulb maturation) in conjunction with the impact of three N application rates (75, 105, and 135 lb/acre N) on yield and quality in Vidalia onion. Soil N levels were affected by N rate, year, and onion growth stage. In 2020, up to 135 lb/acre N was required to maximize onion yields, and in 2021, onion yields were unchanged among N fertilizer treatments. Final N applications at bulb initiation resulted in greater yields than applications made during bulb growth or bulb maturation. In addition, as the N rate increased and the time of final application occurred later in bulb development, pungency values increased. Incidence of sour skin (Burkholderia cepacia) and center rot (Pantoea sp.) diseases were greater in 2020 compared with 2021 and seemed to be affected by environmental conditions more than N fertilization.

Onions (Allium cepa) are among the most significant vegetables grown in the United States, with $1.6 billion in value obtained from 127,200 of acres of production reported by the US Department of Agriculture (USDA 2022) in 2022. In the southeastern United States, the state of Georgia is recognized for production of Vidalia onions, where more than 11,000 acres were grown in 2021 with an estimated farm-gate value of $167 million (University of Georgia 2022). Vidalia onions are exclusively grown in the southeastern region of the state predominately on sandy loam soils, where mild winters, low sulfur soils, and ample water supply create ideal conditions for the cultivation of sweet onions (Boyhan and Torrance 2002).

The successful production of onions is dependent on the soil nitrogen (N) availability throughout crop development (Brewster 2008). Onions have a shallow root system, where most roots are found in the top 6 inches of soil (Halvorson et al. 2002; Strydom 1964). Significant research has been conducted to determine appropriate levels of N fertilization to optimize the yield and quality of Vidalia onions (Batal et al. 1994; Boyhan et al. 2007; Díaz-Peréz et al. 2003). Previous studies have highlighted the benefits of splitting total recommended N into multiple applications throughout the growing season (Boyhan et al. 2007; Coolong and Boyhan 2017; da Silva et al. 2022). Typically, the first N application occurs at transplanting, followed by a second application during the vegetative growth stage [30 to 50 d after transplant (DAT)] to promote root and leaf growth. The remaining N is typically divided into two additional applications in the latter half of the season.

Because N assimilation is greatest during bulb development, applications of N late in the season may impact onion bulb quality to a greater degree than early season N applications (Geisseler et al. 2022). Increased soil N levels can also lead to elevated bulb pungency, reducing the perceived sweetness of onions (Greenwood et al. 1980; Randle 2000). Reductions in bulb quality during storage have been reported when N is applied during bulb maturation (Batal et al. 1994; Coolong and Randle 2008; Randle 2000; Tekalign et al. 2012). Additionally, increased soil N levels during bulb maturation have been reported to increase susceptibility to some bacterial pathogens (Díaz-Perez et al. 2003). However, late applications of N fertilizers in onion can increase N fertilizer use efficiency and create opportunities to decrease the overall N fertilization rates (de Jesus et al. 2023a). Therefore, the objective of this study was to evaluate the effects of timing of the last N fertilizer application for the season in combination with three N fertilizer rates on the yield and quality of short-day onions.

Material and methods

Field experiments were conducted in 2020 and 2021 at the University of Georgia (UGA) Vidalia Onion and Vegetable Research Center located in Lyons, GA, USA (32°00′58″ N, 82°13′17″ W). The region is classified as having a humid subtropical climate, and the soil is characterized as an Irvington Sandy Loam (USDA 2023) with 0.6% organic matter, pH of 6.4 to 6.8, a bulk density of 1.62 g·cm3, and low water holding capacity. Preplant soil tests (0 to 6 inches) were conducted (Mehlich 1 extract) with values ranging from 216 to 258 lb/acre phosphorous (P), 154 to 200 lb/acre for potassium (K), and 1362 to 1474 lb/acre calcium (UGA Agriculture and Environmental Services Laboratory, Athens, GA, USA). These values are considered very high and high for P and K, respectively (UGA 2020).

Seeds of short-day onion ‘Candy Ann’ (Solar Seeds, Bangalore, India) were grown in nursery beds for ∼8 weeks. Seedlings were removed from transplant beds by hand and foliage cut to a length of ∼4 to 5 inches. Bareroot seedlings were then transplanted to the field on 15 Dec 2019 and 10 Dec 2020. Fields were prepared by harrowing soil to a depth of ∼10 inches, followed by final tillage and bed formation using a tractor mounted rotary tiller. Beds were ∼6 inches tall and spaced at 6 ft center to center, with each bed having four onion rows spaced 10 inches apart, with an in-row spacing of 4 inches between plants (87,120 plants/acre). Experimental plots were 30 ft long containing ∼360 plants each with 5 ft nonplanted buffers between adjacent plots within a row.

Three N fertilizer rates and three final fertilizer N application times were evaluated in a split-plot, randomized complete block design with eight replications in both years. Fertilizer rate treatments were 75, 105, and 135 lb/acre N. Beginning at transplanting all plots received an application of 20% of their season total N, which corresponded to 15, 21, and 27 lb/acre N (5.0N–4.4P–12.5K, Rainbow Plant Food, Timac Agro USA, Reading, PA, USA), representing the 75, 105, and 135 lb/acre N treatments, respectively. Two additional applications of 15, 21, and 27 lb/acre N (5.0N–4.4P–12.5K, Rainbow Plant Food, Timac Agro USA) were made at 23 DAT (2020) or 27 DAT (2021) and 47 DAT (2020) or 49 DAT (2021). These applications corresponded to early and late vegetative growth stages of development. Because the fertilizer applied contained both P and K, which were not balanced among treatments, levels of these nutrients also varied among the three rate treatments. The 75, 105, and 135 lb/acre N treatments received 40, 55, and 71 lb/acre P, respectively. Further, 113, 158, and 203 lb/acre K, were applied to the 75, 105, and 135 lb/acre N treatments, respectively.

The final N application consisted of 40% of the season total N application, which corresponded to 30, 42, and 54 lb/acre N using calcium nitrate (15.5N–0P–0K, Yara Liva, Yara North America, Tampa, FL, USA). There were three final N application times, which corresponded to bulb initiation, bulb development, or bulb maturation and were applied at 64, 74, and 84 DAT in 2020, and 68, 78, and 88 DAT in 2021, respectively. All fertilizers were granular products and broadcast-applied by hand to individual plots.

During each growing season, onions were overhead irrigated using stationary sprinklers. Irrigation volume was determined using an irrigation application (da Silva et al. 2019) that was based on historic onion evapotranspiration (ETc) and current accumulated rainfall. Due to excessive rainfall in 2020, plots received less than 1 inch of irrigation during bulb development and maturation. In 2021, ∼1.7 inches of irrigation water were applied to the study, primarily during bulb maturation. Air temperatures and precipitation were monitored and recorded every 15 min using a nearby on-farm weather station (UGA 2022). At the end of the experiment the irrigation application was used to calculate total ETc for each season (da Silva et al. 2019). Preemergent herbicides oxyfluorfen (0.49 lb/acre) (Goal 2xL; NuFarm, Alsip, IL, USA) and pendimethalin (0.82 lb/acre) (Prowl 3.3EC; BASF, Research Triangle Park, NC, USA) were broadcast applied over onion transplants within 7 DAT in both years. Routine fungicide and insecticide applications were made weekly during the season according to recommendations for the region (Sial et al. 2022).

Soil mineral N availability

Soil samples were collected before each fertilizer application and at harvest, which were 0, 23, 47, 71, and 129 DAT in 2020, and 5, 27, 49, 68, and 131 DAT in 2021, to determine the soil mineral N content. Sampling times corresponded to planting, early and late vegetative growth, bulb initiation, and harvest. Soil samples were comprised of five subsamples collected in each plot at a 0- to 6-inch soil depth. Samples were then homogenized, air dried, and tested for soil mineral N content [ammonium (NH4+) and nitrate (NO3)] at a commercial laboratory (Waters Agricultural Laboratories, Camilla, GA, USA).

Short-day onion harvesting and bulb quality

Onions were harvested when ∼50% of the plants had evidence of pseudostem lodging (tops down), which occurred on 27 Apr 2020 (129 DAT) and 21 Apr 2021 (131 DAT). There were no notable differences in maturity among treatments. At harvest onion tops and roots were manually cut, and bulbs were cured for 1 week with forced air at 38 °C. Subsequently, bulbs were graded into colossal (>3.75 inches), jumbo (3.25 to 3.75 inches), and medium (<3.25 inches) sizes following the standards for grades of Bermuda–Granex–Grano type onions (USDA 2014). Marketable yields were calculated as the sum of colossal, jumbo, and medium bulbs. After harvest, onions were evaluated for sour skin (Burkholderia cepacia) and center rot (Pantoea sp.), in which 20 jumbo-sized onions were cut longitudinally and the percentage of bulbs with visual symptoms of center rot and sour skin damage were determined.

Onion pungency measured as pyruvic acid was determined from a sample of five jumbo-sized onions. The neck, root plates, and dry outer scales of onion bulbs were removed, and the remaining whole onions were blended for juice extraction using a household blender. A 1.5-mL sample of liquid was removed from the blended sample and stored at –20 °C until analysis. The frozen samples were defrosted, and 20 uL of onion juice was used to determine the pyruvic acid content according to Schwimmer and Weston (1961).

Statistical analysis

Data were analyzed using the Standard Least Squares method from JMP Pro 16.0 (SAS Institute Inc., Cary, NC, USA). Total yield, bulb grade (colossal, jumbo, and medium), and bulb quality (i.e., sour skin incidence, center rot incidence, and pungency) were analyzed with total fertilizer N rate (75, 105, and 135 lb/acre N), last N application (bulb initiation, during bulb development, or during bulb maturation), year (2020 and 2021), and their interactions as fixed effects. Soil N content was analyzed with fertilizer N rate treatments, last application timing, year, sampling time (0, 23, 47, 71, and 129 DAT in 2020 and 5, 27, 49, 68, and 131 DAT in 2021), and their interaction as fixed effects. In both analyses, block was treated as a random effect. When the F value of the analysis of variance was significant, multiple mean comparisons were performed using the Tukey’s honestly significant difference test (P < 0.05).

Results

Weather conditions

There were 29.7 and 20.9 inches of rainfall in the 2020 and 2021 seasons, respectively (Fig. 1). In 2020, there were 5.3 inches of precipitation from transplanting to early vegetative stages of growth (21 DAT), and 3 inches from early to late vegetative stages (47 DAT). During bulb initiation, there were fewer but larger rainfall events with a total of 21.5 inches of precipitation. In 2021, there were 15.8 inches of rainfall from transplanting to bulb development (78 DAT) and 5.1 inches of rainfall from bulb development to harvest (131 DAT). Rainfall exceeded ETc in both growing seasons. Onion ETc was estimated to be 10.14 inches and 9.53 inches for the 2020 and 2021 seasons, respectively.

Fig. 1.
Fig. 1.

Daily maximum (max.) and minimum (min.) air temperatures (°F) and rainfall (inches) for ‘Candy Ann’ onion grown during 2020 (A) and 2021 (B) in Georgia, USA. Arrows (↓) indicate date of soil sampling for soil nitrogen determination. Data averages estimated using the University of Georgia Weather Network; (°F – 32) ÷ 1.8 = °C, 1 inch = 2.54 cm.

Citation: HortTechnology 34, 1; 10.21273/HORTTECH05323-23

In 2020, average minimum and maximum air temperatures were 34 °F and 76 °F, respectively. Average minimum and maximum air temperatures were 34 °F and 74 °F, respectively in the 2021 season (data not shown). Although average air temperatures were similar between the two seasons, there were more nights with temperatures below freezing in 2020 than 2021, particularly in the first 2 months of the study (Fig. 1).

Soil mineral N concentrations

Soil mineral N concentrations were affected by a year and fertilizer rate interaction (Fig. 2). In 2020, soil mineral N concentrations were higher in the 135 lb/acre N treatment at early (23 DAT) and late vegetative (47 DAT) stages of growth compared with the 75 and 105 lb/acre N treatments. Soil mineral N concentrations decreased for all treatments later in the season, and no significant differences were seen among N fertilizer rates at any other sampling time in 2020. In 2021, soil mineral N concentrations were again highest in the 135 lb/acre N treatment at the early vegetative growth stage compared with the 75 and 105 lb/acre N treatments. Soil mineral N concentrations decreased during bulb initiation (68 DAT), but the 135 lb/acre N treatment maintained higher soil N levels (4 mg N·kg soil−1) than the other treatments.

Fig. 2.
Fig. 2.

The effects of 135, 105 and 75 lb/acre nitrogen (N) on soil mineral N concentrations at 0, 23, 47, and 71, and 129 (harvest) days after transplant (DAT) in 2020 (A), and at 5, 27, 49, 68, and 131 (harvest) DAT in 2021 (B) for ‘Candy Ann’ onion grown in Georgia, USA; 1 lb/acre =1.1209 kg·ha−1, 1 mg·kg−1 = 1 ppm.

Citation: HortTechnology 34, 1; 10.21273/HORTTECH05323-23

Soil mineral N concentrations at harvest were affected by the interaction of year and timing of the last N fertilizer application (Fig. 3). In general, soil mineral N concentrations were significantly lower in 2020 compared with 2021 (data not shown). In 2020, there were no differences in soil mineral N concentrations among the N timing treatments. However, soil mineral N concentration at harvest in 2021 was greater (4.2 mg N·kg soil−1) when the last N application occurring during bulb growth compared with the last N application at bulb initiation (3.0 mg N·kg soil−1) or during bulb maturation (3.5 mg N·kg soil−1).

Fig. 3.
Fig. 3.

Effect of timing of the last fertilizer nitrogen (N) application on soil mineral N content at harvest in 2020 and 2021 in Georgia, USA; 1 mg·kg−1 = 1 ppm.

Citation: HortTechnology 34, 1; 10.21273/HORTTECH05323-23

Onion yields

Marketable onion yields were impacted by the interaction between year and N rate (Table 1). In 2020, marketable yield of onions increased as the N fertilizer rate increased from 75 to 135 lb/acre N, resulting in yields of 38,905 and 50,968 lb/acre, respectively. However, in 2021, there was no difference in marketable yield of onions among N fertilizer rates. The yield of colossal bulbs was greater in 2020 compared with 2021, regardless of N fertilizer rate. In 2020, there was an increase of colossal-sized onions with the application of 135 lb/acre N (3926 lb/acre), whereas no differences were measured among the applications of 105 lb/acre N (1413 lb/acre) and 75 lb/acre N (817 lb/acre). In 2021, N fertilizer rates did not significantly affect the yield of colossal bulbs. The yield of jumbo-sized onions was also significantly higher in 2020 compared with 2021, regardless of the N fertilizer rate. The yield of jumbo-sized onions was influenced by N rate only in 2020. Specifically, the lowest N application rate of 75 lb/acre N had the lowest yield of jumbo bulbs compared with the application of 105 or 135 lb/acre N. In contrast to the other onion sizes, the yield of medium-sized onions was greater in 2021 compared with 2020. In 2020, the lowest N application rate of 75 lb/acre N had a higher yield of medium-sized onions compared with the other N application rates, whereas there was no significant difference among fertilizer N treatments in 2021.

Table 1.

Interaction between year and fertilizer nitrogen (N) rate on marketable yield and yield of colossal, jumbo, and medium onions grown in Georgia, USA in 2020 and 2021.

Table 1.

There was no interaction between the timing of the last N application and fertilizer rate or year for total marketable yield of onion. However, there was a significant main effect of timing of the last N application on marketable yield and bulb size distribution (Table 2). Marketable yield was higher when the last N fertilizer application occurred at bulb initiation (44,747 lb/acre) than during bulb growth (43,170 lb/acre) or during bulb maturation (41,520 lb/acre). Similar results were measured for the yield of jumbo-sized onions, with the final N fertilizer application at bulb initiation resulting in a greater yield (36,611 lb/acre) than when applied at bulb maturation (31,661 lb/acre). The timing of application did not affect the yield of colossal-sized bulbs.

Table 2.

Effects of the stage of development for the timing of last fertilizer nitrogen (N) on total, colossal, jumbo, and medium onion yields averaged over 2020 and 2021 production seasons in Georgia, USA.

Table 2.

Bulb quality

The incidence of symptoms of center rot and sour skin were unaffected by interactions between fertilizer N rate, timing of last application, and year. However, center rot and sour skin incidence were influenced by the main effect of year, and there was a greater incidence of center rot (5.5%) and sour skin (1.5%) in 2020 compared with 2021 (Table 3). Fertilizer N rate and timing of last application timing had no impact on incidence of onion sour skin and center rot.

Table 3.

Main effects of year, fertilizer nitrogen (N) rate, and stage of development of the last fertilizer N application on the incidence of center rot (Pantoea sp.) and sour skin (Burkholderia cepacia) for onion grown in Georgia, USA, in 2020 and 2021.

Table 3.

Onion pyruvic acid concentration (pungency) was affected by the interaction between N fertilizer rate and timing of last N fertilizer application (Table 4). Onion pungency was lowest in plants receiving 75 lb/acre N with the last N application at bulb initiation. However, as fertilizer N rate increased and as the time of last N application was later, pungency also increased. In general, onion pungency was significantly higher in 2020 (6.28 µmol·mL−1 juice) compared with 2021 (4.20 µmol·mL−1 juice) (data not shown).

Table 4.

Effect of the interaction between nitrogen (N) rate and the stage of development for the last fertilizer N application on the pyruvic acid concentration (pungency) of onion grown in Georgia, USA, in 2020 and 2021.

Table 4.

Discussion

Rainfall has been shown to impact N management for vegetables grown in the southeastern United States (da Silva et al. 2020, 2022; de Jesus et al. 2023b; Zotarelli et al. 2015) and soil mineral N concentrations (Cavero et al. 1999; Gu and Riley 2010; Hasegawa and Denison 2005). Because precipitation exceeded onion ETc, particularly early in the season, it is likely that the rainfall may have leached soluble fertilizer N below the root zone. Bates and Tisdale (1957) reported downward movement (40 cm) of NO3 after the addition of 3.29 cm of water in soil columns using sandy soils. Further, NO3 derived from calcium nitrate, has been shown to migrate to a depth of more than 30 cm in the soil profile in a fine sandy soil 1 d after simulated rainfall in a field setting (Esala and Leppanen 1998).

In the present study, there was generally a lower level of soil mineral N in 2020 compared with 2021 (Fig. 3). Soil mineral N concentrations in 2021 ranged between 4.5 and 9.5 mg N·kg soil−1during early vegetive growth (27 DAT), whereas soil mineral N averaged 2.5 mg N·kg soil−1 for all three fertilizer treatments at the same development stage in 2020. Soil mineral N levels increased in 2021 to an average of 12.5 mg N·kg soil−1 during the late vegetative stage of development (49 DAT), whereas they ranged between 4.5 and 9 mg N·kg soil−1 for the same period in 2020, suggesting that more soil mineral N was available to plants before bulbing in 2021. In 2021, there were fewer rainfall events compared with 2020 between the second and third sampling periods (late vegetative growth), and there was a concomitant increase in soil N concentrations (Fig. 2). de Jesus et al. (2023a) reported an increase in fertilizer N use efficiency during late vegetative and bulb initiation stages of onion growth in a relatively drier year compared with a year with excessive precipitation, suggesting that rainfall can directly affect soil N uptake in onion.

Marketable yields for all fertilizer rates were comparable to or higher than commercial yields for the region (Coolong and Boyhan 2017). In 2020, when rainfall was greater, total marketable yields increased as fertilizer rate increased from 75 to 135 lb/acre N. However, when there were few rainfall events in 2021, there were no differences in marketable yields among fertilizer N rate treatments. In 2021, there were no differences in total marketable yield among N fertilizer rate treatments, suggesting that 75 lb/acre N was sufficient to achieve commercially acceptable onion yields. These findings agree with the results reported by da Silva et al. (2022), which suggest that lower than recommended N fertilizer rates can achieve satisfactory onion yields in Georgia, USA. However, the response of onions to the higher rates of fertilization in 2020 indicates that fertilizer rate should be adjusted according to rainfall patterns during the growing season.

Phosphorous and K levels varied among fertilizer rate treatments. Boyhan et al. (2007) evaluated the impact of N, P, and K fertilization on yield of Vidalia onions over a 6-year period. Although the authors noted a quadratic response for total yield to the application of N fertilizer, there was no impact of P fertilization on total yields of onion. However, increasing rates of P fertilizer did lead to a reduction in the yield of jumbo bulbs (Boyhan et al. 2007). The authors also reported a quadratic response of onion yield to K fertilization, with yields peaking with the addition of ∼83 lb/acre K (Boyhan et al. 2007). In the present study, all fields had either very high or high levels of preplant P and K, respectively, suggesting that the impact of varying P and K on yields was likely minor.

The timing of the last fertilizer N application significantly influenced onion marketable yields, with applications at bulb initiation leading to increased total marketable yields compared with later applications during bulb growth or maturation. This suggests that growers can optimize their final N application by timing it with the bulb initiation stage of growth. Recently de Jesus et al. (2023a) reported increased fertilizer N use efficiency when N was applied during bulb initiation, but reduced N uptake when the final application is delayed until bulb growth.

Symptoms of sour skin and center rot diseases were observed in onions in 2020 but not in 2021, and they were unaffected by fertilizer rate or the timing of the last N application. da Silva et al. (2022) reported similar results, with no differences in sour skin and center rot incidence among N application rates ranging from 75 to 135 lb/acre N. These results contrast with previous research suggesting that high N rates and late fertilizer N applications can increase incidence of bacterial disease in onions (Batal et al. 1994; Wright 1993). It is notable that when only data from 2020 were analyzed, when incidence of center rot was 5.5%, there was an effect of the timing of the last N application on center rot incidence (data not shown). In that season, final N applications at bulb initiation resulted in a significantly lower rate of center rot (2.30%) than applications during bulb swelling (6.25%) and maturation (6.67%) (data not shown). However, when both 2020 and 2021 data were analyzed, there was no significant impact of N rate, application time, or their interaction on the incidence of center rot in bulbs (Table 3). Increased disease incidence in 2020 may have been related to increased rainfall in the weeks leading up to harvest. Onions are vulnerable to bacterial infection near harvest when the neck retains more water, creating a conducive environment for pathogens (Brewster 2008; Stumpf et al. 2021). In seasons when weather conditions are more conducive for bacterial diseases, there may be an impact of N timing on severity of some pathogens; however, this effect may be inconsistent as our data suggest.

Fertilizer N rate and the timing of last fertilizer N application interacted to affect onion pungency. Although previous studies have extensively explored the effects of increased N fertilization on onion pungency, the effect of the timing of last fertilizer N application is limited (Coolong and Randle 2003; McCallum et al. 2005; Randle 2000). In the present study, onions receiving the lowest N rate (75 lb/acre N) with the last fertilizer N application delayed until during bulb maturation had increased pungency compared with when the last N application was made at bulb initiation. This suggests that later fertilizer N applications, after the bulb initiation stage of development, may contribute to increased pungency in onions.

Conclusion

This study evaluated the effects of fertilizer N rates and timing of the last fertilizer N application for in the production Vidalia onions. In 2020, total yields increased with increasing N fertilizer, whereas they were not affected by fertilizer rate in 2021. This may have been due to greater precipitation rates and potential N loss during vegetative stages of growth, which was reflected in lower soil inorganic N concentrations in 2020 compared with 2021. The timing of N fertilizer application also influenced yield, with an application at bulb initiation resulting in more jumbo-sized onions and greater total yields. The impact of N fertilization rate on yield varied according to environmental conditions and growers should consider this when applying fertilizer to onions. In seasons with high levels of precipitation, onion yields increased when using 135 lb/acre N, although in drier seasons acceptable yields were obtained with 75 lb/acre N. Results also suggested that a final application of N during the season should occur at bulb initiation and not later because it led to increased yields and improved flavor potential compared with applications later in bulb development.

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  • Hasegawa H, Denison RF. 2005. Model predictions of winter rainfall effects on N dynamics of winter wheat rotation following legume cover crop or fallow. Field Crops Res. 91(2-3):251261. https://doi.org/10.1016/j.fcr.2004.07.019.

    • Search Google Scholar
    • Export Citation
  • McCallum J, Porter N, Searle B, Shaw M, Bettjeman B, McManus M. 2005. Sulfur and nitrogen fertility affects flavour of field-grown onions. Plant Soil. 269:151158. https://doi.org/10.1007/s11104-004-0402-5.

    • Search Google Scholar
    • Export Citation
  • Randle WM. 2000. Increasing nitrogen concentration in hydroponic solutions affects onion flavor and bulb quality. J Am Soc Hortic Sci. 125(2):254259. https://doi.org/10.21273/JASHS.125.2.254.

    • Search Google Scholar
    • Export Citation
  • Sial A, Johnson A, Cabrera E. 2022. Georgia commercial pest management handbook volume I. Univ. Georgia Coop Ext Serv Spec Bull 28.

  • Schwimmer S, Weston WJ. 1961. Enzymatic development of pyruvic acid in onion as a measure of pungency. Agric Food Chem. 9(4):301304. https://doi.org/10.1021/jf60116a018.

    • Search Google Scholar
    • Export Citation
  • Strydom E. 1964. A root study of onions in an irrigation trial. S Afr J Agric Sci. 7(4):593601. https://hdl.handle.net/10520/AJA05858860_329.

    • Search Google Scholar
    • Export Citation
  • Stumpf S, Leach L, Srinivasan R, Coolong T, Gitaitis R, Dutta B. 2021. Foliar chemical protection against Pantoea ananatis in onion is negated by thrips feeding. Phytopathology. 111(2):258267. https://doi.org/10.1094/PHYTO-05-20-0163-R.

    • Search Google Scholar
    • Export Citation
  • Tekalign T, Abdissa Y, Pant LM. 2012. Growth, bulb yield and quality of onion (Allium cepa L.) as influenced by nitrogen and phosphorus fertilization on vertisol. II: Bulb quality and storability. Afr J Agric Res. 7(45):59805985. https://doi.org/10.5897/AJAR10.1024.

    • Search Google Scholar
    • Export Citation
  • University of Georgia. 2020. Fertilizer recommendations by crops. http://aesl.ces.uga.edu/cropcodelist.pdf. [accessed 18 Oct 2023].

  • University of Georgia. 2022. 2021 Georgia farm gate value report. Univ Georgia Coop Ext Serv Bull AR 22-01. https://caed.uga.edu/publications/farm-gate-value.html. [accessed 25 May 2023].

  • US Department of Agriculture. 2014. United States standards for grades of Bermuda–Granex–Grano type onions.

  • US Department of Agriculture. 2022. National Agricultural Statistics Service. Quick Stats. Dept Agric, Washington, DC. https://quickstats.nass.usda.gov/. [accessed 23 May 2023].

  • US Department of Agriculture. 2023. Soil survey. Web soil survey - Soil survey of Tattnall County, Georgia. https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx. [accessed 12 Sep 2023].

  • Wright PJ. 1993. Effects of nitrogen fertilizer, plant maturity at lifting, and water during field‐curing on the incidence of bacterial soft rot of onions in store. N Z J Crop Hortic Sci. 21(4):377381. https://doi.org/10.1080/01140671.1993.9513796.

    • Search Google Scholar
    • Export Citation
  • Zotarelli L, Rens LR, Cantliffe DJ, Stoffella PJ, Gergela D, Burhans D. 2015. Rate and timing of nitrogen fertilizer application on potato ‘FL1867’. Part I: Plant nitrogen uptake and soil nitrogen availability. Field Crops Res. 183:246256. https://doi.org/10.1016/j.fcr.2015.08.007.

    • Search Google Scholar
    • Export Citation
  • Fig. 1.

    Daily maximum (max.) and minimum (min.) air temperatures (°F) and rainfall (inches) for ‘Candy Ann’ onion grown during 2020 (A) and 2021 (B) in Georgia, USA. Arrows (↓) indicate date of soil sampling for soil nitrogen determination. Data averages estimated using the University of Georgia Weather Network; (°F – 32) ÷ 1.8 = °C, 1 inch = 2.54 cm.

  • Fig. 2.

    The effects of 135, 105 and 75 lb/acre nitrogen (N) on soil mineral N concentrations at 0, 23, 47, and 71, and 129 (harvest) days after transplant (DAT) in 2020 (A), and at 5, 27, 49, 68, and 131 (harvest) DAT in 2021 (B) for ‘Candy Ann’ onion grown in Georgia, USA; 1 lb/acre =1.1209 kg·ha−1, 1 mg·kg−1 = 1 ppm.

  • Fig. 3.

    Effect of timing of the last fertilizer nitrogen (N) application on soil mineral N content at harvest in 2020 and 2021 in Georgia, USA; 1 mg·kg−1 = 1 ppm.

  • Batal KM, Bondari K, Granberry DM, Mullinix BG. 1994. Effects of source, rate, and frequency of N application on yield, marketable grades and rot incidence of sweet onion (Allium cepa L. cv. Granex-33). J Hortic Sci. 69(6):10431051. https://doi.org/10.1080/00221589.1994.11516543.

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  • Hasegawa H, Denison RF. 2005. Model predictions of winter rainfall effects on N dynamics of winter wheat rotation following legume cover crop or fallow. Field Crops Res. 91(2-3):251261. https://doi.org/10.1016/j.fcr.2004.07.019.

    • Search Google Scholar
    • Export Citation
  • McCallum J, Porter N, Searle B, Shaw M, Bettjeman B, McManus M. 2005. Sulfur and nitrogen fertility affects flavour of field-grown onions. Plant Soil. 269:151158. https://doi.org/10.1007/s11104-004-0402-5.

    • Search Google Scholar
    • Export Citation
  • Randle WM. 2000. Increasing nitrogen concentration in hydroponic solutions affects onion flavor and bulb quality. J Am Soc Hortic Sci. 125(2):254259. https://doi.org/10.21273/JASHS.125.2.254.

    • Search Google Scholar
    • Export Citation
  • Sial A, Johnson A, Cabrera E. 2022. Georgia commercial pest management handbook volume I. Univ. Georgia Coop Ext Serv Spec Bull 28.

  • Schwimmer S, Weston WJ. 1961. Enzymatic development of pyruvic acid in onion as a measure of pungency. Agric Food Chem. 9(4):301304. https://doi.org/10.1021/jf60116a018.

    • Search Google Scholar
    • Export Citation
  • Strydom E. 1964. A root study of onions in an irrigation trial. S Afr J Agric Sci. 7(4):593601. https://hdl.handle.net/10520/AJA05858860_329.

    • Search Google Scholar
    • Export Citation
  • Stumpf S, Leach L, Srinivasan R, Coolong T, Gitaitis R, Dutta B. 2021. Foliar chemical protection against Pantoea ananatis in onion is negated by thrips feeding. Phytopathology. 111(2):258267. https://doi.org/10.1094/PHYTO-05-20-0163-R.

    • Search Google Scholar
    • Export Citation
  • Tekalign T, Abdissa Y, Pant LM. 2012. Growth, bulb yield and quality of onion (Allium cepa L.) as influenced by nitrogen and phosphorus fertilization on vertisol. II: Bulb quality and storability. Afr J Agric Res. 7(45):59805985. https://doi.org/10.5897/AJAR10.1024.

    • Search Google Scholar
    • Export Citation
  • University of Georgia. 2020. Fertilizer recommendations by crops. http://aesl.ces.uga.edu/cropcodelist.pdf. [accessed 18 Oct 2023].

  • University of Georgia. 2022. 2021 Georgia farm gate value report. Univ Georgia Coop Ext Serv Bull AR 22-01. https://caed.uga.edu/publications/farm-gate-value.html. [accessed 25 May 2023].

  • US Department of Agriculture. 2014. United States standards for grades of Bermuda–Granex–Grano type onions.

  • US Department of Agriculture. 2022. National Agricultural Statistics Service. Quick Stats. Dept Agric, Washington, DC. https://quickstats.nass.usda.gov/. [accessed 23 May 2023].

  • US Department of Agriculture. 2023. Soil survey. Web soil survey - Soil survey of Tattnall County, Georgia. https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx. [accessed 12 Sep 2023].

  • Wright PJ. 1993. Effects of nitrogen fertilizer, plant maturity at lifting, and water during field‐curing on the incidence of bacterial soft rot of onions in store. N Z J Crop Hortic Sci. 21(4):377381. https://doi.org/10.1080/01140671.1993.9513796.

    • Search Google Scholar
    • Export Citation
  • Zotarelli L, Rens LR, Cantliffe DJ, Stoffella PJ, Gergela D, Burhans D. 2015. Rate and timing of nitrogen fertilizer application on potato ‘FL1867’. Part I: Plant nitrogen uptake and soil nitrogen availability. Field Crops Res. 183:246256. https://doi.org/10.1016/j.fcr.2015.08.007.

    • Search Google Scholar
    • Export Citation
Hanna Ibiapina de Jesus Department of Horticulture, University of Georgia, 1111 Miller Plant Sciences, Athens, GA 30602, USA

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Andre Luiz Biscaia Ribeiro da Silva Department of Horticulture, Auburn University, 124 Funchess Hall, Auburn, AL 36849, USA

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Bhabesh Dutta Department of Plant Pathology, University of Georgia, 2360 Rainwater Rd., Tifton, GA 31793, USA.

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Timothy Coolong Department of Horticulture, University of Georgia, 1111 Miller Plant Sciences, Athens, GA 30602, USA

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Contributor Notes

Funding for this research was provided a Vidalia Onion Committee Commodity Grant titled “What Is the Best Timing for Nitrogen Fertilizer Application in Vidalia Onion Production?”

T.C. is the corresponding author. E-mail: tcoolong@uga.edu.

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  • Fig. 1.

    Daily maximum (max.) and minimum (min.) air temperatures (°F) and rainfall (inches) for ‘Candy Ann’ onion grown during 2020 (A) and 2021 (B) in Georgia, USA. Arrows (↓) indicate date of soil sampling for soil nitrogen determination. Data averages estimated using the University of Georgia Weather Network; (°F – 32) ÷ 1.8 = °C, 1 inch = 2.54 cm.

  • Fig. 2.

    The effects of 135, 105 and 75 lb/acre nitrogen (N) on soil mineral N concentrations at 0, 23, 47, and 71, and 129 (harvest) days after transplant (DAT) in 2020 (A), and at 5, 27, 49, 68, and 131 (harvest) DAT in 2021 (B) for ‘Candy Ann’ onion grown in Georgia, USA; 1 lb/acre =1.1209 kg·ha−1, 1 mg·kg−1 = 1 ppm.

  • Fig. 3.

    Effect of timing of the last fertilizer nitrogen (N) application on soil mineral N content at harvest in 2020 and 2021 in Georgia, USA; 1 mg·kg−1 = 1 ppm.

 

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