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Article

Optimizing Fertilization Rate to Achieve High Onion Bulb Yield and High Nitrogen Fertilizer Productivity in Dry-Hot Valley Region of Southwest China

by
Jiancha Li
1,2,
Kun Li
1,2,
Yilin Li
1,2,
Xuewen Yue
1,2,
Hongye Zhu
3,
Liangtao Shi
1,2,* and
Haidong Fang
1,2,*
1
Institute of Tropical Eco-Agriculture, Yunnan Academy of Agricultural Sciences, Yuanmou 651300, China
2
Yunnan Key Laboratory of Soil Erosion Prevention and Green Development, Yuanmou 651300, China
3
Institute of Agricultural Environment and Resources, Yunnan Academy of Agricultural Sciences, Kunming 650051, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(8), 1822; https://doi.org/10.3390/agronomy15081822
Submission received: 16 June 2025 / Revised: 5 July 2025 / Accepted: 22 July 2025 / Published: 28 July 2025
(This article belongs to the Section Soil and Plant Nutrition)
Browse Figures
Review Reports Versions Notes

Abstract

Excessive fertilization is a widespread issue in onion (Allium cepa L.) production in Southwest China. This practice not only leads to environmental pollution but also decreases the marketable yield and fertilizer productivity of onions. Identifying an optimal fertilization rate is crucial for promoting high-yield and highly efficient onion cultivation. The objective of this research is to determine the appropriate amount of fertilizer by investigating the effects of different fertilization rates on the growth characteristics and bulb yield of onion. The study was conducted over two consecutive growing seasons utilizing a randomized complete block design, which included six treatments: local routine fertilizer application (F1), a 20% reduction from F1 (F2), a 40% reduction from F1 (F3), a 60% reduction from F1 (F4), an 80% reduction from F1 (F5), and no fertilizer application (F0). The results show that, at the mature stage, aboveground dry matter quantity and its accumulation rate of onion under treatment F2 were found to be the highest among all other treatments across both growing seasons. Following the onset of bulbing, dry matter accumulation initially increased but subsequently decreased with reduced fertilizer supply; notably, it was greater under treatment F2 compared to other treatments. Compared with F1, the PFPN (partial factor productivity of nitrogen fertilizer) under treatment F2 increased by 35.2% and 32.0%, and the marketable bulb yield under treatment F2 increased by 8.4% and 5.8% during the 2022–2023 and 2023–2024 growing seasons, respectively. The marketable bulb yield demonstrated extremely significant positive correlations with aboveground dry matter and the dry matter accumulation rate throughout all growth periods in both growing seasons. Furthermore, marketable bulb yield exhibited extremely significant positive correlations with dry matter translocation before the onset of bulbing and dry matter accumulation following bulbing initiation. It was concluded that the appropriate fertilizer application (F2), characterized by a fertilization rate of 339-216-318 kg ha−1 for N-P2O5-K2O, enhanced onion bulb yield and nitrogen fertilizer productivity by promoting post-bulbing dry matter accumulation. This study emphasizes the significance of optimizing the fertilization rate as a crucial factor in achieving high-yield and highly efficient onion cultivation by enhancing dry matter accumulation.

1. Introduction

Fertilization is a crucial human activity that provides essential nutrition for soil and crops, enabling high crop yield and promoting soil health. It plays an important role in meeting the food demands of the continuously growing global population [1]. However, excessive application of fertilizers can lead to agricultural non-point source pollution [2,3,4], hinder crop growth, and degrade soil health [1,4]. Furthermore, it can decrease fertilizer-use efficiency [4,5] and even jeopardize food security and human health [6]. Excessive levels of fertilization can be detrimental to plants and may result in reducing plant yield [7]. Proper accumulation, distribution, and translocation of dry matter are critical factors for achieving high yields in crops. In particular, enhancing the dry matter accumulation of harvested organs plays a vital role in attaining optimal crop productivity. However, previous studies have indicated that unnecessary rates of fertilization negatively impact dry matter accumulation, distribution, translocation, and overall crop yield [8,9]. Hence, it is imperative to conduct studies aimed at determining the optimal regional fertilization rate for crops in order to develop sustainable agriculture and enhance crop yields by optimizing the dry matter partitioning of harvested organs. The optimum fertilization rate for crops highly depends on various factors, including different crops, fertilizer types, weather conditions, soil properties, and geographical regions. Therefore, in pursuit of achieving high yields while reducing fertilizer usage, it is essential to recommend optimal fertilizer application rates for specific crops and further investigate their effects on dry matter production and allocation at a local level.
Onions (Allium cepa L.) are a vital vegetable crop that is beneficial to human cardiovascular health. Cultivated globally across various climatic zones, onions have a long history of cultivation in China. From 1990 to 2019, global onion production experienced rapid growth, increasing from less than 2 million hectares to over 5 million hectares [10]. The gross production value of onions ranks second among vegetable crops worldwide, following tomatoes [11]. The dry-hot valley region of Southwestern China is a typically ecologically fragile zone, characterized by low soil nutrient availability [12] and insufficient water resources [13], despite having abundant photo-thermal resources. Onion plays a crucial role in promoting the development of the agricultural economy in the dry-hot valley region of Southwest China. To achieve consistently high bulb yield and income, farmers often apply increasing amounts of fertilizer throughout the onion-growing season. Most of the fertilizer applied is either retained in the soil or released into the air and water and is not utilized by onion plants. It is imperative to optimize the fertilization rate for onions in order to minimize fertilizer waste and mitigate environmental risks while achieving high bulb yields. A prerequisite for reducing fertilization rates while enhancing crop yields is the establishment of a robust scientific theory and foundation upon which to initiate discussions regarding local policy options [1]. However, there are few published studies that illustrate the impact of fertilization rate on dry matter accumulation, distribution, and bulb yield of onion in the dry-hot valley region of Southwest China. This study aimed to investigate the variations in dry matter accumulation, distribution, and translocation across different organs, as well as onion bulb yield and components of yield under various fertilization rates. Additionally, it sought to explore the relationship between bulb yield formation and dry matter accumulation while determining the optimal fertilization rate for achieving high-yielding and highly efficient onion production. The findings of this research can provide a theoretical foundation and technical guidance for enhancing high-yielding and highly efficient onion cultivation in the dry-hot valley region of Southwest China.

2. Materials and Methods

2.1. Study Site

The study site is situated in a dry-hot valley in the Jinsha River basin located in Southwest China (25°23′ N, 101°35′ E), where the field experiments were performed. The long-term annual average temperature and precipitation are approximately 21.5 °C and 600–800 mm, respectively. During the onion-growing seasons, the daily average maximum temperature and daily average minimum temperature are 25.9 °C and 7.2 °C, respectively. The soil quality is poor, characterized by low organic matter content and degraded structural properties. The physio-chemical properties of soil are detailed in Table 1. Soil texture was determined by the soil densitometer method. Soil bulk density was detected by the cutting ring method. Soil pH was determined using an automatic acid–base titrator based on 1:2.5 (soil/water, w/v) suspensions. Soil organic matter was determined using the potassium dichromate volumetric method, and total nitrogen was analyzed by the Kjeldahl method. Soil available nitrogen was determined using the micro-diffusion method following alkaline hydrolysis. Soil available phosphorus was determined using the Olsen method, and soil available potassium was quantified from 1 M NH4OAc extracts by flame photometry.

2.2. Experimental Layout

The subject of this study was the short-day red onion. This cultivar adapts to lower latitudes and shorter daylengths; however, high temperatures and long daylengths are required to promote dry matter accumulation and bulb development after bulb initiation. Field experiments were conducted over two consecutive onion-cropping seasons in the same area, specifically from November 2022 to April 2023 and from November 2023 to April 2024. Twenty thousand seeds were sown in seedbeds of 30 m2. The seedbeds were completely covered with shading netting for 7 days until the seeds had germinated and emerged through the soil surface, after which the shading netting was completely removed. After 40 days, transplanting of the normal seedlings was conducted on 1 November 2022, and on 7 November 2023, they were transplanted to the designed plots. A total of 18,000 onion seedlings were transplanted per field area of 667 m2. The experiment was designed using the randomized complete block design method and included six different fertilization rates. These fertilization rates comprised the local routine fertilization rate with a N-P2O5-K2O fertilization rate of 339-216-318 kg·ha−1 (F1), a 20% reduction from F1 (F2), a 40% reduction from F1 (F3), a 60% reduction from F1 (F4), an 80% reduction from F1 (F5), and no fertilizer application (F0). Each treatment consisted of four replicates that were randomly distributed across 24 split plots, with each plot covering an area of 60 m2 (length: 12 m, width: 5 m). All experimental treatments were carried out by splitting fertilization five times using drip irrigation under plastic film (Table 2). Five-time fertilization was conducted at 10 days, 40 days, 70 days, 90 days, and 110 days after transplanting, respectively. Using drip irrigation under plastic film ensures that the onion crops receive the required amount of water throughout the entire growing season. The cumulative volume of irrigation applied during this period was recorded as 330 mm.

2.3. Measurements

2.3.1. Aboveground Dry Matter

A total of five plants were randomly sampled in each plot at 60 days, 90 days, 120 days, and 150 days after transplanting. The rest of the plants were harvested at 150 days after transplanting. During the period from 60 to 120 days post-transplanting, each plant was separated into stems and leaves, while the stems (necks), leaves, and bulbs were sampled, respectively, for each plant at harvest. The plant samples were de-enzymed in ovens at 105 °C for 30 min, followed by drying at 75 °C until a constant weight was achieved. Subsequently, the dry weight of the plant samples was measured.

2.3.2. Aboveground Dry Matter Partitioning and Translocation

The characteristic values of aboveground dry matter partitioning and translocation were calculated in accordance with the following established formulations:
(1) The proportion of leaf (PL, %) = leaf dry matter/aboveground dry matter
(2) The proportion of stem (PS, %) = stem dry matter/aboveground dry matter
(3) The proportion of bulb (PB, %) = bulb dry matter/aboveground dry matter
(4) Aboveground dry matter (ADM, kg·hm−2) = onion plant aboveground dry matter (kg·plant) × 18,000 (plants/667 m2) × 15
(5) The aboveground dry matter accumulation rate (ADMAR, kg·day−1·hm−2) = the increment of aboveground dry matter/time increment
(6) Dry matter translocation (DMT, kg·hm−2) = aboveground dry matter (ADM) at onset of bulbing − (leaves + stem) dry matter at maturity.
(7) Dry matter translocation efficiency (DMTE, %) = (dry matter translocation/aboveground dry matter at onset of bulbing) × 100.
(8) Contribution of pre-bulbing dry matter translocation to bulb (Pre CDMTB, %) = (dry matter translocation/bulb dry matter) × 100.
(9) Post-bulbing dry matter accumulation (Post DMA, kg·hm−2) = aboveground dry matter at maturity − aboveground dry matter at onset of bulbing.
(10) Contribution of Post-bulbing dry matter accumulation to bulb at maturity (Post CDMAB, %) = Post-bulbing dry matter accumulation/bulb dry matter at maturity.

2.3.3. Bulb Yield and Its Components

The manual harvesting of onion plants was conducted over an area of 1 m2 in each experimental plot at harvesting, and the number of marketable and unmarketable bulbs was subsequently recorded. The bulbs with a transverse diameter less than 30 mm, decayed, splitting, or bolting were classified as unmarketable bulbs; all others were considered marketable bulbs [14]. The fresh weight, longitudinal diameter, and transverse diameter of a total of 5 marketable bulbs were measured. The bulbs’ longitudinal diameters and transverse diameters were measured using a Varner caliper. The characteristic values of bulb yield were calculated in accordance with the following established formulations:
(1) Small bulb rate (LBR, %) = bulb number with transverse diameter less than 30 mm/total number of bulb per square meter × 100
(2) Splitting bulb rate (SBR, %) = splitting bulb number/total number of bulb per square meter × 100
(3) Bolting bulb rate (BBR, %) = bolting bulb number/total number of bulb per square meter × 100
(4) Marketable bulb rate (MBR, %) = 100-LBR-SBR-BBR
(5) Marketable bulb yield (BY, kg·hm−2) = fresh weight of marketable bulb × total number of onion plants per hm2 × MBR/100
(6) Bulb index (BI) = longitudinal diameter/transverse diameter
(7) Percentage of bulb dry matter (PBDM) = dry weight of marketable bulb/fresh weight of marketable bulb × 100
(8) Partial factor productivity of nitrogen fertilizer (PFPN) = bulb yield(kg·hm−2)/nitrogen application amount during the growing season (kg·hm−2).

2.3.4. Statistical Analysis

The analysis of variance, multiple comparisons, and correlations (p < 0.05) were performed using SPSS 21.0 software. Drawings were undertaken using Origin 2022 software.

3. Results

3.1. Aboveground Dry Matter

Aboveground dry matter refers to the carbohydrate accumulated by plants during photosynthesis, which is closely related to the final production. The variations in the aboveground dry matter of onion in response to different fertilizer applications during the two consecutive growing seasons are shown in Figure 1. Generally, the aboveground dry matter of onion showed a slight increase during the early stages, followed by rapid growth in the middle stages, and ultimately reached the maximum value at harvesting. The aboveground dry matter of onion exhibited a trend of initial increase, followed by a decline as the fertilization rate was reduced. The aboveground dry matter of onions was significantly higher under treatments F1, F2, and F3 compared to the other treatments throughout the entire growth period during the 2022–2023 growing season. Compared to F1, the aboveground dry matter of onion was greater under treatment F2, with the exception of seedlings. During the maturity stage, the aboveground dry matter of onion was significantly greater under treatment F2 than under F1. The aboveground dry matter of onion was significantly greater under treatments F1, F2, and F3 compared to those under other treatments throughout the total growth period during the 2023–2024 growing season. At maturity, compared to F1, the aboveground dry matter under F2 was enhanced by 17.4% and 9.8% during the 2022–2023 and 2023–2024 growing seasons, respectively. Compared to F3, the aboveground dry matter under F2 was enhanced by 17.2% and 8.4% during the 2022–2023 and 2023–2024 growing seasons, respectively. There was no significant difference in the aboveground dry matter among F1, F2, and F3 during period IV of the 2023–2024 growing season.
The aboveground dry matter accumulation rates of onion with fertilizer applications during the two consecutive growing seasons of 2022–2023 and 2023–2024 are shown in Figure 2. Generally, the aboveground dry matter accumulation rate of onion showed a slight increase at the early stages, followed by rapid growth in the middle stages, especially from the onset of bulbing to bulb swelling, and then gradually declined during the maturity period. The aboveground dry matter accumulation rates of onion under treatments F1, F2, and F3 were significantly higher compared with those under other treatments throughout the total growth period during the 2022–2023 and 2023–2024 growing seasons. Although there was no significant difference in the aboveground dry matter accumulation rates among F1, F2, and F3 compared with treatments F1 and F3, treatment F2 enhanced the aboveground dry matter accumulation rate of onion during the maturity period. Specifically, compared to F1, the aboveground dry matter accumulation rate under treatment F2 increased by 60.8% and 33.3% at maturity during the 2022–2023 and 2023–2024 growing seasons, respectively. In comparison with treatment F3, the dry matter accumulation rate under treatment F2 increased by 69.5% and 23.0% at maturity during the 2022–2023 and 2023–2024 growing seasons, respectively.

3.2. Dry Matter Partitioning

At 60 days and 90 days after transplanting, the proportion of leaves remained greater than that of stems under all treatments during the 2022–2023 and 2023–2024 seasons (Figure 3). Furthermore, it was observed that as the fertilization rate decreased, the proportion of leaves decreased, while the proportion of stems increased. F1, F2, F3, and F4 were effective in enhancing the proportion of leaves, but F5 and F0 demonstrated superior performance in enhancing the proportion of stems.
Conversely, at 120 days after transplanting, the proportion of stems was higher than that of leaves across various treatments during the 2022–2023 and 2023–2024 growing seasons. The proportion of leaves was higher under treatments F1, F2, F3, and F4 compared to those under treatments F5 and F0, but the proportion of stems was lower under treatments F1, F2, F3, and F4 than under treatments F5 and F0 during the 2022–2023 growing season. However, there were no significant differences in the proportion of leaves and stems under all treatments during the 2023–2024 growing season.
At 150 days after transplanting, most of the stems transformed into bulbs, and the proportion of bulbs was remarkably higher than that of leaves and stems. Additionally, the proportion of leaves and stems showed an increasing trend with reduced fertilization rates; in contrast, the proportion of bulbs presented a decreasing trend. The highest proportion of bulbs was observed under treatment F5 among all treatments during the 2022–2023 growing season. The proportion of bulbs under F2 and F4 was greatest among all treatments during the 2023–2024 growing season. However, there were no significant differences under F1, F2, and F3 in the proportion of bulbs.
Compared to treatment F1, treatment F2 increased the proportion of stems by 0.2% and 5.7% at 120 days after transplanting; however, the bulb proportion changed by—0.3% and 1.9% at maturity during the 2022–2023 and 2023–2024 growing seasons, respectively. Compared to treatment F3, treatment F2 increased the proportion of stems by 1.0% and 6.5% at 120 days after transplanting while increasing the proportion of bulbs by 0.5% and 3.6% at maturity during the 2022–2023 and 2023–2024 growing seasons, respectively.

3.3. Dry Matter Translocation (DMT)

At the pre-bulbing stage, the dry matter translocation, dry matter translocation efficiency, and contribution of dry matter translocation to bulb development showed gradual reducing trends with reduced fertilizer supply during 2022–2023 and 2023–2024 (Figure 4). The differences in the dry matter translocation, dry matter translocation efficiency, and contribution of dry matter translocation to bulb development were not significant under F1, F2, and F3.
Post-bulbing dry matter accumulation initially increased but subsequently decreased with reduced fertilizer supply in 2022–2023 and 2023–2024 (Figure 5). Post-bulbing dry matter accumulation was greater under F2 compared to other treatments. Specifically, at the post-bulbing stage, dry matter accumulation under F2 was enhanced by 25.0 and 12.8% compared with that under F1 during the 2022–2023 and 2023–2024 seasons, respectively. Compared to treatment F3, dry matter accumulation under F2 increased by 15.8% and 10.3% at the post-bulbing stage during the 2022–2023 and 2023–2024 seasons, respectively. The contribution of post-bulbing dry matter accumulation to bulb development initially increased, decreased gradually, and then improved with reduced fertilizer supply during the 2022–2023 and 2023–2024 seasons. The highest contribution of post-bulbing dry matter accumulation to bulb development was observed in treatment F0, followed by treatment F3 and then F2. The difference in the contribution of post-bulbing dry matter accumulation to bulb development was not significant between treatments F2 and F3.

3.4. Bulb Yield and Partial Factor Productivity of Nitrogen Fertilizer

The fresh weight of bulbs initially increased and then decreased with the reduction in fertilizer supply during both growing seasons (Table 3). The fresh weight of bulbs under treatment F2 was greatest among all treatments. The splitting rate of bulbs was higher under treatment F1 compared to other treatments, but there were no prominent differences across all treatments. The small bulb rate initially decreased and then increased with the reduction in fertilizer supply during the 2022–2023 growing season, and the small bulb rate under F2 was lowest among all treatments. The difference in the small bulb rate was insignificant under various treatments during the 2023–2024 growing season. The bolting rate of bulbs showed a gradual increasing trend as the fertilization rate decreased. The marketable bulb rate initially increased and subsequently decreased with the reduction in fertilizer supply across both growing seasons. The dry matter content of bulbs showed an increasing trend with the reduction in fertilizer supply; however, no significant differences were observed across all treatments. The bulb index initially decreased and then increased with the reduction in fertilizer supply during the 2022–2023 growing season. The bulb index showed an increasing trend with the reduction in fertilizer supply during the 2023–2024 growing season.
Marketable bulb yield initially increased and subsequently decreased with the reduction in fertilizer supply during the 2022–2023 and 2023–2024 growing seasons (Figure 6). Marketable bulb yield under treatment F2 was highest among various treatments in both growing seasons. Compared with F1, marketable bulb yield under treatment F2 increased by 8.4% and 5.8% in 2022–2023 and 2023–2024, respectively. Compared with F3, marketable bulb yield under F2 increased by 24.1% and 5.0% in 2022–2023 and 2023–2024, respectively.
The widespread practice of excessive nitrogen fertilization has led to an imbalance in soil nutrients and a decline in the PFPN. Within the context of high-efficiency agricultural production, enhancing agricultural nitrogen fertilizer productivity is crucial for achieving sustainable agricultural development and ensuring food security. The PFPN showed a gradually increasing trend with the reduction in fertilizer supply during the 2022–2023 and 2023–2024 growing seasons (Figure 6). The PFPN under treatment F2 was higher than that under treatment F1 in both growing seasons. Compared with F1, the PFPN under treatment F2 increased by 35.2% and 32.0% in 2022–2023 and 2023–2024, respectively. The differences in the PFPN between treatments F2 and F3 were not statistically significant.

3.5. Correlation Analysis

A Pearson’s correlation matrix (Figure 7) was constructed for the two growing seasons to assess the relationships between onion marketable bulb yield and dry matter characteristics. The results exhibit significant correlations between onion marketable bulb yield and dry matter characteristics across both growing seasons. Onion marketable bulb yield showed significantly positive correlations with aboveground dry matter and the dry matter accumulation rate throughout the onion growth period during the 2022–2023 growing season (Figure 7a). However, the positive correlation between onion marketable bulb yield and the aboveground dry matter accumulation rate at 150 days after transplanting was not significant during the 2023–2024 growing season (Figure 7b). Additionally, onion marketable bulb yield exhibited negative correlations with the proportion of stems throughout the onion-growth period during the 2022–2023 growing season, while the negative correlation was insignificant at 120 days after transplanting during the 2023–2024 growing season. Furthermore, onion marketable bulb yield exhibited significantly positive correlations with the proportion of leaves from 60 days to 120 days after transplanting but exhibited a significantly negative correlation with the proportion of leaves at 150 days after transplanting during the 2022–2023 growing season. However, the positive correlation between onion marketable bulb yield and proportion of leaves at 120 days after transplanting was insignificant in 2023–2024. Onion marketable bulb yield showed significantly positive correlations with dry matter translocation, dry matter translocation efficiency, and the contribution of dry matter translocation to bulb development before the onset of bulbing in the 2023–2024 growing season. However, an insignificant correlation was observed between onion marketable bulb yield and the contribution of dry matter translocation to bulb development in the 2022–2023 growing season. Onion marketable bulb yield demonstrated significantly positive correlations with dry matter accumulation after the onset of bulbing in both growing seasons.

4. Discussion

4.1. Effects of Various Fertilization Rates on Dry Matter Accumulation of Onion

Plant dry weight was significantly associated with onion bulb yield, which has been verified in previous studies [15]. The achievement of high yield intensively depended on the enhanced aboveground dry matter accumulation [16], which was strongly influenced by fertilization rate [10]. Aboveground dry matter was notably affected by various fertilization rates. In this study, the aboveground dry matter and dry matter accumulation rate of onion initially increased and subsequently decreased with the reduction in fertilization rate, and this variation trend was considerably obvious from the initiation of bulbing to harvest. Among all treatments, treatment F2 exhibited the highest aboveground dry matter and dry matter accumulation rate. Marketable bulb yield initially increased and then decreased with augmenting fertilizer supply [17]. Excessive fertilizer application rates can cause salt damage to onion plants [18], thereby suppressing marketable bulb yield [10]. Similarly, the results of this study indicate that marketable bulb yield manifested an increasing trend at first and then a decreasing trend with the reduction in fertilizer supply during both growing seasons. The productivity of onion bulbs was the highest under treatment F2 with an N-P2O5-K2O fertilization rate of 339-216-318 kg·ha−1. Previous studies have reported that a significantly high nitrogen application rate, as much as 250 kg N·ha−1, is necessary to achieve higher marketable and total bulb yields [19]. The 200 kg·ha−1 nitrogen fertilization rate resulted in the highest yield as well as dry matter accumulation in Portici, located in Naples, Southern Italy [20]. The application of 200 kg N·ha−1 resulted in the highest yield (42.6 t·ha−1)—and the average weight of bulb (193.6 g) in Shewa Robit, North Shewa, Ethiopia [21]. However, an optimized nitrogen application rate of approximately 150 kg ha−1 was recommended when organic matter was added under irrigated conditions in Northern Senegal [22]. The requirement for mineral fertilizer varies across different regions, depending on different factors in the respective growing areas. Effective use of nitrogen depends on soil condition, irrigation system, climatic factors, and management factors. In a range of research reports, it is described that onion is a heavy feeder of N, P, and K, and the application of these mineral elements enhances production significantly [23]. Previous studies showed that the average onion productivity in Ethiopia (9.3 t·ha−1) is significantly lower compared to that of other major onion-producing countries, such as the Republic of Korea (66.15 t·ha−1), the United States (56.13 t·ha−1), the Netherlands (51.64 t·ha−1), Japan (46.64 t·ha−1), and Egypt (36.16 t·ha−1) [24]. However, the results of this study indicate that the average onion yield reaches its maximum value of 118 t·ha−1 under treatment F2 in the dry-hot valley region of Southwest China. The target yield serves as a key determinant in establishing the optimal fertilizer application rate. Meanwhile, optimizing nitrogen fertilizer management strategies not only significantly increased crop yield but also improved the PFPN [25]. In this study, compared with treatment FI, F2 reduced the fertilization rate and improved the onion marketable bulb yield and the PFPN. Hence, the optimal fertilization rate can simultaneously enhance crop marketable yield and partial factor productivity of nitrogen fertilizer, thereby achieving high-yield and high-efficiency cultivation. Treatment F2 is recommended as the optimized fertilizer application rate in the dry-hot valley region of Southwest China. Marketable bulb yield was significantly and positively correlated with aboveground dry matter and the dry matter accumulation rate of onion across all growth periods in both growing seasons. These results suggest that optimizing fertilization rates to enhance aboveground dry matter and the dry matter accumulation rate, particularly during the post-bulbing period, is closely associated with improving marketable bulb yield.

4.2. Effects of Various Fertilization Rates on the Partitioning and Translocation of Aboveground Dry Matter of Onion

Dry matter partitioning and translocation are crucial for crops to attain high and stable yields [26,27]. Improving dry matter production is a prerequisite for achieving high crop yields, while distributing more dry matter to harvested organs plays a vital role [28]. It is essential to enlarge leaf dry matter to enhance bulb yield from planting to the initiation of bulbing [17], followed by expanding bulb dry matter during the bulb developing stage [29,30]. Similarly, the results of this study indicate that the proportion of leaves was higher than the proportion of stems before bulb initiation; however, the proportion of bulbs was higher than the proportion of leaves after bulb initiation. This study shows that marketable bulb yield was significantly positively correlated with the proportion of leaves before bulb initiation. However, a significantly negative correlation was exhibited between bulb yield and the proportion of leaves at harvest. Therefore, an appropriate fertilization rate should accelerate leaf dry matter accumulation from planting to the initiation of bulbing, ensuring that leaves can produce sufficient dry matter to support onion bulb development during the bulb development stage. Conversely, insufficient fertilizer supply leads to an obvious decrease in leaf dry matter from planting to the initiation of bulbing, followed by a manifest decrease in bulb dry matter during the bulb development stage, ultimately resulting in reduced bulb yield at harvest.
Previous studies have demonstrated that the translocation of dry matter is influenced by field management practices [31], including fertilizer supply, which is the primary human factor affecting dry matter translocation [32,33]. The translocation and assimilation of non-harvested organs after the initiation of harvested organs constitute the main source of crop yield formation [34]. This study exhibits that marketable bulb yield exhibited significantly positive correlations with dry matter accumulation and the proportion of bulbs after bulbing initiation, as well as dry matter translocation, translocation efficiency, and the contribution of dry matter translocation to bulbs before bulbing initiation. Fertilization can promote crop yield by regulating the translocation and assimilation of harvested organs and the source–sink relationships of dry matter [31,33,35]. However, the highest fertilization rate disrupted the source–sink relationships of dry matter, leading to a reduction in the marketable yield of plants [33]. Analogously, this study reveals that the dry matter accumulation and the proportion of bulbs both showed trends of initially increasing and subsequently decreasing with the reduction in fertilization rate after initiation of bulbing. The dry matter accumulation and the proportion of bulbs under treatments F2 and F3 were higher than those under other fertilization treatments after the initiation of bulbing. In contrast, the dry matter translocation, translocation efficiency, and contribution of dry matter translocation to bulb development before initiation of bulbing showed gradually decreasing trends as the fertilizer supply was reduced. Previous studies have demonstrated that the optimal fertilizer application level can reduce fertilizer use by 25% without significantly reducing the yield [36]. Therefore, the optimal fertilization rate F2 can improve the source–sink relationships of dry matter, promote dry matter production, and facilitate its allocation to onion bulbs after bulb initiation. The results indicate that optimizing fertilization rate to enhance marketable bulb yield is closely associated with improving dry matter accumulation and the proportion of bulbs after bulbing initiation.

5. Conclusions

Optimizing the fertilization rate improved marketable bulb yield and partial factor productivity of nitrogen fertilizer by promoting dry matter accumulation and increasing the dry matter accumulation rate after bulbing initiation. A fertilization rate of N-P2O5-K2O at 339-216-318 kg ha−1 is highly recommended for onion production in the arid-hot valley of Southwest China. This optimal fertilization rate achieved a 20.0% reduction in fertilizer application amount, a 32.0–35.2% increase in the PFPN, and a 5.8–8.4% improvement in onion bulb productivity compared to the local routine fertilizer application.

Author Contributions

Conceptualization, J.L., H.Z., L.S., and H.F.; Writing—original draft, J.L.; Data curation, J.L.; Formal analysis, J.L., K.L., and Y.L.; Methodology, X.Y., K.L., and Y.L.; Data encapsulation, J.L.; Investigation, J.L., K.L., and Y.L.; Data curation, J.L.; Funding acquisition, H.Z. and L.S.; Supervision, L.S.; Project administration, L.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Agricultural Joint Fund of Yunnan Province, China (202301BD070001-219), the National Key Research and Development Program of China (2023YFD1901205), the Fund of Institute of Tropical Eco-agriculture, Yunnan Academy of Agricultural Sciences, China (2022RQS002) and the Key Laboratory Project of Yunnan Province (202205AK070026-01).

Data Availability Statement

The data presented in this study are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Effects of different fertilization rates on aboveground dry matter of onion at 60 days, 90 days, 120 days, and 150 days after transplanting during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. I, II, III, and IV show times of 60 days, 90 days, 120 days, and 150 days after transplanting. The different lowercase letters indicate significant differences between different fertilization rates within growth periods.
Figure 1. Effects of different fertilization rates on aboveground dry matter of onion at 60 days, 90 days, 120 days, and 150 days after transplanting during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. I, II, III, and IV show times of 60 days, 90 days, 120 days, and 150 days after transplanting. The different lowercase letters indicate significant differences between different fertilization rates within growth periods.
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Figure 2. Effects of different fertilization rates on aboveground dry matter accumulation rate of onion at 60 days, 90 days, 120 days, and 150 days after transplanting during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. I, II, III, and IV show times of 60 days, 90 days, 120 days, and 150 days after transplanting. The different lowercase letters indicate significant differences between different fertilization rates within growth periods.
Figure 2. Effects of different fertilization rates on aboveground dry matter accumulation rate of onion at 60 days, 90 days, 120 days, and 150 days after transplanting during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. I, II, III, and IV show times of 60 days, 90 days, 120 days, and 150 days after transplanting. The different lowercase letters indicate significant differences between different fertilization rates within growth periods.
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Figure 3. Effects of different fertilization rates on aboveground dry matter partitioning of onion at 60 days (a,e), 90 days (b,f), 120 days (c,g), and 150 days (d,h) after transplanting during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. I, II, III, and IV show times of 60 days, 90 days, 120 days, and 150 days after transplanting. The different lowercase letters indicate significant differences between different fertilization rates within growth periods.
Figure 3. Effects of different fertilization rates on aboveground dry matter partitioning of onion at 60 days (a,e), 90 days (b,f), 120 days (c,g), and 150 days (d,h) after transplanting during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. I, II, III, and IV show times of 60 days, 90 days, 120 days, and 150 days after transplanting. The different lowercase letters indicate significant differences between different fertilization rates within growth periods.
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Figure 4. Effects of different fertilization rates on aboveground dry matter translocation, translocation efficiency, and contribution of dry matter translocation to bulb development of onion during 2022–2023 and 2023–2024 seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. The different lowercase letters indicate significant differences between different fertilization rates.
Figure 4. Effects of different fertilization rates on aboveground dry matter translocation, translocation efficiency, and contribution of dry matter translocation to bulb development of onion during 2022–2023 and 2023–2024 seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. The different lowercase letters indicate significant differences between different fertilization rates.
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Figure 5. Effects of different fertilization rates on post-bulbing dry matter accumulation and contribution of dry matter translocation to bulb development of onion during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. The different lowercase letters indicate significant differences between different fertilization rates.
Figure 5. Effects of different fertilization rates on post-bulbing dry matter accumulation and contribution of dry matter translocation to bulb development of onion during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, F5, and F0 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, 80% lower than fertilization rate of local routine, and no fertilizer application, respectively. The different lowercase letters indicate significant differences between different fertilization rates.
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Figure 6. Effects of different fertilization rates on onion marketable bulb yield and partial factor productivity of nitrogen fertilizer during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, and F5 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, and 80% lower than fertilization rate of local routine, respectively. The different lowercase letters indicate significant differences between different fertilization rates within growing seasons.
Figure 6. Effects of different fertilization rates on onion marketable bulb yield and partial factor productivity of nitrogen fertilizer during 2022–2023 and 2023–2024 growing seasons. F1, F2, F3, F4, and F5 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, and 80% lower than fertilization rate of local routine, respectively. The different lowercase letters indicate significant differences between different fertilization rates within growing seasons.
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Figure 7. A Pearson’s correlation matrix between bulb yield, aboveground dry matter accumulation, partitioning, and translocation at different times after transplanting in the 2022–2023 (a) and 2023–2024 (b) growing seasons, respectively. “*”, “**”, and “***” denote significance levels at p < 0.05, p < 0.01, and p < 0.001. I, II, III, and IV show the times of 60 days, 90 days, 120 days, and 150 days after transplanting.
Figure 7. A Pearson’s correlation matrix between bulb yield, aboveground dry matter accumulation, partitioning, and translocation at different times after transplanting in the 2022–2023 (a) and 2023–2024 (b) growing seasons, respectively. “*”, “**”, and “***” denote significance levels at p < 0.05, p < 0.01, and p < 0.001. I, II, III, and IV show the times of 60 days, 90 days, 120 days, and 150 days after transplanting.
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Table 1. The physio-chemical properties of soil at a 0–20 cm depth.
Table 1. The physio-chemical properties of soil at a 0–20 cm depth.
Soil PropertyValue
Soil textureSandy loam
Bulk density1.4 (g∙cm−3)
pH6.4
Soil organic matter6.1 (g∙kg−1)
Available nitrogen39.0 (mg∙kg−1)
Available phosphorus30.4 (mg∙kg−1)
Available potassium129.0 (mg∙kg−1)
Table 2. The fertilization rates under different experimental treatments in this study.
Table 2. The fertilization rates under different experimental treatments in this study.
TreatmentThe First Fertilization Rate (kg·hm−2) The Second Fertilization Rate (kg·hm−2)The Third Fertilization Rate (kg·hm−2)The Fourth Fertilization Rate (kg·hm−2)The Fifth Fertilization Rate (kg·hm−2)
NP2O5K2ONP2O5K2ONP2O5K2ONP2O5K2ONP2O5K2O
F142.327.039.884.654.079.584.654.079.5126.981.0119.384.654.079.5
F233.921.631.867.843.263.667.843.263.6101.764.895.467.843.263.6
F325.416.223.950.732.447.750.732.447.776.148.671.650.732.447.7
F417.010.815.933.921.631.833.921.631.850.932.447.733.921.631.8
F58.45.48.016.810.815.916.810.815.925.216.223.916.810.815.9
F00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0
Table 3. Bulb yield components of onion under different treatments.
Table 3. Bulb yield components of onion under different treatments.
YearTreatmentBulb Fresh Weight (g·plant−1)Splitting Bulb Rate (%)Small Bulb Rate (%)Bolting Bulb Rate (%)Marketable Bulb Rate (%)Dry Matter Content of Bulbs (%)Bulb Index
2022–2023F1780.1 ± 50.5 ab1.2 ± 0.523.8 ± 3.3 ab0.1 ± 1.0 b75.0 ± 3.3 bc9.8 ± 0.3 a1.0 ± 0.2 a
F2913.4 ± 50.5 a0.0 13.1 ± 3.3 c0.2 ± 1.0 b86.7 ± 3.3 a9.8 ± 0.3 a0.9 ± 0.2 ab
F3756.7 ± 50.5 b0.0 19.1 ± 3.3 abc0.2 ± 1.0 b80.8 ± 3.3 ab10.1 ± 0.3 a0.9 ± 0.2 ab
F4799.2 ± 50.5 ab0.0 25.0 ± 3.3 ab0.8 ± 1.0 b74.2 ± 3.3 bc9.3 ± 0.3 a0.9 ± 0.2 c
F5564.0 ± 50.5 c0.028.6 ± 3.3 a6.8 ± 1.0 a64.6 ± 3.3 c9.5 ± 0.3 a0.9 ± 0.2 abc
2023–2024F1647.1 ± 40.1 ab15.3 ± 1.8 a6.3 ± 1.4 a0.9 ± 0.3 b77.5 ± 2.7 a8.0 ± 0.5 a0.8 ± 0.2 b
F2697.9 ± 40.1 a12.0 ± 1.8 a6.7 ± 1.4 a0.9 ± 0.3 b80.2 ± 2.7 a8.2 ± 0.5 a0.7 ± 0.2 b
F3637.4 ± 40.1 ab14.7 ± 1.8 a4.7 ± 1.4 a1.5 ± 0.3 b79.3 ± 2.7 a8.4 ± 0.5 a0.7 ± 0.2 b
F4543.0 ± 40.1 b12.6 ± 1.8 a5.3 ± 1.4 a1.7 ± 0.3 b80.4 ± 2.7 a8.4 ± 0.5 a0.7 ± 0.2 b
F5395.4 ± 40.1 c11.5 ± 1.8 a3.6 ± 1.4 a8.5 ± 0.3 a76.5 ± 2.7 a8.9 ± 0.5 a0.9 ± 0.3 a
Note: Different letters within a column indicate significant differences among treatments within two seasons at 5% level. F1, F2, F3, F4, and F5 represent fertilization rate of local routine, 20% lower than fertilization rate of local routine, 40% lower than fertilization rate of local routine, 60% lower than fertilization rate of local routine, and 80% lower than fertilization rate of local routine, respectively. Values are the mean ± Standard Error.
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Li, J.; Li, K.; Li, Y.; Yue, X.; Zhu, H.; Shi, L.; Fang, H. Optimizing Fertilization Rate to Achieve High Onion Bulb Yield and High Nitrogen Fertilizer Productivity in Dry-Hot Valley Region of Southwest China. Agronomy 2025, 15, 1822. https://doi.org/10.3390/agronomy15081822

AMA Style

Li J, Li K, Li Y, Yue X, Zhu H, Shi L, Fang H. Optimizing Fertilization Rate to Achieve High Onion Bulb Yield and High Nitrogen Fertilizer Productivity in Dry-Hot Valley Region of Southwest China. Agronomy. 2025; 15(8):1822. https://doi.org/10.3390/agronomy15081822

Chicago/Turabian Style

Li, Jiancha, Kun Li, Yilin Li, Xuewen Yue, Hongye Zhu, Liangtao Shi, and Haidong Fang. 2025. "Optimizing Fertilization Rate to Achieve High Onion Bulb Yield and High Nitrogen Fertilizer Productivity in Dry-Hot Valley Region of Southwest China" Agronomy 15, no. 8: 1822. https://doi.org/10.3390/agronomy15081822

APA Style

Li, J., Li, K., Li, Y., Yue, X., Zhu, H., Shi, L., & Fang, H. (2025). Optimizing Fertilization Rate to Achieve High Onion Bulb Yield and High Nitrogen Fertilizer Productivity in Dry-Hot Valley Region of Southwest China. Agronomy, 15(8), 1822. https://doi.org/10.3390/agronomy15081822

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