The Effect of Sowing Date on Soybean Growth and Yield Under Changing Climate in the Southern Coastal Region of Korea

Abstract

Sowing date significantly affects plant growth, development, and yield, holding a crucial role in soybean cultivation. This study was conducted in the southern coastal region of Korea under recent climate change conditions to investigate the effects of five different sowing dates on climatic characteristics, growth, and yield. Compared to historical data, the southern coastal region has experienced a consistent increase in average temperature during the soybean cultivation period, along with frequent abnormal summer climate events such as concentrated heavy rainfall and monsoons. These climate changes prolonged the vegetative growth period in earlier sowings, leading to an increased risk of lodging at maturity due to vigorous vegetative growth. Furthermore, earlier sowing delayed flowering and exposed plants to longer post-flowering photoperiods, consequently reducing the number of pods. Therefore, in the southern coastal region of Korea, it is crucial to re-evaluate conventional sowing practices and establish region-specific optimal dates, with careful consideration given to postponing the soybean sowing date to late June in order to enhance yield stability and improve the feasibility of double-cropping systems by shortening the growing period.

1. Introduction

Soybean (Glycine max L.) is one of the most important food crops globally, cultivated alongside rice, barley, wheat, and corn for centuries. It is widely grown across temperate, tropical, and subtropical regions. Serves as a key plant-based source of protein and fat, with increasing global demand. Its high utility value is reflected in its extensive use in various processed food products [1,2]. Soybeans are typically cultivated as a single crop; however, in the southern regions of Korea, they have traditionally been grown in double-cropping systems alongside winter crops such as onions, garlic, and winter barley, owing to regional climatic conditions. In recent years, however, global warming has led to an increase in soybean cultivation in double-cropping systems involving spring potatoes and spring cabbage [3].

Over the past century, the average temperature in Korea has risen by approximately 1.7 °C—more than twice the global average rate—and this warming trend is projected to continue. Under the Representative Concentration Pathway (RCP) 8.5 scenario, the average temperature may increase by up to 5 °C by the end of the 21st century [2,4,5].

This climate shift presents opportunities and challenges: it may enable the northward expansion of cultivation zones and lengthened growing seasons, and it increases the risk of crop damage from extreme weather events such as heatwaves, localized heavy rainfall, and droughts [4]. Although temperature increases within the optimal range can enhance photosynthesis and crop growth, exceeding this range (25~30 °C) can lead to significant growth inhibition and yield loss. For soybeans, temperatures above 37 °C during the flowering to maturation period can cause substantial reductions in yield [2,3,6,7,8].

Notably, variations in soybean yield have been linked to climatic conditions associated with different sowing dates [9]. Because each growth stage is affected by the prevailing environmental conditions at the time—such as temperature, photoperiod, and precipitation—the sowing date becomes a critical factor in determining growth and yield. Sowing times vary depending on the local climate, cropping system, and soybean variety. Selecting high-yielding varieties and determining their optimal sowing times is a fundamental strategy for ensuring stable cultivation and maximizing yield [1].

To address this, researchers have used the CROPGRO-Soybean model to simulate future soybean productivity under various sowing dates in the Korean Peninsula, considering climate change scenarios [3,10]. Studies also assessed the growth and yield of late-planted soybeans in response to warming trends [11] and identified suitable varieties based on soybean ecotypes and regional differences in key traits and yields across Korea’s central and southern regions [4,12].

However, limited research has focused specifically on adjusting sowing dates in response to climate change for the major soybean varieties cultivated in Korea’s southern coastal areas. Moreover, the standard soybean cultivation practices in this region—established in the late 1970s and early 1980s—are no longer fully aligned with the physiological characteristics of newly developed varieties or the current cultivation environment. This highlights the need to revisit and redefine optimal sowing dates for key varieties in the region [12].

Therefore, this study aims to precisely determine the optimal sowing time to ensure stable cultivation and maximized productivity for major soybean varieties in Korea’s southern coastal region under changing climate conditions.

2. Materials and Methods

2.1. Study Area and Soil Chemical Properties

The experiment was conducted over two years (2023–2024) at the experimental field of Sunchon National University in Jeollanam-do, South Korea (35°00′ N, 127°50′ E), using two soybean varieties: Daewon and Seonpung. These two soybean varieties (non-GMO) are widely utilized as major varieties in the South Korean fermented soybean product industry due to their suitable characteristics for manufacturing and superior quality. The physicochemical properties of the experimental soil are listed in

Table 1. The chemical properties of the topsoil (5–15 cm) in the field before the experiment in 2023 and 2024.

Year	pH (1:5)	EC (ds m−1)	OM (g kg−1)	P2O5 (mg kg−1)	Ex. Cation (cmol kg−1)
					K	Ca	Mg	Na
2023	6.19	0.61	26.0	225	1.12	6.62	2.42	0.13
2024	6.43	0.37	31.2	276	1.16	9.37	3.65	0.16

EC, electrical conductivity; OM, organic matter.

. Soil samples were collected from a depth of 5–15 cm prior to the experiment to analyze their chemical characteristics. The experimental soil, a silty loam, showed a pH ranged from 6.19 to 6.43, and electrical conductivity (EC) ranged from 0.37 to 0.61 dS/m, which fell within the optimal range of below 2.0 dS/m.

2.2. Climate Analysis Method

Monthly average temperature, cumulative precipitation, and growing degree days (GDD) from May to October—the soybean cultivation period—were obtained from the Korea Meteorological Administration’s standard meteorological station located near the experimental site. These data were compared with 10-year climate normals spanning the past 30 years (1993–2002, 2003–2012, and 2013–2022) as well as the average values for 2023 and 2024.

GDD were calculated using daily average temperatures during the vegetative and reproductive growth stages, with the flowering stage serving as the reference point, by averaging the daily maximum and minimum temperatures and subtracting a crop-specific base temperature (10 °C).

2.3. Sowing Dates and Cultivation Methods

Sowing dates were determined based on the optimal sowing window for single cropping in the southern region of Korea, as recommended by the Rural Development Administration (RDA), which falls in early to mid-June. Five sowing schedules were implemented as follows: I (12, 16 May), II (24, 26 May), III (June 5), IV (14, 16 June), and V (24 June) (

Table 2. Sowing dates in 2023 and 2024.

Year	Sowing Date
	I	II	III	IV	V
2023	12 May	26 May	5 June	16 June	24 June
2024	16 May	24 May	5 June	14 June	24 June

). Plants were spaced at 70 cm × 15 cm, with two plants per hill. Fertilizers were applied based on soil analysis, following the standard rate of N–P₂O₅–K₂O at 3.0–3.0–3.4 kg ha⁻¹, and incorporated as a basal application prior to rotary tillage. The experiments were performed in four replicates and randomized block design was used.

The experiments followed a split-plot design, with sowing dates as the main plots and soybean varieties as subplots. Each treatment was replicated four times in plots measuring 25 m² (10 m × 2.5 m). Weed control after sowing was performed using Alachlor granules at a rate of 2 kg per 10a, with additional manual weeding conducted during the growing season. The lodging index was assessed at maturity, with plants inclined more than 45° classified into five grades based on the percentage of lodged plants: 1 (<5%), 3 (6–10%), 5 (11–50%), 7 (51–75%), and 9 (>76%).

2.4. Growth and Yield Characteristic Survey

Crop growth characteristics were assessed at the flowering stage (R2) for each treatment using 10 plants per replication with four replications. Measurements included plant height, stem diameter, number of branches, number of main stem nodes, leaf area index (LAI), and dry weight. At the maturity stage (R8), yield components were assessed using a completely randomized design with four replications per treatment, where ten randomly selected plants from each replication were sampled. The evaluated parameters included the number of pods per plant, the number of seeds per pod, the 100-seed weight, and the total yield per plant. The seed yields of the plots were converted into seed yields per hectare with 14% moisture. Seed size distribution was analyzed using a sieve shaker (Retsch, Haan, Germany, As200) equipped with international standard sieves (ISO mesh sizes) to classify seed sizes into the following categories: <4.75 mm, 5.6 mm, 6.7 mm, and >8.0 mm.

2.5. Statistical Analysis

All data were measured in triplicate, and statistical analyses were performed using ANOVA through SAS software (version 9.4; SAS Institute Inc., Cary, NC, USA). Duncan’s multiple range test was used to test for significant differences between treatments at the 5% significance level of mean values. The correlation between the treatment factors and growth and yield characteristics was analyzed using Pearson’s correlation at the 0.1%, 1%, and 5% levels.

3. Weather Conditions

Recent climate change, including global warming, has significantly affected the growth and development of soybeans, often leading to various physiological disorders. Among the environmental factors, temperature and precipitation play critical roles in determining soybean growth, yield, and quality [13,14]. An analysis of average temperature trends during the soybean growing season (May–October) in the southern coastal region of Korea from 1993 to 2024 (Figure 1A) revealed that the 10-year normal temperatures for the periods 1993–2002 and 2003–2012 were both 21.8 °C. However, the 10-year normal for 2013–2022 rose to 22.5 °C and the annual averages for 2023 and 2024 increased even more, reaching 23.1 °C and 24.2 °C, respectively. These rising temperatures are approaching the optimal range for soybean growth (25–30 °C) [15], suggesting potential improvements in cultivation conditions in the future.

Emergence is a critical stage in soybean cultivation, marking the beginning of independent nutritional life as chloroplasts form in above-ground organs. This stage is highly influenced by environmental factors such as temperature and soil moisture [16]. Healthy emergence supports early crop establishment, enhances resistance to pests and diseases, and improves adaptability to environmental changes. The minimum temperature required for soybean emergence is 8 °C [13]; temperatures below this threshold can lead to cold damage. In 2023 and 2024, minimum temperatures during the sowing period (mid-May to late June) were 12.2 °C and 8.6 °C, respectively, indicating favorable conditions for emergence and early growth without cold damage.

Total precipitation during the soybean growing period (May to October) was 1649 mm in 2023 and 1906 mm in 2024 (Figure 1B), representing increases of 55.3% and 79.0%, respectively, compared to the 30-year average of 1061 mm (based on the periods 1993–2002, 2003–2012, and 2013–2022). These results indicate that precipitation in the past two years was significantly higher than the historical average. In Korea, approximately 50% of the annual precipitation typically occurs during the summer months of June and July [17]. Compared to the 30-year average of 51.9% for this period, the proportion of summer precipitation increased by 15.6% in 2023 (67.5%) and by 9.3% in 2024 (61.2%).

Consequently, during July—the mid-growth stage of soybeans—excessive rainfall, reduced sunlight, and increased pest and disease pressures had a considerable impact [4,17]. Excessive moisture for more than four days after flowering is a major factor contributing to yield reduction [13]. Persistent soil saturation during this period can inhibit root respiration, reduce nitrogen fixation by symbiotic bacteria, and increase vulnerability to root diseases such as Phytophthora root rot and Pythium damping-off, which thrive in wet conditions [18,19]. Studies show that severe instances of these moisture-induced diseases can lead to soybean yield losses ranging from 5% to 10% [20]. In July 2024, humid conditions persisted for 19 days, likely exerting a greater negative effect on soybean yield compared to the 12 humid days recorded in 2023.

Additionally, temporary drought during the pod-filling stage in August is a key factor affecting pod number and seed size [21,22]. In August 2024, total precipitation was just 76.2 mm—79.4% lower than the 371.2 mm recorded in August 2023—likely contributing to reduced seed size and overall yield.

In summary, the warming trend in the southern coastal region of Korea over the past three decades has brought temperatures closer to the optimal range for soybean growth, whereas abnormal climate events such as heavy rainfall during the reproductive stages (June–July) and drought during pod filling (August) are expected to negatively impact seed filling and yield. Soybeans are particularly sensitive to high temperatures and excess moisture, both of which can significantly disrupt reproductive development [18]. This underscores the importance of adopting agricultural management strategies, such as selecting heat- and moisture-tolerant varieties and adjusting sowing schedules to avoid periods of peak environmental stress during soybean cultivation in an increasingly variable climate.

4. Result and Discussion

4.1. Growing Degree Days and Days Required for Major Growth Stages by Sowing Date

The total accumulated temperature by sowing date for each soybean variety in 2023 and 2024 is presented in Figure 2. In 2023, Daewon and Seonpung exhibited a gradual decrease in growing degree days (GDD) from sowing group I to group V. The GDD ranged from 2811–3471 °C for Daewon and 2985–3731 °C for Seonpung, all of which were within or above the 2500–3000 °C range generally considered necessary for economically viable soybean production [4]. Notably, Daewon maintained GDD values above 3000 °C up to sowing group III, whereas Seonpung did so up to group IV.

A similar trend was observed in 2024. The highest GDD values were recorded in sowing group I—3816 °C for Daewon and 3952 °C for Seonpung—whereas the lowest occurred in group V, at 3090 °C and 3126 °C, respectively. Importantly, all sowing groups in 2024 exceeded 3000 °C. While appropriate GDD is essential for crop growth, excessively high GDD can induce heat stress, shorten the growth cycle, exacerbate water stress, and heighten pest and disease pressures, ultimately leading to severe negative impacts on yield and quality.

These results suggest that, owing to recent climate changes, economic soybean production may still be feasible, even with sowing delayed to late June. Accordingly, adjusting the sowing schedule in double-cropping systems involving winter crops in Korea’s southern coastal regions could not only enhance economic returns but also improve management efficiency. Moreover, revising the minimum effective accumulated temperature required for soybean cultivation to ensure economically viable yields under changing climate conditions may be necessary, and further research should explore the upper limit of suitable sowing dates.

Changes in the number of days required to reach the flowering (R2), seed-filling (R6), and maturity (R8) stages by sowing date for each variety are listed in

Table 3. The number of days from sowing to full bloom, full seed, and full maturity about sowing date in 2023 and 2024.

Cultivar	Year	Sowing Date	Number of Days from Sowing to Full Bloom (R2)	Number of Days from Sowing to Full Seed (R6)	Number of Days from Sowing to Full Maturity (R8)
Daewon	2023	I	51	96	138
		II	44	85	126
		III	44	76	118
		IV	38	72	114
		V	39	74	110
	2024	I	52	100	148
		II	47	92	140
		III	47	81	129
		IV	38	79	127
		V	37	76	117
Seonpung	2023	I	69	116	152
		II	59	102	140
		III	58	93	134
		IV	53	82	127
		V	50	84	121
	2024	I	69	114	155
		II	61	106	147
		III	56	98	136
		IV	54	89	127
		V	48	85	119

I: (12, 16 May), II: (24, 26 May), III: (5 June), IV: (14, 16 June), V: (24 June).

. For Daewon and Seonpung, the number of days from sowing to R2, R6, and R8 generally shortened as the sowing date was delayed from group I to group V in both years.

Specifically, for Daewon, the time to reach R8 in sowing group I was the longest—138 days in 2023 and 148 days in 2024—and gradually decreased with later sowing. In sowing group V, R8 was reached in just 110 days (2023) and 117 days (2024), reflecting reductions of 28 and 31 days, respectively. Seonpung showed a similar trend, with 152 and 155 days required to reach R8 in group I during 2023 and 2024, decreasing to 121 and 119 days in group V—a reduction of 31 and 36 days, respectively.

These findings, which demonstrate a gradual reduction in growth duration with delayed sowing, are consistent with the results reported by Oh et al. [1] and Lee et al. [4]. Although influenced by factors such as sunlight hours, temperature, and soil moisture, growth periods shorten under high average temperatures and are further accelerated under short-day conditions [1,23,24]. As shown in Figure 1A of this study, average monthly temperatures increased from mid-May to late June, exposing the crop to high temperatures and shorter daylight durations, which promote flowering [1]. This supports the observed reduction in growth duration from sowing group I to group V.

When comparing total days to R8 across sowing dates and years, Daewon showed a consistent 16-day difference between groups I and V in 2023 and 2024. For Seonpung, the difference was 12 days in 2023 and 15 days in 2024. Despite the ~45-day difference in sowing dates between groups I (mid-May) and V (late June), the actual difference in harvest timing was only 12–16 days. This suggests that early sowing does not substantially advance the harvest date. Consequently, in the context of double-cropping systems with winter crops, earlier sowing offers limited advantages in terms of timing for subsequent crops. Therefore, in the southern coastal regions of Korea, this study underscores the importance of adjusting soybean sowing dates later rather than earlier to optimize economic efficiency, minimize the required growing period, and reduce the total duration of field operations.

4.2. Growth Characteristics, Correlations, and Lodging Index by Sowing Date

The growth characteristics of soybeans at the flowering stage (R2)—including plant height, stem diameter, number of branches, number of main stem nodes, LAI, and dry weight—were analyzed according to sowing date (

Table 4. The change of soybean growth characteristics at flowering (R2) in 2023 and 2024.

Cultivar	Year	Sowing Date	Plant Height	Stem Diameter	No. of Branch	Node No. of Main Stem	LAI	Dry Weight
			cm	mm	Plant−1			g
Daewon	2023	I	63.5 a	6.5 a	3.7 a	13.7 a	4.6 a	14.8 a
		II	59.1 b	6.2 a	3.5 a	13.5 a	4.6 a	14.4 a
		III	57.8 b	5.6 b	3.8 a	13.0 ab	4.1 a	12.3 b
		IV	52.5 c	5.6 b	4.1 a	12.7 b	2.7 b	9.8 c
		V	41.4 d	5.5 c	4.1 a	11.8 c	2.4 b	7.9 d
	2024	I	93.2 a	9.4 a	4.3 a	14.3 a	7.1 a	26.3 a
		II	88.8 b	9.1 b	4.0 a	14.0 ab	6.8 ab	23.7 b
		III	76.0 c	8.9 ab	4.5 a	13.7 b	6.3 b	21.3 c
		IV	75.1 c	7.7 b	4.2 a	13.0 b	4.9 c	17.5 d
		V	44.0 d	6.3 c	4.3 a	11.3 c	4.1 c	15.5 e
Seonpung	2023	I	56.5 a	10.6 a	3.8 a	17.0 a	8.9 a	35.6 a
		II	56.0 a	9.3 b	4.0 a	16.0 b	8.3 b	30.9 b
		III	55.8 b	9.1 b	3.7 a	15.8 b	8.3 b	30.1 b
		IV	55.2 b	8.6 c	4.0 a	15.0 c	7.6 c	27.5 c
		V	53.8 c	8.5 c	4.2 a	14.0 d	6.8 d	25.2 d
	2024	I	76.8 a	9.8 a	4.0 a	16.5 a	8.8 a	35.2 a
		II	75.8 a	9.5 ab	4.3 a	16.3 ab	8.5 ab	32.3 b
		III	73.7 b	8.8 b	4.5 a	15.3 b	7.8 b	28.9 c
		IV	66.2 c	8.5 c	4.3 a	15.7 b	7.2 c	24.4 d
		V	67.2 c	8.5 c	4.4 a	13.8 c	6.1 b	21.9 e

I: (12, 16 May), II: (24, 26 May), III: (5 June), IV: (14, 16 June), V: (24 June). Different lowercase letters in the table indicate statistically significant differences between treatments at p < 0.05 as determined by ANOVA, followed by Tukey’s post hoc test.

). In general, all traits except the number of branches showed a declining trend with delayed sowing.

For the Daewon variety in 2023 and 2024, the tallest plants were recorded in sowing group I, reaching 63.5 cm and 93.2 cm, respectively. As sowing was delayed, plant height decreased progressively, reaching a minimum of 41.4 cm and 44.0 cm in sowing group V—representing reductions of 34.8% and 52.7%, respectively. Stem diameter and the number of main stem nodes also declined with delayed sowing. In sowing group V, these values were reduced by 15.3% (5.5 mm) and 13.8% (11.8 nodes), respectively, compared to group I. Similarly, LAI and dry weight were highest in the early sowing groups (I–II), exceeding 4.6 and 14.4 g, respectively.

A similar trend was observed for the Seonpung variety across both years, with plant height, stem diameter, number of main stem nodes, LAI, and dry weight all declining with delayed sowing. The lowest values were consistently found in sowing group V, except for the number of branches, which remained relatively unchanged.

These patterns were further supported by correlation analysis (

Table 5. Correlation of growth characteristics by year, cultivar, and sowing date treatments. Soil moisture content by subsurface drainage methods.

Treatments	Plant Height	Stem Diameter	No. of Branches	Node no. of Main Stems	LAI	Dry Weight
sowing time	−0.513 ***	−0.390 **	0.205 ns	−0.542 ***	−0.407 **	−0.447 ***
cultivar	−0.053 ns	0.653 ***	0.212 ns	0.750 ***	0.760 ***	0.779 ***
Year	0.678 ***	0.344 **	0.406 **	0.057 ns	−0.009 ns	0.311 *

ns: No significance; ***: p < 0.001, **: p < 0.01; *, p < 0.05.

), which revealed statistically significant negative correlations between sowing date and key R2 growth traits: plant height (r = −0.513 ***), stem diameter (−0.390 **), number of main stem nodes (−0.542 ***), LAI (−0.407 **), and dry weight (−0.447 ***).

In agreement with previous studies by Clovis et al. [25] and Haoyu et al. [26], our results confirm that later sowing dates lead to reductions in plant height, main stem node number, and LAI. This is likely due to the shortened vegetative growth period associated with delayed sowing, which limits photo-assimilate accumulation [27] and thus hinders vegetative development. Overall, the results indicate that earlier sowing promotes superior growth characteristics during the R2 stage in terms of plant height, stem diameter, number of main stem nodes, LAI, and dry weight—excluding the number of branches.

Figure 3 shows the lodging index at maturity according to the planting date for each year. Across two years, planting group I showed the highest average lodging index of 8 in both Daewon and Seonpung cultivars. However, as planting was delayed, the lodging index gradually decreased, showing the lowest index of below 4 in planting group V. This is because early planting leads to a long vegetative growth period and excessive vegetative growth induced by increased photosynthetic activity. This results in taller plants, higher leaf area index (LAI), and greater biomass accumulation, all of which contribute to a higher lodging index [1,4]. Therefore, early planting of soybeans can significantly increase the risk of lodging and consequently negatively impact quality and yield.

4.3. Yield Components and Correlations by Sowing Date

The results of examining the changes in the yield components of soybean varieties according to sowing date are listed in

Table 6. The yield components by sowing date in 2023 and 2024.

Cultivar	Year	Sowing Date	Pod Number	Seed Number	100-Seed Weight
			No. Plant−1	No. Pod−1	g
Daewon	2023	I	32.5 c	1.8 a	21.9 a
		II	40.7 b	1.9 a	22.2 a
		III	41.5 b	1.8 a	22.3 a
		IV	44.7 ab	1.8 a	21.8 a
		V	46.8 a	1.8 a	21.8 a
	2024	I	31.7 c	1.6 a	21.8 a
		II	35.6 b	1.7 a	21.7 a
		III	37.0 b	1.7 a	21.5 a
		IV	38.3 ab	1.8 a	21.5 a
		V	39.5 a	1.8 a	21.9 a
Seonpung	2023	I	30.1 d	1.9 a	22.8 a
		II	41.5 c	2.0 a	22.5 a
		III	41.1 c	2.0 a	22.2 a
		IV	42.8 b	2.0 a	22.5 a
		V	43.8 a	2.0 a	22.6 a
	2024	I	26.5 d	1.9 a	22.6 a
		II	34.6 c	1.9 a	22.4 a
		III	36.9 b	1.9 a	22.3 a
		IV	37.3 ab	2.0 a	22.1 a
		V	39.0 a	1.9 a	22.0 a

I: (12, 16 May); II: (24, 26 May); III: (5 June); IV: (14, 16 June); V: (24 June). Different lowercase letters in the table indicate statistically significant differences between treatments at p < 0.05 as determined by ANOVA, followed by Tukey’s post hoc test.

. First, although the number of seeds per pod and 100-seed weight did not show significant differences across sowing dates in either Daewon or Seonpung over the years, the number of pods per plant tended to increase with later sowing. In the Daewon variety, the number of pods per plant was lowest in sowing group I, with 32.5 and 31.7 pods in 2023 and 2024, respectively. In contrast, the highest values were recorded in sowing group V, with 46.8 and 39.5 pods, representing an average increase of over 31.4% compared to group I. A similar trend was observed in the Seonpung variety, where the number of pods per plant increased with later sowing in both years. The highest values were again found in sowing group V, with 43.8 and 39.0 pods in 2023 and 2024, respectively. These results indicate that the number of pods per plant—a key yield component—increased progressively with delayed sowing and peaked in sowing group V.

Seed yield also exhibited an increasing trend with later sowing (Figure 4). For the Daewon variety, the highest yields in 2023 and 2024 were recorded in sowing group V, showing increases of 42.2% (3230 kg ha⁻¹) and 40.0% (2940 kg ha⁻¹), respectively, compared to group I. Likewise, in the Seonpung variety, sowing group V produced the highest yields of 3530 kg ha⁻¹ in 2023 and 3120 kg ha⁻¹ in 2024—representing yield improvements of over 44.4% relative to sowing group I, which recorded the lowest yields.

These trends were clearly supported by the correlations between yield components and sowing date (

Table 7. Correlation of yield component by year, cultivar, and sowing date treatments.

Treatments	Pod Number	Seed Number	100-Seed Weight	Yield
Sowing time	0.710 ***	0.254 ns	−0.277 ns	0.719 ***
Cultivar	−0.146 ns	0.808 ***	0.757 ***	0.427 ns
Year	−0.486 *	−0.359 ns	−0.378 ns	−0.100 ns

ns: No significance; ***: p < 0.001; *: p < 0.05.

), where highly significant positive correlations were found for the number of pods per plant (r = 0.710 ***) and seed yield (r = 0.719 ***).

Therefore, in Daewon and Seonpung the number of pods per plant increased with later sowing dates, leading to the highest yields in sowing group V. This finding contrasts with the results reported by Haoyu et al. [26] and Spyridon et al. [28], who suggested that earlier sowing promotes rapid initial growth, resulting in a longer vegetative phase, faster node formation, earlier flowering, and consequently, increased pod numbers and yields.

This discrepancy is likely due to differing climatic conditions. Shandong Province in China, studied by Haoyu et al. [26], has a cooler average summer temperature (14.0 °C) and a temperate monsoon climate with annual rainfall of 600–800 mm. In contrast, the southern coastal region of Korea experiences a subtropical climate, with high average monthly temperatures during the growing season (26.3 °C in 2024), along with strong winds, heavy monsoon rains, and typhoons in June–July.

Earlier sowing in this region leads to higher cumulative effective temperatures and a prolonged vegetative period, resulting in excessive vegetative growth. Concentrated rainfall during the flowering period (Figure 1B), caused by extreme weather events, reduces photosynthesis and disrupts nutrient translocation, increasing lodging risk [4] and ultimately decreasing grain filling rates and yield.

Although earlier sowing extends the vegetative phase, it can delay flowering and expose plants to longer photoperiods post-flowering. Because soybeans are short-day plants—where flowering is induced under shorter daylight conditions—this can reduce pod formation and negatively impact yield, consistent with findings from other studies [1,11,29,30].

In conclusion, under the recent climate conditions in Korea’s southern coastal region, the number of pods per plant and seed yield—key yield components of soybeans—increased with later sowing dates, with the highest yields observed in the latest sowing group (group V).

4.4. Seed Quality by Sowing Date

The results of comparing the proportion of seed sizes according to soybean sowing date are listed in

Table 8. The change of soybean seed size by sowing date in 2023 and 2024.

Cultivar	Year	Sowing Date	<4.75 mm	4.75~5.6 mm	5.6~6.7 mm	6.7~8.0 mm	>8.0 mm
		%
Daewon	2023	I	4.2	9.8	19.0	67.0	0.0
		II	3.6	6.3	15.1	75.0	0.0
		III	1.9	2.9	15.9	77.3	2.0
		IV	0.5	1.0	18.2	78.4	1.9
		V	0.5	0.3	14.1	80.9	4.2
	2024	I	0.3	2.1	49.8	45.7	2.1
		II	0.5	2.0	46.1	49.7	1.7
		III	0.4	2.9	44.7	50.0	2.0
		IV	0.6	1.6	41.6	55.1	1.1
		V	0.4	1.9	35.2	59.4	3.1
Seonpung	2023	I	1.5	3.3	38.4	56.8	0.0
		II	0.6	3.0	35.1	60.2	1.1
		III	0.8	1.0	31.7	64.5	2.0
		IV	0.6	0.4	27.8	69.2	2.0
		V	0.5	0.2	25.3	71.5	2.5
	2024	I	0.0	1.0	47.1	51.1	0.8
		II	0.0	0.9	43.9	54.7	0.5
		III	0.3	1.4	47.8	50.3	0.2
		IV	0.3	0.7	39.6	58.6	0.8
		V	0.3	0.3	37.5	60.9	1.0

I: (12, 16 May), II: (24, 26 May), III: (5 June), IV: (14, 16 June), V: (24 June).

. In terms of seed diameter, the proportion of medium-sized seeds (≥ 6.7 mm) tended to increase with later sowing dates. In Daewon, the proportion of seeds with a diameter of ≥6.7 mm in sowing group V increased by an average of 11.4% across the years compared to sowing group I. Seonpung exhibited a similar trend, with an increase of up to 13.6% in sowing group V relative to group I over the years.

This trend is likely due to the effects of sowing earlier than the conventional optimal time. Early sowing leads to a longer vegetative growth period and increased accumulation of photo-assimilates, as noted by Lee et al. [4] and Violeta et al. [27]. This excessive growth increases the likelihood of lodging, which in turn reduces photosynthetic efficiency and disrupts nutrient distribution. These factors ultimately decrease the grain-filling rate and negatively impact seed size. This interpretation is further supported by the observed decline in the proportion of seeds with a diameter of ≥6.7 mm when moving from sowing group V to group I. Therefore, the proportion of medium-sized seeds (≥6.7 mm) in soybeans increased with later sowing and was highest in sowing group V.

5. Conclusions

This study aimed to determine the optimal soybean sowing date for the southern coastal region of Korea under current climate change by examining its effects on growth, yield attributes, and seed quality. Firstly, the southern coastal region has experienced a consistent increase in average temperature during the soybean cultivation period compared to historical data, coupled with frequent abnormal summer climate events such as concentrated heavy rainfall and monsoons. This climate change prolonged the vegetative growth phase in earlier sowings, leading to increased lodging risk at maturity due to vigorous vegetative growth. Furthermore, it delayed flowering and exposed plants to longer post-flowering photoperiods, consequently reducing the number of pods. Additionally, these effects also resulted in a decline in seed quality.

Therefore, for this region, careful consideration should be given to postponing the soybean sowing date to late June—later than the traditionally recommended early- to mid-June period. Future research should focus on actively implementing strategies such as irrigation and the utilization of disease-tolerant cultivars to minimize yield losses resulting from increasingly frequent extreme weather events, and on modeling optimal sowing windows that incorporate projected climate scenarios to ensure sustainable production.